Book - School of Science and Technology

Condensation 43Table 2.7 Vapour resistivities of some common materialsMaterialDensity(kg/m 3 )Thermalresistivity(mK/W)Common brick 1700 1.19 35Dense concrete 2100 0.71 200Lightweight concrete 600 4.55 a 45Dense plaster 1300 2.00 50Glass fibre slab 25 28.6 10EPS slab 25 28.6 100Vapourresistivity[N s/(kg m 10 9 )]Notea Derived from 0.19 (Table 1:1) 15% for sand cement joints ˆ 0:22 W/m K and thus 1/0:22 ˆ 4:55 m K/W.These values may be thought of as properties parallel to the values of thermal resistivity(1/k) listed with them for reference and, like them, to be multiplied by the materialthickness to provide individual resistances. Table 2.8 provides approximate values forthe resistance of films. The individual vapour resistances may be added to provide a totalfor a building structure and, although the total should include for surface and air gapresistances, these are so relatively small that they may be ignored.As to the rate of vapour transfer, by mass, this may be computed for either an elementor a whole structure from:m ˆ p v /Gwherem ˆ rate of vapour transfer per unit area (kg/m 2 s)p v ˆ vapour pressure difference across material or structure (Pa)G ˆ vapour resistance of material or structure (N s/kg)ApplicationThe application of these details in evaluation of a problem is best illustrated by anexample and the curtain wall structure used earlier in Figure 2.2 is repeated here forconvenience as Figure 2.9. All the temperature data given there are retained and expandedonly to include values of percentage saturation, 100% externally and 40% within theroom which were not relevant to the thermal calculation. For the necessary listings ofvapour pressures, reference would be made in practice to the psychrometric tables in theGuide Section C1 or some other source of tabulated data.In exactly the same manner as that used to calculate the individual surface temperaturesusing thermal resistances, (p. 27), the vapour pressure gradient through the structure mayTable 2.8 Vapour resistance of some common filmsMaterialThickness(mm)Polythene 0.05 125Gloss paint ± 8Varnish 0.05 5Aluminium foil ± > 4000Vapour resistance(N s/kg 10 9 )