68 - AI 223 PRODUCTS & TECHNOLOGY DOUBLE SKIN F AçADE S Double Skin Faades: Design and Technology for the Irish Climate This is the frst in a series o technical articles promoting a better understanding o sustainable and low-energy design solutions, with particular ocus on their appropriateness to the Irish climate. Double skin açades have long been a eature o architectural competitions and easibility studies, however, until recently, ew designs in Ireland were realised. This was due to a number o actors, including unavourable cost reporting / payback periods; an un-readiness to reduce plant size and associated costs due to lack o perormance certainty ; or client scepticism. Within the last 12 months, a number o buildings, particular in Dublin, are proudly displaying their new ‘double skin’, and there are as many on the drawing boards. What has driven this change? Have market orces demanded a higher per ormance açade, or is this a consequence o the new Part L Regulations, or a greater desire to develop low energy solutions? The development o the double skin açade has be en promoted as the answer to providing a ully glazed curtain wall and as an eective way o controlling heat, light, air and noise through the building envelope as well as reducing energy consumption. These claims depend, however, on the particular climate, site, and orientation that the double skin is placed. So what are the main attributes o double skin açades in relation to the Ir ish climate, and what congurations are most suitable? Reduction of Solar Heat Gain The primary attribute o a double açade is its ability to reduce solar heat gain. Allowing or occupancy and equipment, the solar gain to a building is by ar the largest heat element, which must be reduced in order that it may be naturally ventilated or utilise low energy techniques to provide a comortable working environment. The simplest way to reduce solar heat gain to a building is to reduce the surace area o the glazing, shade the glazing or use body tints / ritting to improve the solar perormance o the glazing. An oce building in Ireland will typically be successully naturally ventilated i the solar heat gain to the foor is limited to the region o 15 – 25 W/m 2 . In the case o a west açade with low-E double glaz ing, this would equate to a maximum percentage glazing o 25-40%. For many building types such as open plan o ce spaces, this may prove too restrictive. Where large surace areas o glazing are preerred, solar heat gain can be reduced by providing shading or solar coating to the glazing. External shading is potentially the most eective means o reducing solar gain, as all h eat is removed prior to reaching the building. However, by their nature, these shading systems will o ten 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 (50-60 o ). However, horizontal external shading alone is not e ective to east/west açades in Ireland, due to l ow 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 b e a relative decrease in light transmission and daylighting quality and physiologically can create a sense o separation rom the ex terior with a reliance on articial lighting to create a comor table environment. Body tints will not necessarily deal with gl are and thereore internal blinds are oten used, which ur ther 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 than external 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 b e used to naturally ventilate the internal space, it will typically consist o ex ternal louvres and dampers, by A ndrew Morrison and David Walshe 1