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R O D U C T S & T E C H N O L O G Y
D O U B L E S K I N F A ç A D E S
ouble Skin Faades:
esign and Technology for the Irish Climate
is is the frst in a series o technical articles promoting a better
derstanding o sustainable and low-energy design solutions, with
rticular ocus on their appropriateness to the Irish climate.
ouble skin açades have long been a eature o architectural
mpetitions and easibility studies, however, until recently,
w designs in Ireland were realised. This was due to a
umber o actors, including unavourable cost reporting
payback periods; an un-readiness to reduce plant size
d associated costs due to lack o perormance certainty ;
client scepticism. Within the last 12 months, a number
buildings, particular in Dublin, are proudly displaying
eir new ‘double skin’, and there are as many on the
awing boards. What has driven this change? Have market
rces demanded a higher perormance açade, or is this
consequence o the new Part L Regulations, or a greater
esire to develop low energy solutions?
e development o the double skin açade has been
omoted as the answer to providing a ully glazed curtain
all and as an eective way o controlling heat, light, air and
oise through the building envelope as well as reducingnergy consumption. These claims depend, however, on
e particular climate, site, and orientation that the double
in is placed. So what are the main attributes o double
in açades in relation to the Ir ish climate, and what
ngurations are most suitable?
eduction of Solar Heat Gain
e primary attribute o a double açade is its ability
reduce solar heat gain. Allowing or occupancy and
quipment, the solar gain to a building is by ar the largest
eat element, which must be reduced in order that it may
e naturally ventilated or utilise low energy techniques to
ovide a comortable working environment.
e simplest way to reduce solar heat gain to a building is
reduce the surace area o the glazing, shade the glazing
use body tints / ritting to improve the solar perormance
the glazing. An oce building in Ireland will typically be
ccessully naturally ventilated i the solar heat gain to the
oor is limited to the region o 15 – 25 W/m2. In the case o a
est açade with low-E double glazing, this would equate to a
aximum percentage glazing o 25-40%. For many building
pes such as open plan o ce spaces, this may prove too
strictive.
here large surace areas o glazing are preerred, solar heat
in can be reduced by providing shading or solar coating to
e glazing. External shading is potentially the most eective
means o reducing solar gain, as all heat is removed prior
to reaching the building. However, by their nature, these
shading systems will oten compromise views and have
more complex maintenance issues. This is particularly true o
south açades, where a horizontal brise soleil is eective in
removing solar heat gain in summer-time when the sun is at
a relatively high altitude (50-60o).
However, horizontal external shading alone is not eective
to east/west açades in Ireland, due to low sun angles in
summer. The solar altitude at 10:00/ 16:00 in August would
only provide 500mm shading or a 1-m deep brise soleil or
east/west açades respectively. West açades are particularly
problematic, as they have to be shaded during the hottest
time o the day, the aternoon.
One option to achieve reduction in solar gain is to improve
the solar properties o the glazing itsel, such as tinted
or refective glazing. While solar coatings have improved
in recent years, there will be a relative decrease in light
transmission and daylighting quality and physiologically can
create a sense o separation rom the ex terior with a relianceon articial lighting to create a comortable environment.
Body tints will not necessarily deal with glare and thereore
internal blinds are oten used, which urther reduce daylighting.
Double skin açades are thereore particularly suited in
Ireland to west (and east) açades, where horizontal shading
such as brise soleil cannot be eectively implemented. The
benet o the double skin açade is that a vertical shield,
such as Venetian blinds, eective or all solar angles, is
provided. Furthermore, the blinds can be raised during
cloudy conditions, which is particularly important in terms o
maximising daylight or Ireland’s variable climate. As the blind
is contained within the double açade cavity, it is not exposed
to the elements, and vastly easier to maintain and clean thanexternal shading elements.
Ventilation
Ireland is particularly suited to naturally ventilated buildings,
as external air temperatures rarely exceed design internal
temperatures [see Figure 2]. The advantage o the two layers
o glazing allows controllable natural ventilation without the
gusts and breezes associated with openable windows. This is
particularly true or buildings at an exposed site or or taller
urban buildings where wind pressures increase at higher
storeys. The extended air path into the building also controls
issues such as noise inltration, dust and insect inltration.
