Concrete Floor Solutions for Passive and Active Cooling

Design options for low energy buildings

Concrete floor solutions for passive and active cooling

2Concrete fl oor solutions for passive and active cooling

ContentsIntroduction 3

Fabric Energy Storage (FES) and thermal mass 4

How much thermal mass do you need? 5

Other design considerations 7

Passive and active fl oor options 9

Exposed slab, naturally ventilated building. Case study: Cambridge Federation of Womens Institute 10

Exposed slab, underfl oor mechanical ventilation. Case study: PowerGen Headquarters 12

Exposed hollowcore slab with cores supplied by mechanical ventilation. Case study: Innovate Green Offi ce 14

Exposed in-situ or lattice girder slab with embedded air ducts supplied by mechanical ventilation. Case study: 4 West Building, University of Bath 16

Exposed slab with embedded cooling/heating pipework. Case study: CAFOD Headquarters (Romero House) 18

Exposed hollowcore slab with embedded heating/cooling pipework. Case study: Vanguard House 20

Exposed composite lattice girder soffi t slab with embedded pipework. Case study: Manchester Metropolitan University Business School 22

Chilled beams with an exposed or partially exposed soffi t. Case study: Conquest House 24

Further case studies

55 Gee Street 26

Watermead Business Park 27

Loughborough University, East Park Design Centre 28

Greenfi elds Community Housing head offi ce 29

160 Tooley Street (speculative offi ce development) 30

97 Milton Park (speculative offi ce development) 31

Bermondsey Square (speculative offi ce development) 32

Toyota Headquarters (UK) 33

Canon Headquarters (UK) 34

References 35

Cover images

Main image: 160 Tooley Street, a 20,000m2 mixed-use development, see page 30.

Photo: courtesy of Timothy Soar

Top insert: A coff ered slab with underfl oor ventilation

Bottom insert: 4 West Building, University of Bath, which uses the Concretcool system, see pages 16 and 17.

Photo: courtesy of Cowlin Construction Ltd.

This page: CAFOD Headquarters atrium, see page 18. Photo: courtesy of Black Architecture.


IntroductionThe last few years have seen low energy, high thermal mass offi ce construction make a decisive move into the mainstream market. What was once largely the preserve of the one-off , owner occupier client has matured to become an increasingly common approach for minimising the cooling load in speculative offi ces.

Growth is largely the result of three key drivers:

1. Tougher Part L energy/CO2 targets and a signifi cant increase in the effi ciency requirements for mechanical cooling plant.

2. Rising energy costs which, more than anything else, is driving demand among occupiers1. This has signifi cantly strengthened the business case for investment in low energy buildings, particularly offi ces. Many investment institutions now accept low energy designs as the best way of future proofi ng their property assets2 and avoiding climatic obsolescence as temperatures continue to rise3.

3. A growing appreciation of visual concrete and the role it can play in the fabric fi rst approach to energy effi cient design.

The use of thermal mass to optimise fabric performance centres on the principle of Fabric Energy Storage (FES), which uses concrete fl oor slabs to absorb unwanted heat, helping stabilise the internal temperature and cut the CO2 emissions associated with mechanical cooling, see Figure 1. This is achieved either passively or in combination with a more active approach such as mechanical ventilation or chilled water to augment performance (sometimes referred to as a Thermally Active Building System or TABS). Using fl oors in this way makes good sense, as they typically provide the greatest source of thermal mass in non-residential buildings. The overall ability of concrete fl oors to provide thermal mass whilst also fulfi lling structural and aesthetic roles, make it a hard working material, capable of saving signifi cant capital and operating costs over the life of the building.

About this publicationSince this guide was fi rst published in 2005 existing FES techniques have evolved and new systems developed. This updated guide details all the various systems currently available, from the simple passive approach to the more sophisticated active systems.

Case studies are included to highlight recent projects featuring the various systems covered. This guide focuses on issues including cooling capacity, system control, visual appearance and buildability/spans etc. and aims to assist designers select the best fl oor option to meet specifi c project needs.

At one end of the spectrum is the entirely passive approach, which uses natural ventilation in combination with a fl at soffi t to meet the cooling needs of fairly undemanding environments, for which close control is not critical. At the other end of the spectrum is the high load environment, requiring the use of active slabs with water and/or mechanical ventilation as the cooling medium to provide greater cooling output and control. These options and the others that sit between share many design features, but also off er qualities of their own, all of which are summarised in the guide.

Figure 1: Benchmark CO2 emissions from offi ce buildings4. Bermondsey Square, London, which features exposed hollowcore slabs, see page 32.

Photo: courtesy of Igloo Regeneration Limited.

Kg CO2 /m2/year

Typical air-conditioned office

Good practiceair-conditioned office

High thermal massoffice with passive

and/or active cooling

0 25 50 75 100 125 150 175 200


How FES worksduring the daytime:The way FES works is quite simple. Concrete fl oor slabs have a high level of thermal mass, in other words they can store a lot of heat. This means that on warm days an exposed concrete soffi t will soak up much of the unwanted heat in a building, helping to maintain a comfortable, stable temperature and reduce the energy used in air-conditioned environments. The main way in which the soffi t absorbs heat is by radiation from adjacent surfaces i.e. objects and people at a higher temperature radiate heat to the comparatively cool concrete. Comfort is determined by a combination of radiant temperature, air temperature and air movement, so the presence of a comparatively cool soffi t makes a signifi cant contribution to summertime performance. The radiant cooling eff ect will continue throughout the day even though a signifi cant amount of heat may be absorbed. This is a consequence of the high level of thermal mass (i.e. heat capacity) provided by the slab which ensures the surface temperature increases very little across the day, maintaining the benefi cial temperature diff erence between soffi t and occupants. The proximity of the soffi t to the occupants makes it particularly suitable for radiant cooling because, to be eff ective, occupants need to be within line of sight of a comparatively cool surface that is nearby andcannot become obscured in the same way that walls and fl oors often are e.g. by furniture, carpets etc. Floor slabs also provide some convective cooling i.e. to air that comes into contact with the surface. This is more signifi cant where fl oors form part of a mechanical ventilation system (e.g. underfl oor ventilation or the TermoDeck system, see page 14.

In addition to stabilising the internal temperature an exposed concrete fl oor can delay its peak by around 6 hours, which in an offi ce environment, will typically occur in the early evening when the occupants have left for the day, see Figure 2.

How FES worksduring the night time:As the evening approaches and the working day comes to an end, heats gains from the sun, internal equipment and occupants start to diminish. During the night, particularly the early hours of the morning, the heating/cooling cycle goes into reverse and stored heat is given up by the fl oor slab. The simplest way of facilitating this is by ventilating the building with cool night air, which can be achieved with either natural and/or mechanical ventilation. The purging of accumulated heat ensures the fl oor slab is ready to repeat the cycle the following day.

This process is quite eff ective in the UK, as the variation in diurnal temperature (diff erence between day and night) is rarely less than5 degrees, and is usually much higher, making night cooling an eff ective means of removing heat. However, the urban heat island eff ect in the centre of large cities can reduce the eff ectiveness of night time cooling. An alternative or addition to night ventilation uses water to provide a more active approach to cooling. This off ers greater fl exibility and control of the process, since the amount of heat removed is not solely determined by night time temperatures.

Fabric Energy Storage(FES) and thermal mass

Figure 2: Stabilising effect of thermal mass on the internal temperature

Internal temperaturewith high thermal mass

Internal temperaturewith low thermal mass

Externaltemperature

Peak temperaturedelayed by up tosix hours

Up to 8 degrees differencebetween peak externaland internal temperature

30oC

15oC

Day DayNight


How much thermal mass do you need?The signifi cance of heat capacity and heat fl owConcrete fl oors of all types have the ability to store heat, but the degree to which this can be usefully exploited largely depends on two key factors:

The level of thermal mass the greater the thermal mass, thegreater a fl oors potential for storing heat; the question is whether the volume/depth of concrete will be suffi cient to handle the buildings cooling demand and the range of external conditions experienced over a typical summer?

The rate of heat fl ow - i.e. the rate at which the fl oor slab can absorb and release heat. This needs to be suffi ciently high to make a worthwhile contribution to the daily cooling needs of the building.

These two factors are intrinsically linked since increased heat fl ow allows more thermal mass to be utilised resulting in greater cooling potential, whilst poor heat fl ow has the opposite eff ect. What this means in practical terms is explained below, fi rstly for naturally ventilated buildings and then for air-conditioned buildings.

