The Influence of façade design on the optimum window to wall ratio and overheating rate in UK Passivhaus dwellings for current and future climates

Yahya Lavafpour and Steve Sharples School of Architecture, University of Liverpool, Liverpool L69 7ZN, United Kingdom

Abstract: Many researches of overheating in buildings have focused on the optimisation of the window to wall ratio (WWR). However, the physical form of the building envelope has not been widely considered as a factor in optimisation investigations. This study examines if the optimum WWR might vary in response to slight geometrical changes to a façade’s design. An evolutionary parametric design method was used to generate a range of inverted pyramidal shaped envelopes with different inclination angles for each façade. This was implemented for an existing UK Passivhaus. The WWR was set as the main variable along with a sub variable of different weather data (both current and future climate scenarios). This paper, in particular, focused on minimising the overheating during the summer while reducing the primary energy use of the building. These two parameters were set as the objectives to be optimised using the parametric tool in the dynamic thermal simulation software DesignBuilder. The calculation used batches of Energy Plus simulations using a genetic algorithm. The results showed that the optimised WWR varied significantly when implementing different façade arrangements. Optimum self-shading shapes could allow more glazing to be used while still achieving thermal comfort in summer climate.

Keywords: overheating, WWR, self-shading, warming summer

Introduction

In the mid-20th century the core principle of modernist architecture was that the shape of a building should be primarily based on its function – hence the phrase “Form Follows Function”, which was coined by the American architect Louis Sullivan. At that time, a building's intended use and its function were the most important criteria in designing the form of a building. The integration of the form and its relationship to a building’s energy consumption means that the whole issue of a building's use needs to be considered differently in a sustainable context (Cody, 2006). A new architectural language introduced by Cody (2010) emphasised that the design of the building envelope was one of the key drivers for achieving energy efficiency in the built environment (‘Form Follows Energy’). He argued how energy will become a new design parameter for future architecture. Research has demonstrated that buildings with different external envelope areas but similar internal floor areas can have different energy demands (Zerefosa, et al., 2012; Capeluto, 2003;Loonen, et al., 2013). Zerefosa et al, (2012) believed that the energy consumption of two buildings with the same materiality, volume, wall area, openings and operating programmes, displayed difference only due to the shape of their external envelopes. They examined the energy behaviour of a case study in Athens with polygonal and prismatic envelope shapes. The research pointed out that the prismatic formed building had lower solar gain compared to its orthogonal counterpart and consumed less energy in an annual

cycle. (Gupta & Gregg, 2012; Laouadi, 2010) showed that shadings was the most effective single intervention for reducing overheating in homes for the future UK warming climate. Monitoring results from the Camden Passivhaus in London showed that summer overheating occurred between 15.00-19.00, with the maximum at 17.00 (Ridley, et al.,2013), suggesting that the overheating happened perhaps due to unnecessary solar gain.

Large south facing windows are common in Passivhaus designs as they provide free solar heating and daylight utilisation. Research has underlined that the optimum window to wall ratio (WWR) is around 25% for the Passivhaus standard. However, some Passivhaus buildings employ up to 60% south facing WWR (SF-WWR). This has raised the concern that the large areas of glazing would lead to increased discomfort hours under UK future climates, especially in London. By contrast, other façades have a very small percentage of windows, and there often is no windows on east or west Passivhaus elevations. Although this will alleviate unwanted solar gain, it will also potentially reduce cross ventilation, daylighting and view. Many researches have focused on the optimisation of WWR, while the form of the building envelope has not been included in the optimisation process. This study aims to investigate if the optimum WWR would vary as a result of slight amendments to the geometry of a façade’s design. In particular, this study examined the concept of self-shading geometries.

Building shape and solar exposure

The greatest source of internal heat gain is usually from solar radiation, which enters the building directly through windows. In addition to various commonly used shading approaches, inclined facades have also been implemented to create a self-shadow on the building’s glazing and envelope. The incidence angle between the direct solar beam and the building’s surface changes as a result of changing the orientation of the building or the inclination of a façade, resulting in different values of solar gain. With no obstacle and shading factor, solar heat gains ¿ on a flat surface with a given tilt angle and orientation can be calculated using Equation 1:

Φsol

¿̌=α s AT [ Id ρ(1−cos∑ )+ Id(1+ cos∑ )+ 1π Ib (cos∑cosLs+cosѱ sin∑ cosLs+√sin 2ѱsin2∑+(cos∑cos Ls+cosѱ sin∑cos Ls)2)]¿

Equation 1where A (m2) is the area of the flat surface, I b and I d are the direct and diffuse solar irradiation (W/m2) respectively, ∑ is the tilt angle of the surface, ѱ is the orientation angle,ρ is the reflection coefficient from the ground, and Ls is the equivalent latitude, which depends on the specific month. This study used a computer-modelling tool, which makes it relatively straightforward to calculate and evaluate large numbers of design alternatives for solar gain.

