Book - School of Science and Technology

24 The building in winterFor the inner skin material of a cavity wall and for internal partitions, etc., the protectedvalues are used. In circumstances where materials are exposed to either driving rain orcondensation, the 5% content noted above is no longer valid and it would seem thatconductivities increase by about 4% for each 1% increase in moisture content.U valuesThe rate of heat transmission, in watts per square metre and kelvin (W/m 2 K) is, for thepurpose of heat loss calculation, termed the thermal transmittance coefficient (U). This isthe reciprocal of the sum of all individual resistances, thus:U ˆ 1/(R si ‡ R so ‡ r 1 L 1 ‡ r 2 L 2 ‡ r 3 L 3 ‡ R a )whereR si ˆ inside surface resistance (m 2 K/W)R so ˆ outside surface resistance (m 2 K/W)R a ˆ air space resistance (m 2 K/W)r 1 , r 2 , etc: ˆ resistivities (m K/W)L 1 , L 2 , etc: ˆ thicknesses (m)Pre-calculated values for transmittance coefficients, applicable to a range of typicalforms of construction, are provided in Table 2.2. The values listed are for what is callednormal exposure but the much more comprehensive list of examples given in the GuideSection A3 includes alternative data for other conditions, defined as follows:ShelteredNormalSevereUp to 3rd floor of buildings in city centres.4th to 8th floors of buildings in city centres; up to 5th floor of suburban andrural buildings.9th floor and above in city centres; 6th floor and above in suburban and ruraldistricts; most buildings on coastal or hill sites.In general terms, the values listed in Table 2.2 will be increased by up to 20% for severeexposure but, in the particular case of single glazing, the increase is much greater at about45%.*Table 2.2 Typical transmittance coefficients (U values) for `normal exposure' (including allowance formoisture content, as appropriate)Construction (dimensions in mm) U value (W/m 2 K)External wallsSolid walls, no insulation600 stone, bare 1.72600 stone, 12 plasterboard on battens 1.35Continued on facing page* These increased values are of particular relevance only with respect to heat losses in UK winters. Windsin summer may be assumed to have less influence and values for normal exposure are thus appropriate.