Thermal mass in naturally ventilated buildingsA question often asked by architects and designers is how much thermal mass do you need?The answer largely depends on the extent to which you want to optimise the building design. It is sometimes suggested that 100mm of concrete is suffi cient, but this does not take into account the way buildings respond to real weather patterns. For example, a naturally ventilated offi ce with exposed 100mm fl oors

(e.g. steel decking/soffi t with in-situ concrete topping) should have suffi cient heat capacity to cope with a simple 24 hour heating and cooling cycle. However, in addition to a buildings daily cycle, there are also longer cycles related to a typical hot spell (usually three to fi ve days) and also the weekly cycle of fi ve working days, from which heat will reach diff erent depths within the available thermal mass. For example, in a non-air-conditioned building the greater the slab depth, the longer the time period it responds to; the core of a 300mm thick concrete slab responds to the monthly average condition and draws heat in deeper over an extended period of hot weather. For longer time periods these factors are important because it is the longer-term average room temperatures that defi ne the thermal storage core temperature and hence the temperature gradient that draws heat in5. So, whilst 100mm of concrete off ers some useful thermal mass, thicker slabs provide greater temperature stability and increased cooling performance across a range of conditions including hot periods.

To help visualise what this means in practice, Figure 3 shows how the soffi t temperature of exposed 100mm and 300mm concrete fl oors in a naturally ventilated offi ce respond to the onset of hot weather. This was produced using fi nite diff erence modelling6 to calculate the heat transfer at the soffi t and through the fl oor slab over the course of several days, during which there is an increase in the external temperature7.

In the case of the 300mm slab, it can be seen that the soffi t temperature slowly increases over the course of several days in response to the onset of hot weather, during which it continues to provide a useful amount of cooling by virtue of its comparatively low soffi t temperature. In contrast, the 100mm fl oor warms up more rapidly, off ering less resilience to the hotter conditions, resulting in a greater risk of overheating. It is also less eff ective at delaying and moderating the daily peak internal temperature. The 300mm slab will of course take longer to cool down when conditions moderate, although as the external temperature drops windows can be opened more freely to optimise comfort, something that is best avoided during the preceding hot weather.

Figure 3: Change in soffi t temperature in response to a period of hot weather

018

19

20

21

22

23

24

25

26

Day

Star

t of h

ot w

eath

er

Soffi

t tem

pera

ture

(oC)

1 2 3 4 5 6 7 8 9 10 11 12 13

Exposed 300mm concrete floor

Exposed 100mm lightweight floor(steel decking with concrete topping)

The 300mm floor stays around twodegrees cooler than the 100mmfloor for several days and thenstabilises at a lower temperatureduring the day time.


Thermal mass in air-conditioned buildingsIn air-conditioned buildings the heating/cooling cycle is generally limited to 24 hours. This is because the tighter control of temperature should ensure there is no build up of heat in the fabric over periods much longer than a day. This might suggest that 100mm of concrete would provide the optimal level of thermal mass needed to reduce the air conditioning load. Whilst this is true for a concrete fl oor slab exposed on one side (i.e. the soffi t), in practice, slabs are often exposed on multiple surfaces i.e. combinations of top, bottom and internal surfaces formed by cores running through the slab. This greatly increases the surface area for heat absorption and the eff ective depth of concrete that can be utilised in a 24 hour period, which in turn, signifi cantly increases the cooling output of the slab, see Figure 6 on page 9.

A doubling of surface area is commonly achieved by combining an exposed soffi t with underfl oor mechanical ventilation, enabling heat transfer from top and bottom surfaces. Similarly, another option is to have an exposed soffi t in conjunction with hollowcores through which air is channelled, greatly increasing the heat transfer area and cooling output. TermoDeck and Concretcool are examples of this system (see pages 14 and 16). The use of mechanical ventilation also enables the air passing over the concrete to be made relatively turbulent, signifi cantly increasing the rate of heat transfer at the surface and, as a consequence, the depth heat will penetrate in the limited time available12; it is worth noting that the 100mm benchmark often quoted assumes the use of natural ventilation i.e. smooth/laminar air fl ow across a single surface.

Another active cooling option increasing in popularity is the use of water rather than air to regulate slab temperature, made possible by embedding plastic water pipes in the soffi t. Heat fl ow between the water and the concrete is by conduction, making it relatively rapid and enabling all the available thermal mass in the slab to be exploited. Hydrodeck and Climaspan are examples of this approach, see page 20.

Surface emissivity why this matters For heat to be stored or released from an exposed soffi t it must of course pass between the concrete slab and occupied space below. Although heat moves relatively quickly through concrete, the rate is generally much slower at its surface, which tends to act as a thermal bottle neck. Where possible, encouraging turbulent air fl ow across the soffi t will improve the convective heat fl ow. However, the rate at which the radiant heat fl ow occurs is determined by a diff erent property called surface emissivity. This relates to how refl ective or dull/matt a surface is, and is measured by a factor ranging between 0 and 1. Emissivity matters because matt surfaces, such as that of concrete, have a high emissivity level of between 0.85-0.95, making them very good at absorbing and emitting radiant heat. In contrast, comparatively refl ective surfaces such as fl oors with steel decking forming the soffi t, have a much lower emissivity of around 0.22-0.28, which limits radiant heat fl ow. Since the overall heat fl ow at the soffi t is typically about two thirds radiant and one third convective, a low surface emissivity will have a signifi cant impact on the already limited movement of heat to and from the surface.

In practical terms, low surface emissivity restricts the FES performance of exposed fl oors with steel decking, which is further reduced if permeable ceiling tiles are used to visually screen the steel decking. This can be seen in the cooling output of around 10-14 W/m2, dropping to around 4-9 W/m2 when a permeable ceiling is used. This contrasts with a value of around 15-25 W/m2 for a plain, exposed concrete soffi t. The cooling output for these and other fl oor options are summarised in Figure 6 on page 9.

A water cooled concrete soffi t is used at the Manchester Metropolitan Business School, see page 23.Photo: courtesy of Feilden Clegg Bradley Architects.

Regardless of whether a passive or active approach to cooling is used, the thermal mass in precast or in-situ concrete fl oor slabs with depths in excess of 100mm can be fully utilised to maximise cooling output and provide enhanced performance. The suggestion that 100mm of concrete provides suffi cient thermal mass is too simplistic and fails to take into account how fl oors are used in practice and the range of conditions under which they must perform.


Embodied CO2Detailed studies8,9 looking at the comparative embodied CO2 of concrete frame and steel frame offi ce buildings have been undertaken by the trade bodies for these materials. Both studies concluded that generally there is very little diff erence in the embodied CO2 of either option. So, not withstanding the importance of designing and specifying steel and concrete frames with a view to minimising their carbon footprint, the focus should be on the operational emissions, as this provides the greatest opportunity to reduce the whole life CO2 emissions.

Operational CO2 Figure 1 (page 3) compares the operational CO2 emissions of a typical and good practice air-conditioned offi ce with that of a high thermal mass offi ce with passive and/or active cooling; the reduced CO2 emissions for the latter approach is quite apparent. There are obviously some environments that dictate the need for air conditioning, although emissions can still be signifi cantly reduced through the use of the building fabric as described in this guide. A further incentive for adopting a more passive approach where possible, is the increasingly demanding emissions targets imposed by Part L2 of the Building Regulations, along with challenging new seasonal energy effi ciency requirements for cooling plant, if used.

Adaptation to future climateFES can help mitigate climate change through reduced operational CO2 emissions, whilst also off ering a degree of adaptation to the warming climate. Central to this is the ability of concrete frame buildings to very eff ectively combine passive and active cooling measures, varying the balance between the two as conditions dictate. This applies equally to both daily weather patterns and the much longer-term trends linked to climate change. Concrete fl oor slabs also off er a means of designing out the risk of climatic obsolescence in a number of other ways including:

The ability to embed dormant water cooling pipes for future use. The relative ease in which chilled beams can be retrofi tted with an

exposed soffi t.

The ability to activate dormant thermal mass by removing false ceilings if fi tted.

The ability to retrofi t micro-bore cooling pipes to the soffi t for increased performance.

Greater eff ectiveness of night cooling as an adaptation measure compared to buildings with lighter weight fl oors (night cooling is likely to be used as a future adaptation measure in many existing buildings).

BREEAM There are three key BREEAM categories in which concrete fl oors have the potential to infl uence the overall rating of a building. These are Health & Wellbeing, Energy and Materials. A breakdown of these categories and how they may relate to concrete fl oors is shown in Figure 4.

Other Design Considerations

Figure 4: Infl uence of concrete fl oors on a BREEAM rating for offi ces.

