Book - School of Science and Technology

Heat losses 23The values noted in Table 2.1 for external and internal surface resistances relate tonormal building materials, most of which have high emissivity values. An exception mayarise in the case of the resistance offered by an air gap in a cavity construction (R a ) wheretwo sets of values are given, one for the situation where the facing surfaces within the airgap have high emissivity and the other to meet the case where a bright metallic surface,such as aluminium foil, is inserted to provide low emissivity.ConductivitiesValues for the thermal properties of a selection of common building materials were givenin Table 1.1, and others are listed in the Guide Section A3. The following definitions maybe helpful:ConductivityThe standardised value, in watts for one metre thickness per square metre and kelvin:k ˆ Wm/m 2 K ˆ W/m KConductanceThe value for a stated thickness (k/L):C ˆ W/m 2 KResistivityThe reciprocal of the conductivity (1/k)r ˆ m K/WResistanceThe reciprocal of the conductance (L/k):R ˆ m 2 K/WFor example, the thermal conductivity (k) of expanded polystyrene is 0.035 W/m K andthe thermal conductance (C) of a slab of this material, 25 mm thick, is thus 0:035/0:025 ˆ1:4 W/m 2 K. Similarly, the thermal resistivity (r) of the material is 1/0:035 ˆ 28:6 m K/Wand the thermal resistance (R) of the slab is 1/1:4 (or 28:6 0:025) ˆ 0:71 m 2 K/W.Moisture in masonry materialsA review at the Building Research Establishment* has shown that the conductivities of allmasonry materials bear the same general relationship to their dry densities and follow thesame proportional pattern with increasing moisture content. Corrected practical values formoisture content by volume were selected for the Guide Section C3 in consequence, as follows:. brickwork, protected from rain 1%. concrete, protected from rain 3%. both materials, exposed to rain 5%.*Loudon, A. G., `U values for the 1970 Guide', JIHVE, 1968, 36, 167.