Book - School of Science and Technology

44 The building in winterTemperature °C–0.5–0.21.615.418.619.82000Vapour pressure Pa15001000–1 °C100% RH562Pa21 °C40% RH1009Pa500Saturated vapour pressureVapour pressureFigure 2.9 Condensation. Vapour pressure gradientsbe found using the vapour resistances from Guide Section C1. The overall resistance willbe made up thus:Inside air to surface resistance13 mm plaster 0:013 50 ˆ 0:6575 mm lightweight block 0:075 45 ˆ 3:3850 mm glass fibre slab 0:050 10 ˆ 0:5025 mm air gap40 mm concrete panel 0:040 200 ˆ 8:00Outside air to surface resistance12:53 N s/kg 10 9The total change in vapour pressure across the structure will be, read or interpolatedfrom Guide Section C1, the vapour pressure at the inside surface condition (19.8 C and42% RH) minus the vapour pressure at the outside surface condition ( 0:4 C and 96%RH) (994 562) ˆ 432 Pa and this must be allocated in line with the resistances of eachcomponent of the structure as follows,Room side surface of plasterInterface, plaster to concrete blockInterface, concrete block to glass fibreInterface, glass fibre to air gap994 Pa972 Pa856 Pa838 Pa