BREEAM category

1. Health & wellbeing

2. Management

3. Transport

4. Water

5. Energy

6. Pollution

7. Waste

8. Land use & ecology

9. Materials

Innovation

Total

Weighting

15%

12%

8%

6%

19%

10%

7.5%

10%

12.5%

100%

10%

No. of sub-categories

6

5

5

4

9

5

4

5

5

1. Visual comfort2. Safety & security3. Thermal comfort4. Water quality5. Acoustics performance6. Indoor air quality

Scores up to 2.8% (daylight, glare, view etc)

Scores up to 1.9% (thermal modelling of design)

Scores up to 1.9% (meets acoustic standards)Scores up to 5.6% (0.95% for nat. vent. capability)

% contribution to overall BREEAM rating

1. Reduced CO2 emissions2. Energy monitoring3. Efficient external lighting4. Low/zero carbon tech.5. Efficient cold storage6. Efficient transport systems7. Efficient laboratory systems8. Efficient equipment (process)9. Drying space

Scores up to 8.1%

Scores up to 2.7% (0.54% for night time cooling)

1. Life cycle impacts2. Hard landscaping3. Responsible sourcing4. Insulation5. Designing for robustness

Scores up to 4.8% (based on Green Guide rating)

Scores up to 2.9%

0


Visual in-situ soffi tsMaking in-situ concrete soffi ts visual gives rise to a number of specifi cation questions. The fi nish required determines the formwork. If a particular fi nish, such as board marking, is required then clearly the appropriate formwork should be used. For a plain, fl at fi nish, the design and formwork supplier should jointly determine the level of variations in tone and texture that are acceptable. The National Building Specifi cation and the National Structural Concrete Specifi cation for Building Construction include standard in-situ concrete fi nish specifcations.

The concrete mix is usually specifi ed by the structural engineer based on strength and durability requirements. But for visual concrete the mix is adjusted to aid placing, compaction and to achieve a consistent fi nish. The siting of the project is important because the constituents of concrete are locally sourced and vary depending on the local geology. With all this determined, a coordinated concrete specifi cation should be prepared, giving structural and architectural requirements. It is best practice at the early stages to consider having test panels made. Consideration should also be given to panel layouts. Joints between formwork panels are often visible, so the architect should detail how the panels are to be laid out. Choose reinforcement spacers that minimise visual impact while the falsework design should minimise potential defl ection which could be particularly noticeable in plain walls. While high quality fi nishes are achieved in concrete, a completely uniform fi nish as struck is unrealistic as some variation and blemishes are part of concretes visual character. If a blemish-free surface is required, consider a plain concrete fi nish, allow for blowholes to be fi lled and the surface made good with a fi nishing coat.

For detailed information see How to achieve visual concrete, available from www.concretecentre.com

Locating building servicesA diff erent approach to the design of overhead services is needed when exposing the soffi t. There are a number of standard solutions that can be used including grouping systems such as lighting, fi re alarms, and sensors into modular services rafts. Floor voids, perimeter bulkheads and ceiling voids in corridors can be used to locate ventilation ductwork. Other options include using the cores in hollowcore slabs for pipes and cabling or integrating services into the design of in-situ or precast slabs. Rebates can be cast into the soffi t for extract grilles, smoke alarms, lighting etc. Water-based slab cooling and heating off ers a very neat option for concealing all the associated pipework and avoiding or minimising the need for heat emitters such as radiators (see page 18). More information on locating services can be found in a companion guide entitled Utilisation of Thermal mass in Non-Residential Buildings 13 which is available from The Concrete Centre website.

Acoustic considerationsConcrete soffi ts provide little acoustic absorbency, resulting in increased sound refl ection and longer reverberation times. However, there are a range of options that can be used to address this whilst minimising or avoiding the need for sound-absorbing materials to be located on the soffi t. More information on acoustics can be found in a companion guide entitled Utilisation of Thermal mass in Non-Residential Buildings, which is available from The Concrete Centre website.

System controlIn terms of control, the main objective is to take maximum advantage of night cooling whilst avoiding over-cooling, which can result in uncomfortable conditions at the start of the day and may cause the heating to be activated. Experience gained in the operation of buildings with FES over the last few years has helped refi ne and standardise the general approach used. More information on ventilation control can be found in a companion guide entitled Utilisation of Thermal mass in Non-Residential Buildings, which is available from The Concrete Centre website.

PowerGens HQ optimises acoustic performance by focusing sound

refl ected off the soffi t onto the acoustically absorbent wings located on

the lighting rafts suspended beneath each coffer. See page 13 for more

information.

Photo: courtesy of Peter Cook


Passive and active fl oor optionsAll of the standard types of concrete fl oor slab can form the basis of a passive or active FES system.

The properties of these fl oors are summarised in Figure 5, including details of the specifi c FES systems they are compatible with. A detailed overview of these systems is provided in this section of the guide, starting with the most basic, passive approach and ending with the more sophisticated water-based systems.

Flat slab

Construction: In-situ

Span: 5m to 12m, but most economicup to 9m. Can be 6m to 13m withpost-tensioning.

Compatible FES systems: All systems except TermoDeck.

Comments: Quick, versatile and easyto construct.

Profi led slab e.g. coff ered

Construction: In-situ or precast

Span: Up to 13m, but typically 10-11m.

Compatible FES systems: All systems except TermoDeck and Concretcool.

Comments: Lighter weight enables longer spans. Increased surface area improves FES performance. Compared to fl at slabs, formwork costs are higher and more time may be required for construction. Can be post-tensioned.

Composite lattice girder soffi t slab

Construction: Precast soffi t slab with in-situ concrete topping.

Span: Up to 5m without void formers or 12m with void formers.

Compatible FES systems:All systems except TermoDeck.

Comments: High quality precast soffi t. Quick to construct.

Hollowcore slab

Construction: Precast with optional in-situ concrete topping.

Span: Economic across a wide span range, but the maximum economic span is typically around 14-16m.

Compatible FES systems:All systems except Concretcool.

Comments: Relatively low cost option. Structurally effi cient and suitable for a wide range of building types. The addition of a structural concrete topping can be used to enhance performance.

Figure 5: Main types of fl oor slab

Figure 6: Summary of cooling outputs from the main FES/slab options

0 5 10 15 20 25 30 35 40 45

Approximate cooling output W/m250 55 60 65 70 75 80 90 100

Exposed steel compositefloor with a permeable ceiling

Exposed steel composite floor

Exposed slab, naturally ventilated building

Exposed slab, underfloormechanical ventilation

Exposed hollowcore slab with cores suppliedby mechanical ventilation (TermoDeckTM)

Exposed in-situ or lattice girder slab with embedded,finned aluminium ducts, supplied by mechanicalventilation (ConcretcoolTM)

Exposed flat slab with embedded cooling/heatingpipework (CoolslabTM, HydroDeckTM, ClimaspanTM)

Exposed profiled slab with embedded cooling/heating pipework

Exposed hollowcore slab with cores supplied bymechanical ventilation plus water cooling (TermoDeckTM

combined with HydroDeckTM)

A summary of the approximate cooling output that can be expected from the main FES/fl oor options is provided in Figure 6. It should be noted that in some buildings additional cooling of around 25 W/m2 may be provided by natural ventilation10 i.e. from openable windows, although this will diminish during hot weather.

10

Concrete fl oor solutions for passive and active cooling

Construction:

Works with all slab types i.e. in-situ, precast and composite.

Description:

Flat or profi led slab cooled at night by natural ventilation from perimeter windows and assisted in some cases by an atrium with high level openings providing stack ventilation.

Maximum slab cooling output (approximate):

15-20 W/m2 (fl at slab)20-25 W/m2 (profi led slab)

Key benefi ts:

Highly energy effi cient if building is controlled well. Relatively simple to design and operate. Little to no maintenance. Works with all concrete slab types. Applicable to new build and existing concrete buildings where the

slab can be exposed.

Cooling output can be increased if required e.g. with the addition of chilled beams or by embedding dormant pipework that can be used in the future if required.

Key considerations:

Use is limited to spaces with relatively low heat gains and occupant density.

Cooling performance is more weather dependent than other FES options.

External noise, pollution and/or security issues may preclude the use of natural ventilation.

Good occupant understanding and control needed to optimise year-round performance.

Case Studies:

Headquarters of the Cambridge Federation of Womens Institutes (see opposite).

The most basic form of FES system uses an exposed concrete soffi t with night cooling via openable windows, to regulate slab temperature during the day. Whilst the cooling output is modest, it is generally suffi cient to maintain comfortable conditions in buildings with good solar shading and relatively low internal heat gains.

Flat slabs provide the simplest and most cost-eff ective fl oor solution and are economical for spans up to about 9 m, or slightly more with post-tensioning. Profi led slabs off er an alternative e.g. with a coff ered, troughed or wave-form soffi t, which reduces the weight and helps optimise the span that can be achieved. These can be either precast or cast in situ with the option of post-tensioning (see PowerGen case study on page 13 and Canon on page 34).