Thermal discomfort

In the UK there is no universally agreed overheating criterion for residential building. This is perhaps because in the UK the retention of heating has been the main focus of thermal design and so overheating has not historically been a concern (Lomas & Porritt, 2017). The Housing Health and Safety Rating System from the Housing Act 2004 (Health and SafetyRating System , 2006) stated, “a healthy indoor temperature is around 21°C. As temperatures rise, thermal stress increases, initially triggering the body’s defence mechanisms such as sweating. High temperatures can increase cardiovascular strain and

trauma, and where temperatures exceed 25°C, mortality increases and there is an increase in strokes. Dehydration is a problem primarily for the elderly and the very young”.

The Passivhaus standard requires that indoor air temperatures should not exceed 10% of occupied hours above 25°C for living spaces. CIBSE TM36 (2005) set the ‘warm’ and ‘hot’ temperature thresholds of 25°C and 28°C for living rooms and 21°C and 25°C for bedrooms respectively. CIBSE Guide A (2007) said operative temperatures should not exceed 10% of occupied hours above 25°C for living spaces (both living room and bedrooms). Operative temperature also should not exceed 1% of occupied hours above 28 and 26°C for living rooms and bedrooms.

Methodology

To generate a proposed self-shading form a parametric design method was used to generate a range of reversed pyramidal shape envelopes with different tilts on each façade. The form generation was carried out using evolutionary parametric tools Grasshopper and Galapagos (Rhino Plug-ins) to create an extensive range of façade inclinations to achieve a desirable fitness. A simplified box building (9m long x 7m wide x 3m high) was created as a base case for an initial pilot study. Eight points were formulated to create this box in different X, Y, and Z axes. The four points that created the base were kept fixed throughout the parametrisation analysis. The other four points (Figure 1), which were located at 3m high in the Z direction, were created using the number slider to manipulate the points parametrically. Each façade was divided into grids and the sum of the solar radiation received by each grid was calculated.

The Galapagos evolutionary computing tool was used to find the best fitness to minimise the solar radiation from 1 June to 30 August while choosing the optimised forms that received the higher solar radiation between 1 October and 30 March. Data analysis derived from the pilot study hypothesised inclination angles of 115° and 122° to be an effective shading intervention for the south and west walls respectively. The east-facing wall had a more complex shape, where the inclination angle for point D was steeper (117°) than for point B (103°). For the north facade the wall leant inward slightly at 87°. This geometry was then implemented in to the case study.

Implication to an existing Passivhaus case study

According to the Passivhaus Institute, the frequency of overheating increases sharply with a WWR of more than 20%. However, larger windows are often recommended (Schneider,2006). The study used the design of an actual Passivhaus built by bere:architects called Larch House (Figure 2), which employed a SF-WWR of 55% (Bere, 2014). This house was modelled using the advanced dynamic simulation software DesignBuilder. Selected tilted walls from the pilot unit replaced the original vertical walls of the Larch House case study. Dynamic thermal simulations were used to examine the impact of these inclinations for the Passivhaus dwelling to make use of the self-shading that this form created. A sensitivity analysis of internal temperatures and thermal comfort conditions in the dwelling as a function of building facade inclination and prevailing climatic conditions was undertaken. The simulations were calculated for the living room and main bedroom. Windows assigned to the living room and the main bedroom on relevant facades were subject to the parametric analysis of WWR. This paper focused on minimising the thermal discomfort overheating during the summer while reducing the primary energy use of the building. These two parameters were set as the main objectives to see if the optimum WWR would

vary by manipulating the façade geometry. The Optimisation Tool in DesignBuilder v5 was used to identify the optimum combination.

