Book - School of Science and Technology

26 The building in winterTable 2.2 ContinuedPitched roofsDomestic and commercialTiles on battens, roofing felt and rafters, bare plasterboard ceiling below joists 2.6Tiles on battens, roofing felt and rafters, 50 glass fibre mat between joists,plasterboard ceiling below 0.6Tiles on battens, roofing felt and rafters, 100 glass fibre matbetween joists, plaster ceiling below 0.35IndustrialCorrugated double-skin decking with 25 glass fibre mat 1.1Calculation of a U valueThe wide range of constructions currently encountered, built up from the many compositeelements available, often requires that transmittance coefficients (U values) arecalculated from first principles. For example, consider a curtain wall constructioncomprising:Outside 40 mm thick pre-cast concrete panelAir gap 25 mm wideInsulation 50 mm glass fibre slabInside 75 mm lightweight concrete block yPlaster 13 mm lightweightThe resistances, from the physical data provided in Tables 1.1, 2.1 and 2.3, may besummated as follows:Outside surface, R so ˆ 0:06Concrete panel (1/1:4) (40/1000) ˆ 0:029Air space, R a ˆ 0:18Insulation (1/0:035) (50/1000) ˆ 1:43Concrete block (1/0:22) (75/1000) ˆ 0:34Plaster (1/0:16) (20/1000) ˆ 0:125Inside surface, R si ˆ 0:12and thusU ˆ 1/2:28 ˆ 0:44 W/m 2 KR ˆ 2:284 m 2 K/WSurface temperaturesIt is often necessary, for a variety of reasons, to establish either a surface or an interfacetemperature for some form of composite construction. The temperature gradient acrossthe structure may be plotted, as shown in Figure 2.2, using the resistances of the variouselements, with the simple case of an unplastered and uninsulated 210 brick wally0.19 (from Table 1.1) ‡15% for sand/cement joints ˆ 0:22 W/m K.