Another benefi t of profi led slabs is the increased surface area of the soffi t, which enhances the convective heat fl ow with the space below, enabling it to be doubled in some instances. However, radiant heat fl ow to and from the soffi t is largely unaff ected by the extra surface area, so the overall increase in cooling output is limited to around 25% or 5 W/m2 compared to a fl at slab. Other benefi ts of a profi led slab can include enhanced daylight penetration and acoustic control, along with generally pleasing aesthetic qualities.

System 1:Exposed slab, naturally ventilated building

11


Location: Cambridge

Year: 2005

Client: Cambridge Federation of Womens Institutes

Architect: EllisMiller

Structural engineer: Whitbybird

M&E engineer: Roger Parker and Associates

Main contractor: Britaniabuild

FES system: Exposed, precast concrete slabs and natural ventilation.

The RIBA award winning headquarters of the Cambridge Federation of Womens Institutes is a good example of a simple, slow response, high thermal mass envelope, used to help regulate internal conditions. The site originally comprised two large pig sheds which were gifted to the Womens Institute. One was demolished to create space for a car park, whilst the other was developed into their headquarters. To comply with environmental and structural requirements, much of the original structure had to be rebuilt.

Cross ventilation is achieved from a combination of perimeter windows on the main facade and vents on the adjacent rear wall. The narrow fl oor plan makes the use of natural ventilation a particularly eff ective means of cooling the building during the summer months. External insulation was added to the walls, which were then clad in timber. The roof was replaced by 1.2m wide precast concrete panels also insulated externally and then covered with a profi led metal sheet. The underside of the slab has no fi nish, leaving the concrete soffi t fully exposed. The panels slope from front to back of the narrow fl oor plan, forming a shallow mono pitch roof. Internal walls are mostly fair-faced brick and block with a painted fi nish. Heating is provided by a gas boiler and radiators, whilst a modest amount of PV (700W) reduces the electrical demand. The total contract value for the project was a modest 307,000 and has been skilfully executed by architect Jonathan Ellis-Miller, with an outcome that the occupants are very pleased with11.

Headquarters of the CambridgeFederation of Womens Institutes

Photography: courtesy of Timothy Soar Louvre grilles in the back wall enable cross ventilation.

12


Construction:

Works with all slab types i.e. in-situ, precast and composite, although the composite option is not compatible with a profi led soffi t (illustrated).

Description:

The addition of underfl oor mechanical ventilation enables some convective heat transfer to and from the top surface of the slab, enhancing FES performance. The overall control strategy is typically mixed-mode i.e. natural ventilation from windows is used when summertime conditions permit, with the mechanical system operating only when needed. During the heating season, the mechanical system enables the ventilation rate to be closely controlled, helping minimise heat loss.


20-25 W/m2 (fl at slab)25-35 W/m2 (profi led slab)

Key benefi ts:

Additional heat fl ow via the upper slab surface increases cooling performance.

Cooling performance can be further enhanced by ensuring air circulating in the fl oor void is turbulent.

Floor outlets off er good fl exibility, enabling changes in building layout to be easily accommodated.

Night cooling is less reliant on there being adequate wind speed. If required, a cooling coil can be added to the air handling plant,

off ering a degree of future adaptation to climate change or an increase in the cooling output.

Key considerations:

Space requirements for air handling plant. Higher capital and operating costs than a naturally ventilated

building (energy and maintenance).

Mixed-mode ventilation has the potential to achieve greater control and better overall performance than provided by natural ventilation alone, but must be appropriately designed, commissioned and controlled13.

Case Studies:

PowerGen Headquarters, Coventry (see opposite). Greenfi elds Community Housing head offi ce, Essex (see page 29). Toyota Headquarters, Surrey (see page 33). Canon Headquarters, Reigate (see page 34). RSPCA Headquarters, West Sussex. Inland Revenue Headquarters, Nottingham.

When considering mechanical ventilation as part of an FES design, the case for using underfl oor ventilation is quite compelling for offi ce environments, not least because exposed soffi ts reduce the options available for air distribution and the routing of services. It also has the advantage of enabling fl oor outlets to be easily relocated to accommodate future layout changes. In terms of FES performance, underfl oor ventilation off ers a number of benefi ts. Firstly, it enables direct contact between the air and the top of the slab, helping unlock some of the thermal mass in the upper part of the slab. In other words, underfl oor ventilation combined with an exposed soffi t enables thermal linking of the slab from both sides, increasing the depth of concrete that is utilised during the day and increasing the overall cooling output.

Secondly, a further increase in cooling performance can be achieved if the air moving across the fl oor void is made turbulent12. Turbulence helps to break up the relatively static layer of air that otherwise clings to the surface of the concrete, restricting heat fl ow. The optimal rate of heat transfer is dependent upon achieving a balance between the mean air velocity, time spent in the fl oor void and fan power. Standard techniques to help achieve this can be applied to the design of underfl oor systems13. Other advantages off ered by an underfl oor supply over ceiling-based systems include14:

A reduction in the construction materials needed. The ability to provide a higher proportion of fresh air to the

occupants.

Lower maintenance. Lower energy consumption.

Several notable offi ces that feature exposed coff ered slabs and mixed-mode, underfl oor ventilation, have been built over the last twenty years, for example the UK Headquarters of PowerGen (page 13), Toyota (page 33) and Canon (page 34). These all share a similar layout, characterised by long narrow fl oor plates arranged over three storeys, with an open balcony arrangement onto a central atrium, which enhances both daylight penetration and natural ventilation.

System 2:Exposed slab, underfl oor mechanical ventilation

13


Location: Coventry

Year: 1994

Client: PowerGen

Architect: Bennetts Associates

Structural engineer: Curtins Consulting Engineers

M&E engineer: Ernest Griffi ths & Son

FES system: Exposed, coff ered slabs and mixed-mode ventilation.

In many respects the 13,000m2 PowerGen Headquarters, completed in 1994, represents a landmark in high thermal mass, passive offi ce design and has provided a successful template for many subsequent buildings. The design off ers a good balance between daylighting, natural ventilation, thermal mass and offi ce layout which has proved eff ective in providing a comfortable, low-energy environment.

The layout consists of two parallel fl oor plates separated by a central atrium and lies on an east-west axis, providing good daylighting and air-fl ow. The structural frame is made from reinforced concrete with exposed, in-situ coff ered fl oors which are central to the buildings use of thermal mass to provide a stable internal temperature. This proved to be very eff ective during the summer of 1995, one of the hottest on record, during which the building performed very well. The recorded internal temperatures closely matched predictions from thermal modelling undertaken at the design stage.

Detailed analysis of PowerGens overall design intent established that the offi ce space requirements would be best met by a series of narrow fl oor plates. This would allow connection across the offi ce space and encourage personal communication between occupants. The size of the fl oor plates also needed to accommodate a variety of departmental offi ces and allow for future fl exibility. Within the 10.8m x 7.2m structural grid are three coff ers, each 2.4m wide, which span from atrium to external window. The coff ers elliptical cross-section is designed to improve the acoustic performance of the offi ce space by focusing unwanted noise onto the acoustically absorbent wings of the interior lighting rafts suspended beneath each coff er. The lighting rafts partially up-light the coff ers to enhance their sculptural form. They also incorporate smoke detectors and the PA system. In long section, the coff ers taper towards their ends to increase the penetration of natural light into the offi ce space from the external windows and the atrium.

Partial post-tensioning was used to minimise early thermal shrinkage eff ects and so ensure that there were no visible cracks in the exposed concrete coff ers. The maximum designed crack width was 0.1-0.2mm so that standard emulsion paint could be applied to the soffi t without the cracks showing through. The fl oor plates are supported by 400mm diameter circular columns. Whilst the design was being developed, key aspects were tested by modelling and mock-ups. A 1:40 scale model of a typical cross-section through the building was made to develop the atrium glazing, offi ce glazing and concrete profi les for maximum lighting performance. A full-size mock-up of a 7.2m x 10.8m structural bay was also built using glass-reinforced plaster to form the coff ers. This was invaluable for confi rming and tuning the design of the coff er profi le and light fi ttings, and for testing the acoustic performance and artifi cial lighting levels.

When Laing Midlands were appointed as design and build contractors, they adopted the approved scheme and worked closely with the design team. The on-site mock-ups played an important role in incorporating key refi nements such as the prefabrication of slab reinforcement into the fi nal design solution. The choice of natural ventilation required the service engineers Ernest Griffi ths & Son to consider all aspects of the building design. The arrangement of relatively narrow open plan offi ce areas on either side of the three-storey atrium provided the ideal layout for good natural ventilation. The building management system controls the top row of windows, which are opened at night to allow cool air to fl ow over the coff ered concrete soffi t. Computer simulations by environmental-modelling specialist EDSL were used to accurately model the offi ce environment and predict peak internal temperatures, taking into consideration external eff ects, internal heat loads and the passive cooling eff ects of the exposed concrete. The modelling also helped to develop the design strategy and establish the right mix of thermal mass and natural ventilation. It also showed that night time ventilation was able to exploit the long-term thermal dynamics of the fl oor. The latter were provided by the careful use of exposed concrete with suffi cient thickness to absorb heat gains over many days.

