Book - School of Science and Technology

Interface, air gap to concrete panelOutside surface of concrete panel838 Pa562 PaThese values may now be plotted, as shown by the heavy full line in Figure 2.9, toillustrate the vapour pressure gradient through the structure.To represent the dewpoints at the various surfaces and interfaces, 100% saturationvapour pressure values may be again read or interpolated from Guide Section C1 for thevarious temperatures noted at the top of Figure 2.9 and a gradient of saturation vapourpressure may then be plotted, as shown by a heavy broken line. For ease these saturatedvapour pressure values as interpolated are as follows:-Room side surface of plasterInterface, plaster to concrete blockInterface, concrete block to glass fibreInterface, glass fibre to air gapInterface, air gap to concrete panelOutside surface of concrete panel2314 Pa2146 Pa1744 Pa685 Pa602 Pa583 PaAir infiltration 45As will be noted, the two lines cross over one another at vapour pressure values of 840 and615 Pa, suggesting that condensation will occur between these points. Since, of course, it isnot possible for the vapour pressure level in the structure to exceed that of the saturationvapour pressure, the former will adjust to the gradient shown by the chain dotted line andindicate that excess moisture will condense out at the interface indicated.As will be appreciated, the external conditions assumed for this example were theheating design conditions used earlier in this chapter, whilst they serve to illustrate onemethod used to trace and evaluate interstitial condensation they could be consideredsevere.Design conditions are generally selected to suit the purpose of the analysis, which isusually to determine the long term build up of condensation within the thickness of theconstruction. BS 5250 suggests the following that relate to a 60-day period for the UnitedKingdom in winter.. outdoors: 5 C, 95% RH. indoors (dry±moist occupancy): 15 C, 65% RH. indoors (moist±wet occupancy): 15 C, 85% RH.However it could be considered that these conditions are rather less searching. For furtherguidance, the reader is referred to two excellent publications by the Building ResearchEstablishment* and Guide Section A7.Air infiltrationThe subject matter of the last few paragraphs has related to conduction heat loss throughthe building fabric, a matter which is capable of examination on a rational basis. As wasexplained much earlier in this chapter, however, the matter of air infiltration from outsidethe building must be considered also. Leakage through windows and doors, an upwarddraught through an unsealed flue, and leakage through the structure itself, particularly ina factory-type sheeted building, will each have an influence.* Digest 110, Condensation, October 1969 (1972 Edition). Report BR 143, Thermal Insulation: AvoidingRisks, 1989.