Book - School of Science and Technology

Conservation of energy 67of air infiltration are clearly beneficial in either season. It would be inappropriate inconsequence to repeat here the comments made in the previous chapter. Nevertheless, theabsolute significance of thermal transmittance for opaque building materials is, in theBritish Isles, less in summer than in winter simply because the temperature difference,inside to outside, is smaller in that season. Typical values might be 22 K ( 1to‡ 21 C)in winter and 6 K (29±23 C) in summer. Similarly, the effect of infiltration by outside airis less in summer, not only as a result of the reduced temperature difference, outside toinside, but also because wind speeds are usually less then than in winter.Extraneous influencesThe salient difference between the two seasons is of course that heat gains from people,artificial lighting and equipment or machinery combine with heat from the sun to create apenalty during the summer months, as far as human comfort is concerned. These heatgains may lead to high temperatures within a building and, in situations where theoccupants are exposed to direct sunshine, there is the added effect of an unpleasantlyhigh radiant intensity from surrounding glazed areas. The control of solar effects iscritical to limitation of heat gains in summer and, in consequence, much of this chapteris devoted to that subject.Not all of the gains noted above related wholly to sensible heat, i.e. that which leads toan increase in air temperature. Some part of the output may be in release of latent heatwhich leads to an increase in the amount of moisture in the air and, if the temperatureremains constant, to an increase in the relative humidity. Sources of latent heat gains arebuilding occupants, moist air infiltrated from outside and vapour from a variety ofactivities such as cooking and bathing, to name the simplest.Conservation of energyFor the first time, the 2002 edition of Part L requires designers to give attention to theperformance of the building in summer. The requirements affect both the design of theenvelope and the engineering systems used to service the building. Previous editions of Part Lhad prescribed maximum areas of glazing, but this had been to limit excessive conductionlosses rather than to control solar gains. The 2001 edition introduces a requirement to limitsolar overheating. A number of ways of demonstrating compliance are provided, rangingMaximum permissible areas of single glazed windows and roof-lights (Regulations 1992)Type of building Windows Roof lightsDwellingsWindows and roof-lights together 15% of total floor areaOther residential(including hotels and institutional) 25% of exposed wall area 20% of roof areaPlaces of assembly, offices and shops 35% of exposed wall area 20% of roof areaIndustrial and storage 15% of exposed wall area 20% of roof areaNotes:1. In any building the maximum glazed area may be doubled where double glazing is used and trebled where treble glazing is used.2. Double glazing provides with a low emmissivity costing may be considered equivalent to treble glazing. (Emissivity 0.02).3. Display windows in shops do not count towards the maximum single glazed area.$