Internal heat gains are minimised by placing areas that require air-conditioning, such as the computer suite and kitchens, at the east and west ends of the building. The larger, heat-generating offi ce equipment, such as photocopiers, is grouped into segregated rooms, out of the open-plan space. Staff have considerable control of their environment as the lower windows may be opened manually during the day.

PowerGenHeadquarters

Coffered slabs with as struck fi nish prior to painting. Photo: courtesy of Peter Cook.For an image of the completed project see page 8.

14


Construction:

Hollowcore is a precast option only. An in-situ concrete topping of around 50mm is typically placed on the slab to enhance the structural performance.

Description:

Precast, hollowcore slab with fresh air from a mechanical ventilation system channelled through the cores, enabling good heat transfer between the air and slab. Cooling/heating is provided by a combination of the ventilation supply and radiant output of the exposed soffi t (which is more signifi cant). The system is typically referred to by the trade name of TermoDeck.


40 W/m2 (basic system)50 W/m2 (with cooling)60 W/m2 (with cooling and switch-fl ow system)

Key benefi ts:

Widely used and well proven system. Provides radiant and convective cooling. Economic across a wide range of spans of up to 16m. Provides an alternative to water as a means of actively cooling

concrete slabs.

Air is typically introduced to the space at ceiling level but can also be introduced at low level if the fl oor to ceiling distance is high.

Soffi t fi nish is suitable for painting on site if required. Can be used as part of a mixed-mode ventilation strategy.

Key considerations:

The slab cores may require periodic cleaning (access points are provided).

Soffi t fi nish may be slightly more utilitarian than some of the alternative options in this guide.

Units typically limited to a maximum width of around 1200mm. TermoDeck is a registered trade mark and the suppliers expertise is

an important element in ensuring the installation works eff ectively. They provide a service that covers design (including controls), thermal modelling and expertise in the treatment of the slabs e.g. drilling, capping and sealing etc.

Case Studies:

Innovate Green Offi ce, Leeds (see opposite). Peel Park, Blackpool. The Ionica building, Cambridge. The Elizabeth Fry building, University of East Anglia. Meteorological Offi ce, Exeter. Jubilee Library, Brighton.

Hollowcore slabs are made from pre-tensioned, precast concrete with continuous hollowcores to reduce self-weight and achieve structural effi ciency. This type of slab can be used very eff ectively as part of an active cooling system, using mechanical ventilation to channel air through the cores before entering the occupied space. Air passes through the cores at low velocities, allowing prolonged contact between the air and concrete for good heat transfer. The operating principal is straightforward; on summer days, the soffi t provides radiant cooling in the usual way, whilst the ventilation supply is cooled as it passes through the cores prior to entering the occupied space. At night, heat fl ow eff ectively reverses with cool ambient air removing heat from the concrete as it circulates through the slab and lowers its temperature ready for the next day. During the heating season, air extracted from the building is used to pre-heat the fresh air supply before it is further warmed by the fl oor slab, which is able to absorb and recycle some of the internal heat gains. If required, additional heat is supplied by a heating coil in the air handling plant. At night the external air supply damper is shut and the system can be operated in recirculation mode so the fl oor can absorb internal gains ready for the next day.

The temperature diff erence between the slab and the air leaving the cores is not more than 12 degrees. The slabs are usually 1200mm wide and approximately 250400mm deep (depending on span), incorporating up to fi ve smooth-faced extruded holes along the length. Three of these are used to form a three-pass heat exchanger in each slab, linked to a supply diff user located on the soffi t. Alternatively, displacement ventilation can be used by ducting the air into an underfl oor ventilation system, which may be a preferred option in spaces with a high fl oor to ceiling distance.Air supply to the slabs is via a duct, typically located in an adjacent corridor above a false ceiling. As with other FES systems, a large proportion of the cooling is radiant, provided by the exposed underside of the slab. Supply diff users are located about 12m from windows to prevent potential down-draughts and/or clashing with partitions. Pre-drilled and sealed openings can be incorporated at mid-span, making it possible to relocate diff users in the future. This enables conference rooms or similar spaces to be accommodated in the centre of the building if required.

The TermoDeck system can be confi gured to suit a variety of applications and cooling duties. In its basic form, loads of up to 40W/m2 can be handled; although experience at the Meteorological Offi ce in Exeter

System 3:Exposed hollowcore slab with cores supplied by mechanical ventilation (TermoDeck)

15


shows that higher loads of around 47W/m2 are possible15. The addition of mechanical cooling can increase the cooling capacity of the basic system to around 50W/m2. Performance can also be increased through indirect evaporative cooling, which cools the supply air without increasing its moisture content. The cooling provided by an evaporative system is dependent on ambient conditions, and the effi ciency of the humidifi er and heat exchanger, but can lower the air temperature by several degrees under average conditions (evaporative cooling is also applicable to other systems with mechanical ventilation detailed in this guide). The highest

cooling performance is achieved by using the TermoDeck switch-fl ow system. This enables the temperature to be adjusted in individual rooms and can be used in conjunction with mechanical or evaporative cooling. The system is regulated by a switch unit incorporating a changeover damper to re-route the supply air; when a room needs extra cooling, the air-supply route through the slabs is changed directly to the core that contains the ceiling diff user, rather than the normal route through all three cores. The shorter distance helps to prevent the supply air taking heat from the slab.

Innovate Green Offi ce: speculative offi ce development Location: Leeds

Year: 2007

Client: Innovate Property

Architect: Rio Architects

Structural engineer: Scott Wilson

M&E engineer: King Shaw Associates

FES system: Exposed hollowcore slabs and mechanical ventilation (TermoDeck).

The Innovate Green Offi ce, completed in 2007, is a speculative development that achieved an impressive BREEAM rating of 87.55%; the highest score ever awarded. Designed for Innovate Property by Rio Architects and King Shaw Associates, at fi rst glance the offi ce does not look particularly green. There are no wind turbines or solar panels, yet the building emits 80% less CO2 than a typical air-conditioned offi ce, producing around 22kg of CO2 per m2 per year. This equates to a saving of roughly 350 tonnes of CO2 every year. The approach developed by the project team achieves these savings by fully applying the principals of fabric energy effi ciency, rather than relying on the addition of renewable systems. An engineering led exercise produced an environmentally and commercially sustainable plan, with the sustainability credentials of each element being considered in conjunction with the clients need for fl exible and economically viable offi ce space.

Yorkshire Forward, the regional development agency, worked with Innovate Property to fund a proportion of the cost of prototyping sustainable construction methods. The building services are designed to produce minimal emissions helped by low energy lighting, heating and cooling. Electricity is provided by a 30kW base load combined heat and power (CHP) system with a matched absorption chiller taking advantage of the waste heat in the summer.

Alongside the effi cient services, the passive design features contribute signifi cantly to the low annual emissions. The whole building was designed as a thermal store; the structural frame consists of load bearing precast concrete wall panels and hollowcore fl oor and roof units, which use the TermoDeck system to fully utilise the thermal mass in the units.

The concrete mix incorporates fl y ash as a cement replacement to reduce embodied CO2, and Lytag, which is a lightweight aggregate

also made from fl y ash. The external concrete walls provide an airtight, weatherproof envelope in a single component and have been designed to ensure solar gains from window openings are minimised. The walls achieve a very good U-value of 0.15 W/m2K and are externally insulated to allow the concrete to be exposed internally for its thermal mass.

With the high level of insulation and airtightness achieved, heat loss is reduced to a point where internal gains from people and computers etc. are almost suffi cient to maintain comfortable conditions from autumn to spring. The mechanical ventilation system recovers these gains, which are stored in the fl oor slabs for benefi cial re-use. Summer cooling is provided by a combination of passive night-cooling and active cooling from the chiller, using the TermoDeck system as a thermal store in a strategy similar to ice storage i.e. to attenuate the peak cooling load and cut the size of the cooling plant.

The offi ce achieved an average daylight factor of 4.5% so artifi cial lighting is only required for about 20% of the working year. External vertical shades and internal blinds control excess gains and glare. Other sustainable elements of the scheme include a vacuum drainage system that utilises harvested rainwater for fl ushing the toilets. This virtually eliminates the need for treated mains water to convey sewage. The overall volume of waste discharged from the building is reduced by 75%. Permeable paving and a natural wetland area prevents storm water fl ooding.

