Book - School of Science and Technology

Atypical construction features 33thermal bridges. Sets of robust details are being prepared that should give satisfactoryperformance; alternative details can always be used, but they should give comparablelevels of performance. This emphasis on cold bridging is necessary as general insulationstandards improve, since the overall performance of the envelope can be significantlyimpaired by relatively small areas of high conduction loss.Figure 2.4 summarises the fabric insulation standards and allowances for windows, doorsand roofs given in the elemental method.Minimum standards of airtightnessAs insulation standards improve, the energy loss through air infiltration becomes increasinglysignificant. Air infiltrating through the wall/roof assemblies not only results in asignificant direct energy loss, but also reduces building performance in other ways.. It can reduce the temperature on the warm side of the insulating layer, thereby reducingthe effectiveness of the insulation.. It can also bring substantial amounts of moisture into the assembly (at a rate muchgreater than that by diffusion). This can cause wetting of the insulation and a consequentdegradation of performance. Over the long term, it can also result in structuraldamage.. It can create problems of draughts in cold weather.. It can upset the balance of mechanical ventilation systems, especially in windy or coldweather.Measurements of air leakage have shown that UK buildings are generally much leakierthan their Scandinavian or North American counterparts. In order to stimulate improvementof this critical aspect of energy performance, a maximum air permeability limit of10 m 3 /hr m 2 at an applied pressure difference of 50 Pa has been set. Air permeability isthe air leakage averaged over the whole envelope area (including ground floors), and is adifferent measure to the oft-quoted air leakage index, which excludes floors in contactwith the ground in the area normalisation. As with avoidance of thermal bridging,achieving a satisfactory air leakage performance requires careful design of the air barrierand of the sealing between elements. The robust details referred to earlier also offersolutions to this problem.An important addition to the regulations is the requirement to pressure test thecompleted assembly to ensure the required airtightness standard has been achieved. Thisshould be carried out according to the procedures of CIBSE TM23. Should the buildingfail to meet the standard, identification and sealing of leakage sites will be required untilthe standard is achieved. The initial airtightness standard of 10 m 3 /hr m 2 @50Paisarelatively modest one, with best practice standards being lower by a factor of two or three.It is therefore likely that standards will be made significantly more demanding at subsequentrevisions of Part L, especially in non-domestic buildings.Atypical construction featuresThe thermal characteristics of the majority of materials and the various composite constructionelements built up from them are not very complexand their thermal transmittance,for steady state energy flow, may be calculated by application of simple arithmeticas has been shown earlier on p. 26. Brief notes on some of the exceptions follow here.