Book - School of Science and Technology

32 The building in winterExposed elementU-value(W/m 2 K)Pitched roof with insulation between rafters 0.20Pitched roof with insulation between joists 0.16Flat roof 0.25Walls 0.35Floors and ground floors 0.25Windows, roof windows and doors (area weightedaverage for the whole building), glazing in metal frames 2.2Windows, roof windows and doors (area weighted averagefor the whole building), glazing in wood or PVC frames 2.0Rooflights 2.2Vehicle access and similar large doors 0.7Coupled with these insulation standards are a set of maximum opening areas that canonly be exceeded if compensating measures are taken elsewhere in the design. Themaximum areas of openings are as shown in the following tableBuilding typeWindows and doors aspercentage of the internalarea of exposed wallRooflights aspercentage ofarea of roofResidential buildings (where peopletemporarily or permanently reside) 30 20Places of assembly, offices and shops 40 20Industrial and storage buildings 15 20Vehicle access doors and displaywindows and similar glazing As requiredFor dwellings, the standard area allowance is 25% of the total floor area.In order to provide design flexibility, it is possible to move away from these standards,provided that the rate of heat loss from the actual design does not exceed that of anequivalent building that does meet the elemental standards. Further trade-off opportunitiesare provided by considering the heating system efficiency. Lower insulationstandards can be offset by improved heating system efficiency and vice versa. However,limiting U-values are defined for roofs, walls and floors, principally to minimise the riskof condensation. Even greater degrees of trade-off are allowed by considering otherbuilding services systems such as ventilation, cooling and lighting. Clearly, the greaterthe extent of trade-off, the more complex the calculations needed to demonstrate compliancebecome.Underpinning these `headline numbers' are some important changes that require carefulconsideration of the construction design details:. Firstly, the opaque U-values have to be calculated using the combined method, whichtakes greater account of the effects of repeating thermal bridges. Similarly, windowstandards are based on whole window U-values, including the effects of the frame. Thisis especially significant, since centre-pane U-values are usually better than that of theframe, especially for metal windows or curtain walling systems.. Secondly, the design of junctions between elements (wall/floor, wall/wall, wall/roof)and around openings (windows and doors) should be such as to eliminate significant