The success of this speculative offi ce development can be attributed to the integrated approach given to fabric energy effi ciency and structural requirements, which have been met using readily available construction materials and technologies.

Photo: courtesy of Rio Architects

16


Construction:

Applicable to in-situ and composite fl at slabs.

Description:

An in-situ or composite lattice girder slab, incorporating proprietary aluminium ventilation ducts16. The fresh air supply is channelled through the ducts in a similar manner as the TermoDeck system (see page 14), with cooling/heating provided by a combination of the ventilation supply and the radiant output of the soffi t.


65 W/m2

Key benefi ts:

High cooling output. Provides radiant and convective cooling. Widely used in mainland Europe. Flexible system that can be confi gured to meet a range of design

requirements.

Provides an alternative to water as a means of actively cooling concrete slabs.

Corrugated fi ns inside the ducts (see image below) increase heat transfer between the air and the slab.

Key considerations:

Soffi t located supply diff users cannot be easily reconfi gured to refl ect changes to internal layout.

Performance is partially dependent on ambient conditions. The internal fi ns enhance heat transfer, but also have modest impact

on fan power.

Case Studies:

4 West Building, University of Bath (see opposite).

The proprietary aluminium ductwork and ancillary components used in Concretcool are manufactured by Kiefer who are a German air conditioning company with a UK distributor16. This relatively new system has been used in a number of European projects, with a total fl oor area of over 260,000m2 to date17. In the UK, the fi rst building to employ Concretcool was the 4 West Building at the University of Bath, completed in 2010. In many ways it is similar to the TermoDeck system, but diff ers in its ability to be used with in-situ concrete fl oors (TermoDeck uses precast hollowcore slabs). It can also be used with composite lattice girder slabs with the option of installing the ductwork at the precast factory.

The aluminium ducts are cast into the slab to form a series of parallel U shape runs, each of which is between 7-10m in length (see front cover image). The ducts are linked at one end to a mechanical ventilation system, whilst the other end feeds a supply diff user that is typically located on the soffi t, but can be on a perimeter bulk head. If located in the fl oor it can provide displacement ventilation. The ductwork is made by an extrusion process and is available in a diameter of either 60mm or 80mm, both incorporate a number of corrugated internal fi ns, eff ectively tripling the internal surface area. This enhances the heat transfer rate between air and concrete, achieving an effi ciency of up to 90%18. It also results in a pressure drop of around 7 Pa/m or 50-70 Pa for a typical duct run. However, the overall fan pressure for an installed system is not unduly high. Plastic spacers are used to ensure the correct position of the ducts between the upper and lower reinforcement and this prevents the ducts fl oating up when concrete is poured. However, a small amount of upwards movement is encouraged as this lifts the plastic spacers clear of the formwork, preventing the ends from remaining visible when the formwork is struck. To stop the ingress of water/wet concrete, the ductwork elbows and other joints incorporate a watertight seal. The addition of ductwork into the slab does not aff ect the structural performance as it is located in the neutral zone i.e. in the central area where the slab experiences least tension/compression.

The basic operating principle is very similar to that of the TermoDeck system (see page 14); on summer nights the air handling plant channels cool external air through the embedded aluminium ducts, removing heat from the concrete ready for the following day, when the heat fl ow naturally reverses and the fl oor cools the fresh air supply, whilst the soffi t provides radiant cooling. During the heating season, the fl oor warms the air supply to almost the same temperature as the soffi t i.e. around 21C, which can be achieved with minimal or no supplementary heating for much of the season.

System 4: Exposed in-situ or lattice girder slab with embedded air ducts supplied by mechanical ventilation (Concretcool)

Internal fi ns greatly increase the surface area, enhancing heat transfer.

17


University of Bath4 West Building Location: Bath

Year: 2010

Client: University of Bath

Architect: Stride Treglown Tektus

Structural engineer: Ramboll

M&E engineer: Roger Preston & Partners

Main contractor: Cowlin Construction

FES system: Exposed, in-situ slabs incorporating theConcretcool system.

The 4 West Building at the University of Bath accommodates general teaching areas, student services and offi ce space. It was completed on time and within budget in April 2010 and achieved a BREEAM rating of Excellent. Built over six fl oors, the concrete frame construction has in-situ concrete fl oors incorporating the Kiefer Concretcool system, which uses mechanical ventilation to regulate the temperature of the exposed soffi ts.

Prior to this systems development there were only two methods for using mechanical ventilation to directly cool the slab. Firstly, by using an underfl oor ventilation supply, which typically off ers a modest increase in cooling output, or secondly by using the TermoDeck system (see page 14) which performs well, but is limited to precast hollowcore fl ooring. The German Kiefer system provides a third option and can be used with in-situ concrete fl oors. It comprises a series of circular aluminium ducts embedded in the slab to create U shape runs of up to 10m in overall length. Each duct

is linked at one end to the mechanical ventilation system, whilst the other end feeds a supply diff user typically located on the soffi t. The ducting is available in diameters of 60mm and 80mm, with the latter used in the 4 West Building. It is made using an extrusion process, enabling corrugated internal fi nes to be formed, eff ectively tripling the internal surface area for maximum heat transfer.

On summer days, the buildings fresh air supply is cooled by the fl oor slabs as it passes through the ducts prior to entering the occupied space. At night, the heat fl ow reverses, and the cool fresh air supply removes the accumulated heat from the slab, so the cycle can be repeated the following day. During the heating season, the fl oor warms the air supply to almost the same temperature as the soffi t. Further heating is provided by TRV controlled radiators. Windows can be opened to supplement the mechanical ventilation and enhance the level of control occupants have over the internal environment.

Whilst the Concretcool system has been widely used in mainland Europe, the 4 West project is the fi rst application of this technology in the UK. Cowlin Construction, the main contractor, worked closely with LTi Advanced System Technology who are the UK distributor for this system. Two months before work started on the concrete frame, LTi gave a presentation to the construction team so they could understand the system and develop a plan to ensure it was fully integrated into the works. The aluminium ducts were pre-cut at the German factory, so all the components arrived ready for installation. 31 days were allowed for the work, but ease of installation enabled this to be reduced to 15 days. The project manager for Cowlin Construction commented that any initial uncertainty about working with the new product was soon dispelled as it proved to be very robust and user-friendly.

Photo: courtesy of Cowlin Construction Ltd Photo: courtesy of University of Bath

18


Construction:In-situ or precast slab, with either a fl at or profi led soffi t.

Description:

A fl at or profi led slab with PEX or polybutylene plastic pipework confi gured in a serpentine layout and embedded close to the soffi t. This is supplied with chilled water in summer and, if required, warm water during the heating season. The active cooling/heating system works with thermal mass and the large area of the soffi t to maintain a stable surface temperature close to that of the air in the room. This enables the fl ow temperature to be optimised for maximum energy effi ciency i.e. relatively high for cooling and low for heating.


65 W/m2 (fl at slab)80 W/m2 (profi led slab)19

Key benefi ts:

High cooling output. Ability to continually regulate the soffi t temperature. Unobtrusive and silent. Can be integrated with virtually any slab design. Modest fl ow temperature permits use of energy effi cient sources of

heating and cooling.

Key considerations:

Void formers can be incorporated into slab to optimise structural effi ciency i.e. weight/span.

Good on site practice needed to ensure accidental damage to pipes from drilling etc. is avoided. The in-situ option is potentially more vulnerable to general on site damage before the concrete is placed.

Expertise is available to assist with the design and delivery of bespoke water-based systems.

Case Studies:

CAFOD Headquarters, London (see opposite). IFDS House Barclays Bank/Basilica, Basildon.

Alongside the proprietary water-based precast systems described in this guide (see pages 20 and 22), there is also the option of embedding pipework in in-situ concrete slabs or bespoke precast units. An example of the in-situ option can be found at the CAFOD Headquarters in London (see opposite). Similarly, IFDS House, (originally built for Barclays Bank and formally called the Basilica) provides an example of the bespoke precast option13. Both options off er good design fl exibility, including the ability to specify a profi led slab which can increase the cooling output; an option otherwise not available with the current proprietary options (which are all based on a fl at slab). However, this system requires greater upfront design

and there is an increased potential for on site damage to the pipework when in-situ slabs are being constructed. Consultants and suppliers20 off er expertise to help with the design and ensure the build programme runs smoothly. If required, they can also provide a single package for the installation and warranty of the system.

