Book - School of Science and Technology

76 The building in summerTable 3.2 Representative design factors for solar gain through opaque materials (dimensions in mm)Construction (dimensions in mm)U value(W/m 2 K)Time lag(h)DecrementfWalls105 brick, unplastered 3.27 3.0 0.88220 brick, 13 lightweight plaster 1.9 7.0 0.46220 brick, 50 air space, plasterboard on battens 1.43 7.1 0.3919 render, 200 dense concrete,25 polyurethane, 12 plasterboard 0.71 8.0 0.2280 pre-cast concrete panel, 50 EPS, 100 dense concrete,12 plasterboard on battens 0.55 8.2 0.29105 brick, 50 air space, 105 brick, 13 dense plaster 1.36 7.9 0.4105 brick, 50 EPS, 100 concrete block, 13 dense plaster 0.56 9.1 0.25105 brick, 50 UF foam, 100 lightweight block,13 dense plaster 0.47 8.4 0.44105 brick, 50 air space, 19 plywood, 95 studding withmineral fibre between studs, 12 plasterboard 0.29 6.1 0.58RoofsLight colour paving slab, 50 extruded polystyrene,waterproof membrane, 75 screed, 150 cast concrete,13 plaster 0.51 10.0 0.17Waterproof covering, 35 polyurethane, vapour barrier,19 timber decking, unventilated air space, 12 plasterboard 0.52 1.9 0.93Tiles on battens, roofing felt and rafters, 50 glass fibremat between joists, plasterboard ceiling below 0.61 1.0 1.0Tiles in battens, roofing felt and rafters, 100 glass fibremat between joists, plaster ceiling below 0.35 1.0 1.0In the special case of lightweight curtain walling, as shown in Figure 2.6, the effect ofsolar radiation is to raise the outer surface temperature very rapidly when the finish isdark. It can be demonstrated that in the British Isles this temperature may be well over50 C in the early afternoon on a south-west facade and, since the time lag of such a formof construction is an hour or less, with negligible decrement, it is inevitable that complaintsof discomfort due to radiation will arise.Flat roofs are subjected to solar radiation during the whole of the daylight hours and arelatively massive construction with a light coloured finish is advantageous. Roof insulation,as may be seen from Table 3.2, does little to increase the time lag but does of coursereduce the U value. In the case of pitched roofs, the heat gain will depend upon the angleof the slope and may be more severe on one face than another, depending upon orientation.It is worth while making a check on the orientation of so-called `north-light' roofssince it is not unknown for them to belie their name!Solar gain through glazingWindows are by far the most significant route by which solar heat enters a building, notleast because that entry is without time lag. This is not to say, however, that the effect isinstantaneous since this may well depend upon the nature and mass of the internal