I the açade is to be used to naturally ventilate the internal
space, it will typically consist o ex ternal louvres and dampers,
by Andrew Morrison and David Walshe
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Full Height Ventilated Flue vs. Single Storey
entilated Faade
e advantages o the single-storey ventilated açade are that
s generally a simpler solution in terms o construction, re
paration, and noise transer between foors. A single storey
çade is more suitable or naturally ventilated buildings as
esh, cool air is available or occupants at each foor level.
contrast, a ull height ventilated fue would have solar
eat build up over a number o foors, which may mean that
mperatures at the top o the fue are too excessive to be
ed or natural ventilation to the occupied space.
s essential that ull height fues utilised in naturally
ntilated buildings are used solely or exhaust air, cross
ntilating rom opening windows on other açades. In this
nguration, the fue is acting in a similar manner to an
rium and the exhaust louvres normally required to extend
above the roo level in order to ensure that hot air does not-circulate to the highest foor level. The principle o this is
milar to ensuring a smoke layer in an atrium. The ull height
ue does, however, have the aesthetic advantage o not
quiring louvres at each foor level.
ptimum double skin açade fue widths are 200 to 400mm
r single-storey fues, which require all sections o the
ternal skin to be openable or maintenance. Full height
ntilated fues are o the order o 80 0 to 1200mm, and an
cess platorm or maintenance can be provided, with less
pening required to the inner skin, and less disruption to the
ternal space, during maintenance.
External Skin - Double Glazed or Single Glazed
sing a single glazed external skin can create a very
ansparent external appearance, particularly when using
w iron glazing. Blinds and solar shading devices (preerably
ntrollable) can then be placed within the cavity and the
lar gain on the blinds can be removed by ventilating the
vity. Openable windows into the fue as a means o providing
tural ventilation must be careully considered, as there is a
k o warm air rom the occupied areas causing condensation
n the cold external glazed skin. By placing the double glazed
nits on the external ace, condensation can be avoided.
ouble glazed units are best suited to be positioned to the
ternal skin or naturally ventilated solutions, whereas
ybrid / mechanical schemes can be achieved with a single
azed external skin.
3. Double Skin Faades with Air Conditioning/ Mechanical
Ventilation vs Naturally Ventilated Systems
The double skin açade can also be used in conjunction with
an air conditioned or mechanically ventilated building. In
this instance, the primary purpose o the double açade is to
reduce solar heat gains and subsequently air conditioning
cooling loads. The external fue can be used either as a
stand-alone system or removing solar heat gains by natural
convection (with the building sealed rom the açade) or used
as a return air path or the mechanical ventilation network,
which has the advantage o the heat trapped in the double
skin fue being available or heat reclaim at central plant.
While the above mechanically ventilated/ hybrid
congurations are most popular world-wide, it is also oten
possible in most building applications in Ireland to utilise
a double skin açade with an entirely naturally ventilated
building, due to the moderate climate.
The benets o a ull naturally ventilated system are energy
consumption, increased uel costs, and the cost saving o
mechanical plant, as well as carbon emissions, and low
energy credentials. There are also comort issues as many
people preer naturally ventilated spaces, provided the
ventilation is controllable and overheating is not a problem.
Another option is to provide a mixed mode system. This has
two meanings; either that the building is partially naturally
ventilated/partially air conditioned, or that the building
operates with natural ventilation in spring/autumn, and with
air conditioning in summer.
However, regardless o preerred system, it must be noted
that in order to achieve successul operation, occupant users
need to play a role in operating and maintaining the systems,
and their eectiveness, will oten depend on a combination
o user control and a sophisticated Building Control System.
Note:
Graphs courtesy o IN2 Engineering Design Partnership.
Acoustic data courtesy o AWN Consulting Engineering.
Cost data courtesy o KSN.
Recent examples o double skin
açades in ofce buildings in Dublin
Riverside One,
Scott Tallon Walker Architects
Riverside Two,KMD Architecture
Photos by Dennis Gilbert
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