The use of water rather than air to regulate the slab temperature will generally provide a slightly higher cooling capacity, although the Concretcool system can off er a similar level of performance (see page 16). Water-based systems also have the ability to continually regulate the soffi t temperature to maintain close control of internal conditions; it takes around 30 minutes for a change to the fl ow temperature to have a discernable eff ect on the soffi t temperature21. This relatively rapid response is made possible by the low resistance to heat fl ow between the water and the concrete, which is around 100 times less than exists between air and concrete22. The large surface area of the soffi t and stabilising eff ect of the thermal mass enables the system to operate with fl ow temperatures close to that of the occupied space, allowing the use of effi cient sources of heating and cooling, such as ground water and ground coupled heat pumps. The ability to spread loads over a 24 hour period also reduces plant size and lends itself to district heating and cooling systems.

Not withstanding any limitations on cooling output imposed by the system supplying the chilled water, the main restriction on performance is the risk of condensation forming on the soffi t if the surface temperature is allowed to drop too low. The point at which this occurs is governed largely by the relative humidity of the air in the room, and the lower limit for the surface temperature is generally about 19-20C, although it may be slightly lower if mechanical ventilation is used with the ability to regulate humidity. In summer, the fl ow temperature to the slab is typically between 14-20C, increasing to around 25-40C during the heating season.

Although water cooling may be the primary method for regulating temperature in summer, night time ventilation can also be used to remove accumulated heat from the slab, with the option of supplementary water cooling used as necessary to achieve the required soffi t temperature at the start of the next day. The CAFOD Headquarters is a good example of a building that combines both techniques to optimise overall performance and energy effi ciency.

The basic design sequence23 of a bespoke water-based system is as follows:

Determine the zoning requirements of the building and associated heating and cooling loads.

Defi ne the available areas for the coils, taking into account any structural constraints.

Calculate the cooling circuit requirements to match the zoning and active soffi t area.

Incorporate the coil layout into the construction plans. Design the interconnecting pipework between the coils and the

chilling and heating plant.

System 5:Exposed slab with embedded cooling/heating pipework

19


Location: Southwark, London

Year: 2010

Client: Catholic Agency for Overseas Development (CAFOD)

Architect: Black Architecture

Structural engineer: WSP

M&E engineer: King Shaw Associates

Main contractor: Volker Fitzpatrick

FES system: Exposed, in-situ slabs with embedded water cooling pipes and mixed-mode ventilation.

CAFODs 8.6m Headquarters is situated on a former car park adjoining St Georges Cathedral in Southwark. The fi ve storey 3,000m2 building refl ects the charitys core values through its restrained design and low environmental impact. The building makes use of a range of passive and low energy systems, including rainwater harvesting, stack ventilation and a thermal mass system that combines passive and active features. Overall, the offi ce achieved a BREEAM rating of Excellent.

Romero House is composed of three distinct elements. The triangular open plan offi ce fl oors are separated from the ancillary spaces by an atrium circulation spine. This arrangement encourages people to leave their fl oor to use the breakout spaces, toilets and meeting rooms. A half level stepped section gives occupants the option to use facilities on upper and lower fl oors and so increases the opportunities for communication and social interaction. Overall, the design helps promote a sense of community within the organisation while fulfi lling CAFODs spatial needs.

Concrete forms an important part of the overall design strategy, and the complex geometry on each fl oor resulted in the use of an in-situ concrete slab, which was formed using simple plywood

shuttering. The concrete mix incorporates cement replacement in the form of ground granulated blast furnace slag (GGBS), helping lower embodied CO2. With the exception of the stairwell, the exposed concrete is painted white to improve refl ectivity and help increase the daylighting toward the centre of the fl oor plan. The exposed stair wall has a rough, needle-gunned fi nish providing a powerful visual contrast to the painted concrete and galvanised steel stair. The exposed concrete soffi t on each fl oor incorporates the Velta Thermally Active Building System (TABS) i.e. embedded PEX cooling pipes, which off set some of the buildings heating and cooling load; the system is only designed to handle what cant be achieved passively. The network of embedded pipes are linked to a ground source heating and cooling system incorporating fi ve closed loop boreholes and a heat pump. Each borehole is 125m deep and produces water at around 12C. Some additional heating is provided by trench heaters on each fl oor. Roof mounted PV panels generate around 3,500 kWh per year, whilst hot water is provided by a solar hot water system.

A mixed-mode ventilation strategy allows occupants to open high-level windows (using winders) for natural ventilation during the summer, with mechanical underfl oor ventilation taking over during periods of particularity high or low external temperatures when windows are best kept shut. Exhaust air is drawn passively across the soffi t by the stack eff ect created by warm air rising through the atrium and exiting at roof level. Monitoring during the summer of 2010 showed that the active thermal mass and ventilation strategy achieved a very stable internal temperature that only varied by around 2-3 degrees across each day24 and typically stayed around 22-23C. During the hottest period when the external temperature reached 32C, internal conditions did not exceed 26C. This impressive performance should be further enhanced by minor adjustments to the internal layout carried out to improve air fl ow; on one fl oor storage units were too high, preventing air from circulating properly25.

CAFOD Headquarters (Romero House)

Photo: courtesy of Black Architecture

20


Construction:

Hollowcore slabs are a precast option only. An in-situ concrete topping of around 50mm is typically placed on the slab to enhance the overall structural performance.

Description:

Precast, hollowcore slabs with PEX plastic pipework embedded near the soffi t for heating and cooling. In contrast to the TermoDeck system, the cores are not active i.e. do not form part of the ventilation supply. However, the two systems can be combined, increasing the overall cooling output, see Figure 6 on page 9.


65 W/m2

Key benefi ts:

High cooling output. Ability to continually regulate the soffi t temperature. Provides the benefi ts of a precast solution. Relatively low cost option. Economic across a wide range of spans of up to 15m. Modest fl ow temperature permits use of energy effi cient sources of


Soffi t fi nish suitable for painting on site if required. Can be combined with the TermoDeck system to increase the cooling

capacity.

Key considerations:

Soffi t fi nish may be slightly more utilitarian than some of the alternative fl oor options.

Units typically limited to a maximum width of around 1200mm. Good on site practice needed to ensure accidental damage to pipes

is avoided.

System limited to fl at slabs.

Case Studies:

Vanguard House, Daresbury Science and Innovation Campus, Warrington (see opposite).

For a general overview of water-based systems see: Exposed soffi t with embedded cooling/heating pipework on page 18.

Hollowcore slabs are made from pre-tensioned, precast concrete with continuous hollowcores to reduce self-weight and provide good structural effi ciency. Overall, the weight is around 32% less than for an equivalent solid slab. In this water-based system, plastic pipework (PEX) is cast into the slab about 60mm above the soffi t. As with the TermoDeck system, services such as cables and pipes for sprinklers etc. can be routed through the cores. Alternatively, cables can be placed in the in-situ concrete topping. The joint between slabs is visible from below and can be plastered over or partially obscured by the lighting raft, however it is not particularly conspicuous and is typically ignored.

HydroDeck and Climaspan hollowcore slabs incorporate top strand reinforcing which removes the camber typically associated with hollowcore, giving a fl at soffi t.

Along with other water-based options in this guide, the cooling/heating output is chiefl y radiant as ventilation is handled separately. However, since this system option uses hollowcore slabs, it off ers the opportunity of utilising the otherwise dormant cores to channel mechanical ventilation into the space and use some of the thermal mass to regulate the air temperature. This provides greater convective cooling, which may increase the overall cooling output to approximately 90 W/m2 26. This system is commercially available by specifying a combination of HydroDeck and TermoDeck systems.

System 6: Exposed hollowcoreslab with embeddedcooling/heating pipework(HydroDeck, Climaspan)

21


Vanguard HouseLocation: Daresbury Science & Innovation Campus, Cheshire

Year: 2011

Client: North West Development Agency

Architect: Fletcher Architects

Structural engineer: Pick Everard

M&E engineer: Atkins / Giff ords

FES system: Exposed, hollowcore slabs with embedded cooling/heating pipework (Climaspan)

Vanguard House provides three-storeys of offi ce and laboratory space, covering 3,600m. The client, North West Development Agency wanted a low carbon, BREEAM Excellent facility (which was achieved) for its latest phase of development at the Daresbury Science & Innovation Campus; a UK centre of excellence in accelerometer research.

The building uses a combination of passive design and geo-thermal technology to provide heating and cooling. During the summer months water extracted from a 120m borehole supplies the precast hollowcore fl oor slabs with plastic pipework embedded close to the soffi t. These were manufactured by Creagh Concrete, under the product name Climaspan. Each precast unit spans up to 10m, and is 1.2m wide x 0.3m deep. The cooling/heating pipework is located at a depth of 60mm from the soffi t and, to ensure it could not be accidentally damaged during construction, site operatives were issued with shanked drill bits with a limited drilling depth. The units make use of top strand reinforcing which virtually eliminates the camber that can sometimes be visible with exposed hollowcore fl oors.