Figure 1. A snapshot of the parametric analysis (pilot unit)

Figure 2. Existing Larch House (bere:architects) and 3D model of the case study and modified geometry.

Weather data

This study used probabilistic climate change scenarios from the UK Climate Change Projections to determine the overheating risk in an existing Passivhaus dwelling under a high emission 50-percentile scenario in London. The PROMETHEUS team (Eames, et al., 2011) have developed future weather files in EnergyPlus format (.epw) using a UKCP09 weather generator. These hourly weather data files are available under low, medium and high emission scenarios with different percentile probabilities for both Test Reference Year (TRY) and Design Summer Year (DSY) weather data. They are available for three future intervals i.e. 2030’s, 2050’s and 2080’s. DSY tend to give warmer summer days while TRY is more representative of the whole year. For the modelling in this study a high emission 50 percentile probability (A1F1 50%) was chosen. Since this study considered a whole year’s energy performance it was felt that the Test Reference Year (TRY) data would be more representative of the year. London (Islington) weather files were used in the simulations as

London is projected to experience the greatest temperature rise due to climate change and the impacts of the urban heat island (GLA, 2006).

Overheating criteria

There are two ways of assessing summertime thermal comfort - static and adaptive. This paper used CIBSE guide A (2007) static criteria, defined as overheating when indoor living spaces hit the ‘warm’ benchmark of 25°C for more than 10% of the occupied time and exceeds the ‘hot’ threshold of 28°C in the living room and 26°C in the bedroom for more than 1% of the annual occupied hours. Assessing thermal comfort based on the static criteria allows a focus on one specific parameter i.e. in this case the geometrical implication of the house. However, in reality, individuals will also adapt their behaviour to changing temperatures.

Results and Discussion

Table 1 shows a breakdown of the simulations, indicating the total hours that operative temperatures exceeded the CIBSE Guide A threshold. This study used a generic UK household occupancy schedule. The bedroom was occupied (with different fractions) from 22.00 to 08.00 weekdays and from 23.00 to 09.00 at weekends. The living room was occupied from 17.00 to 23.00 weekdays and 09.00 to 23.00 weekends. In the current London climate, a similar house to the Larch House case study would experience warm indoor temperature (Lavafpour and Sharples, 2015). Considering the 55% WWR, the house with the original form in London current climate experienced 35% of the occupied hours over 25°C (hθ >25°C) in the living room, and 30% in the bedroom. Overheating in the living room was reduced to 12% using the optimised self-shading geometry. Tilting the façade on the south elevation, and the prismatic shape of the east façade, eliminated the 28°C/26°C overheating threshold but slightly exceeded the 25°C threshold limit. However, by reducing 5% of the WWR (i.e. down to 50%) the proposed geometry eliminated overheating in both rooms. Because of changing the façade angle, the average operative temperature dropped for both summer and winter periods. There was a slight increase in heating load for the optimum shape, which is greater in the current climate. Despite this increase, heating demand always remained within the Passivhaus limitation of 15 kWh/m2 while assuring the minimum indoor temperature of 20°C.

The optimum shape building with 55% of glazing has almost the same Indoor operative temperature as the original shape with 25% WWR. The overheating percentage for the future climate also had a similar trend. A larger WWR on the optimum shape had an equal overheating risk than the original building shape with smaller WWR (Figure 3). The gap between the curves indicating overheating rate for different façade forms grows when the WWR exceeded 35% (Figure 4). The impact of the self-shading geometry is more apparent in the future weather scenarios. It can be seen that the original shape with more than 35% glazing area will experience a high risk of overheating even in the current climate. The optimum shape with 40%WWR had an acceptable overheating rate until 2050s weather projections; however, the proposed geometry could not eliminate overheating.

