Book - School of Science and Technology

34 The building in winterFigure 2.4 Summary of elemental method (extract from Building Regulations Part L ± 2002)Non-homogenous constructionsThese present unique problems arising from discontinuities at corners, at junctions and atheat bridges generally. A rule-of-thumb approach to their solution, which errs on the sideof safety, suggests that where insulating blocks are jointed with sand/cement mortar, andwhere insulation in a timber framed wall is bridged by studs and noggins, the conductivityof the block in the first case and that of the insulation fill in the second, both as quoted inTable 2.3, should be increased notionally by about 15%.As for hollow blocks, the Guide Section A3 provides an analytical method of calculationto take account of the resistance of the enclosed air gap (or gaps) but this does not makeallowance for mortar infill due to careless workmanship on site. Observation suggests thatit is wise to add 50% to any quoted conductivity to allow for the cumulative effect ofjointing and of mortar dropped into those cavities.Ground floors, solid and suspendedIn cases where a building covers other than a small area, the heat loss from the floor willoccur largely around the perimeter since the temperature of the earth at the centre will,over a period of time, approach that within the room. This situation applies whether thefloor is a slab on the ground, a suspended concrete construction or suspended joists andboarding. Whereas previous practice was to select a U value according to the floor shapeand dimensions, it has been shown by Anderson* that a more flexible alternative using theperimeter/area ratio of the floor has much to commend it (see Figure 2.5).* Anderson, B. R., The U-value of ground floors: application to building regulations. BRE InformationPaper IP90, April 1990.