The water cooling system, often referred to as a Thermally Active Building System or TABS, was designed by Velta and can provide up to 65 W/m2 of cooling. At Vanguard House it is used in conjunction with night cooling to optimise summertime performance. During the heating season, the temperature of the ground water is boosted using a heat pump before being supplied to the fl oor units. Tenants have individual control of the system to regulate heating and cooling in their space.

Photography: courtesy of Creagh Concrete Products Ltd

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Construction:

Composite i.e. precast soffi t with in-situ concrete topping. Void formers can be used to optimise the structural effi ciency and increase the span.

Description:

Factory produced, high quality precast soffi t that acts as permanent formwork to an in-situ concrete topping. PEX plastic pipework for cooling/heating is embedded in the precast soffi t.


65 W/m2

Key benefi ts:

High cooling output. Ability to continually regulate the soffi t temperature. Combines the benefi ts of a high quality precast fi nish with an in-situ

concrete slab.

Soffi t can be left unpainted if preferred. Quick to construct and provides a safe working platform that

requires little or no propping.

Available in a broad range of widths. Modest fl ow temperature permits use of energy effi cient sources of


Key considerations:

Void formers can be incorporated to optimise weight and structural effi ciency.

Good on site practice needed to ensure accidental damage to pipes from drilling etc. is avoided.

Early design consideration needed so holes for services can be allowed for.

System limited to fl at slabs.

Case Studies:

Manchester Metropolitan University Business School (see opposite).

For a general overview of water-based systems, see: Exposed soffi t with embedded cooling/heating pipework on page 18. The precast units are typically around 1.5-2.4m wide and include most, if not all, of the bottom reinforcement. The top reinforcement is fi xed on site prior to placing the in-situ concrete topping. Void formers can be used to reduce weight and increase the span up to a maximum of around 12m. The surface fi nish of the soffi t is typically very good, and the concrete mix design can be tailored to achieve a specifi c visual appearance as an alternative to painting. For example, at the Manchester Metropolitan University Business School the concrete mix used a combination of white cement and ordinary Portland cement.

The PEX plastic pipes are located on top of the reinforcement, making the reinforcement work more effi ciently and provide some additional protection against accidental drilling of the pipework. During the manufacturing process, the pipework is pressure tested with air before the concrete is placed to ensure there are no leaks. Once installed on site, it is kept pressurised with water so its integrity can be monitored during the construction process and enable any accidental damage to be more easily found and rectifi ed.

System 7: Exposed composite lattice girder soffi t slab with embedded pipework (Hanson Coolslab)

Photo: courtesy of Hanson UK

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ManchesterMetropolitan University- Business School27

Location: Manchester

Year: 2011

Client: Manchester Metropolitan University

Architect: Feilden Clegg Bradley Studios

Structural engineer: WYG

M&E engineer: AECOM

FES system: Exposed, lattice girder soffi t slabs with embedded water cooling pipes (Hanson Coolslab).

The new Business School at Manchester Metropolitan University is split into three interconnected structures of four, six and eight storeys containing lecture theatres, seminar rooms and working spaces. Linking the structures are atriums housing multi-use workspaces. Design of the 65m project was driven by the Universitys ambition to create a low energy building utilising thermal mass, natural daylight and controlled ventilation. The architects Feilden Clegg Bradley Studios developed it as a building comprising three elements: a base, a solid exposed concrete frame, and a glass veil.

Visually the colour and fi nish of the concrete was of particular importance to the design, but the major technical challenge was to control the temperature of the exposed concrete to 20C. This has the eff ect of regulating the rooms air temperature between 21C and 26C, which makes signifi cant energy savings. During the feasibility stage, M&E consultant AECOM determined that it was possible to adopt an open-loop temperature regulation system using groundwater boreholes near to the site. Early involvement of the contractor Sir Robert McAlpine enabled them to work with the project team and embed the water pipes.

The contractor hoped to install a fully precast system with embedded water pipes, but following the tender process Hansons Coolslab system was selected as the best option. Coolslab is a development of Hansons Omnicore product, a precast soffi t panel which acts as permanent formwork, to an in-situ concrete topping containing polystyrene void formers. The pipework is cast into the panels at the precast factory and is pressure tested before being despatched. The concrete specifi ed for the panels uses a combination of 75% white cement and 25% ordinary Portland cement to create a light fi nish. Whilst this increased the cost of the mix, overall there was a saving as it was cheaper than the alternative option of painting the soffi t. Hanson worked with cooling and heating systems developer Velta, which has seen its products used widely across Europe. Velta specialises in Thermo-Active Building Systems (TABS), which uses plastic water cooling/heating pipes embedded in the building structure. For Manchester Metropolitan University, Sir Robert McAlpine and Hanson developed a bespoke 1.5m wide slab with an overall depth of 475mm spanning 12m. Once on site, the slabs were craned into position and the joints propped underneath to achieve a 10mm positive camber, as engineers had calculated a maximum sag of 20mm due to self weight.

The biggest operational issue was connecting the pipes to the centralised chilled water fl ow and return pipework. So as not to compromise fl oor space, a box housing the pipe terminations was installed within the fl oor slab. Velta designed a steel box that is fl ush with the top surface and able to withstand the fl oor loading. Following the installation of the reinforcement, the concrete topping was poured. To prevent accidental damage to the pipework from contractors working on site, Hilti drill bits of the correct length were issued. The pipework was kept under pressure during the construction phase so its integrity could be monitored at all times.

Mechanical ventilation is provided to the majority of areas using the raised access fl oor as a supply plenum. Return air passes into the atriums via cross-talk attenuators, where it rises to the air handling plant at roof level. Cooling is provided by a combination of the ventilation and Hanson Coolslab system. The air handling plant is supplied with chilled water at 6C from heat pumps, which are in turn supplied indirectly with cool water from a bore hole. A plate heat exchanger ensures the two water circuits are kept separate. Ground water is extracted at around 12C and leaves the heat exchanger at approximately 14C. The supply to the Coolslab system is served directly by the heat exchanger i.e. without the need for the heat pump to modify the fl ow temperature, which at 14C is ideal for water cooled soffi ts operating under peak summertime conditions. Not withstanding this, the heat pump can be used to regulate the fl ow temperature if needed. Another energy saving feature of the installation is the ability of the system to use heat extracted from IT rooms etc. to warm other areas of the building or pre-heat the domestic hot water supply.

Photo: courtesy of Feilden Clegg Bradley Architects

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Construction:

Works with all slab types i.e. fl at, profi led, in-situ and precast.

Description:

Flat or profi led concrete soffi t with chilled beams suspended below. These may also incorporate other services including lighting, smoke detectors, PIR sensors, and sound absorbent acoustic panels etc.


15-25 W/m2 (depending on FES technique used). 100-160+ W/m2 from the chilled beams.

Key benefi ts:

High cooling output and good system control. Combines the benefi t of radiant cooling from the soffi t with

convective cooling from the chilled beams.

All overhead services can be integrated within the chilled beam, off ering an effi cient and easy to install prefabricated system.

Shallow unit depth works well in refurbishment projects with a low slab to slab height.

Modest fl ow temperature permits use of energy effi cient sources of cooling.

Key considerations:

For optimal energy effi ciency, FES requirements must not be overlooked when designing the chilled beam installation and associated controls.

Water fl ow temperatures must be carefully controlled to avoid risk of condensation.

Where possible, beams should be positioned so that air fl ow across the soffi t is maximised.

If used in conjunction with a permeable ceiling, the open area must be optimised to promote air fl ow in the void.

Case Studies:

Conquest House, London (see opposite) Barclaycard Headquarters, Northampton Empress State Building, London

The combination of chilled beams with an exposed concrete soffi t has become a popular option in new and retrofi t projects. In particular, multi-service chilled beams have found favour with many architects and clients for a combination of reasons including their energy effi ciency, simplicity, low maintenance and avoidance of suspended ceilings. They also work well in conjunction with a concrete soffi t, as cooling from the beams is mostly convective and is therefore complimented by the radiant output from the soffi t. A chilled beam is a simple, long rectangular unit enclosing a fi nned tube through which chilled water is pumped. The beams are mounted at a high level where surrounding air is cooled, causing it to lose buoyancy and travel downwards into the occupied space below. As the cooling is largely convective, good air fl ow around the beams is essential. The cooling output varies with the type of beam used i.e. active or passive. Ventilation is essentially a separate provision and can be natural or mechanical, which in some systems is ducted directly to the beam. FES can be employed using the basic principals described in this guide, with the chilled beams operating during the daytime to boost the overall cooling capacity as necessary. Depending on the energy effi ciency of the system used to provide the chilled water, it may be advantageous to also

Concrete Floor Solutions for Passive and Active Cooling

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