Table 1. Impact of form on WWR and overheating percentage for current and future weather, London

WW

R

Current 2030s 2050s 2080sOriginal shape

Optimum* shape

Original shape

Optimum shape

Original shape

Optimum shape

Original shape

Optimum shape

15%

Living room>25°C 80 hrs 21 hrs 543 hrs 242 hrs 823 hrs 406 hrs 1490 hrs 1010 hrsLiving room>28°C 0 0 0 0 24 hrs 11 hrs 128 hrs 19 hrsBedroom>25°C 22 hrs 0 314 hrs 90 hrs 482 hrs 176 hrs 857 hrs 559 hrsBedroom>26°C 0 0 67 hrs 3 hrs 151 hrs 27 hrs 460 hrs 185 hrs

25%

Living room>25°C484 hrs 116 hrs

1124 hrs 532 hrs 1379 hrs 788 hrs 2197 hrs 1449 hrs

Living room>28°C 0 0 108 hrs 9 hrs 232 hrs 40 hrs 832 hrs 251 hrsBedroom>25°C 259 hrs 21 hrs 676 hrs 267 hrs 876 hrs 408 hrs 1317 hrs 805 hrsBedroom>26°C 40 hrs 0 318 hrs 58 hrs 486 hrs 135 hrs 927 hrs 424 hrs

35%

Living room>25°C875 hrs 202 hrs

1531 hrs 752 hrs 1707 hrs 1002 hrs 2296 hrs 1596 hrs

Living room>28°C 36 hrs 0 253 hrs 54 hrs 470 hrs 111 hrs 1079 hrs 420 hrsBedroom>25°C 531 hrs 53 hrs 931 hrs 347 hrs 1066 hrs 519 hrs 1402 hrs 902 hrsBedroom>26°C 178 hrs 2 hrs 530 hrs 112 hrs 706 hrs 203 hrs 1048 hrs 524 hrs

45%

Living room>25°C1327 hrs 411 hrs

1903 hrs 1023 hrs 2046 hrs 1205 hrs 2522 hrs 1710 hrs

Living room>28°C 153 hrs 2 hrs 622 hrs 116 hrs 836 hrs 224 hrs 1420 hrs 599 hrsBedroom>25°C

839 hrs 491 hrs1198 hrs 531 hrs 1281 hrs 717 hrs 1576 hrs 1048 hrs

Bedroom>26°C 459 hrs 207 hrs 834 hrs 231 hrs 952 hrs 379 hrs 1238 hrs 707 hrs

55%

Living room>25°C 1578 hrs 525 hrs 2231 hrs 1051 hrs 2376 hrs 1240 hrs 2772 hrs 1766 hrs

Living room>28°C 433 hrs 0 1020 hrs 132 hrs 1212 hrs 224 hrs 1688 hrs 614 hrs

Bedroom>25°C 1059 hrs 515 hrs 1390 hrs 679 hrs 1439 hrs 846 hrs 1737 hrs 1158 hrs

Bedroom>26°C 694 hrs 269 hrs 1051 hrs 334 hrs 1136 hrs 495 hrs 1399 hrs 826 hrs

*Geometry obtained by this study (not global optimum)

15% 25% 35% 45% 55%0

102030405060

current

Livi

ng ro

om>2

5°C

Ove

rhea

ting

(%)

15% 25% 35% 45% 55%

2030s

15% 25% 35% 45% 55%0

102030405060

2050s

Original shape Optimum* shape

Window to Wall Ratio

Livi

ng ro

om>2

5°C

Ove

rhea

ting

(%)

15% 25% 35% 45% 55%

2080s

Original shape Optimum* shape

Window to Wall Ratio

Figure 3. Percentage overheating time and WWR for current and future climates - ‘warm’ threshold overheating rate in living room

10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%05

10152025303540

Current

Ove

rhea

ting

(%)-

Livi

ng ro

om>2

8°C

10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%

2030s

10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%05

10152025303540

2050s

Original shape Optimum* shape

Window to Wall Ratio

10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%

2080s

Original shape Optimum* shape

Window to Wall Ratio

Figure 4. Percentage overheating time and WWR for current and future climates - ‘hot’ threshold overheating rate in living room

Conclusion and future work

Optimal form of a building can significantly reduce direct solar gain without reducing the total solar heat gain needed for mild and cold regions, in particular for buildings with large glazing areas. The results from this study showed that the WWR varied significantly when implementing different façade arrangements. Self-shading facades allowed a greater window to wall ratio on the south and east facades and a steeper inclined wall for the west façade of the case study house, which originally did not have any windows, allowed for a small area of windows and potentially could increase cross ventilation cooling during the

summer. The study was not trying to generate a complete optimal building form and cannot define finite performance criteria. However, the extent of a small design changes can be estimated using the result of the paper.

The shape of the external building envelope will also affect the daylight and airflow patterns. Future work will investigate the consequences that the proposed geometry may have on natural ventilation and daylighting.

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