Book - School of Science and Technology

22 The building in winter20GlassTemperature, °C100StillairMovingair50 50Distance, mmFigure 2.1 Heat transmission gradient through glassThe effect of these boundary layers is defined in terms of what is described as surfaceresistance, and representative values are as set out in Table 2.1. The notation here is thatgiven in the Guide Section A3 and, as noted later, the value of the outside surfaceresistance (R so ) varies with the degree of exposure such that a sheltered surface has ahigher resistance to heat flow than one which is exposed to severe wind and other effects.In the extreme case of a very tall building, it might well be supposed that wind forces weresuch that the boundary layer is totally dispersed and that the value of the surfaceresistance is zero, i.e. the temperature of the external surface is, effectively, that of theoutside air. The figures listed for internal surface resistances (R si ) vary, it will be noted,only with respect to the disposition of the area concerned and the direction of heat flow.Table 2.1 Surface resistances (`normal' exposure)Surface resistance (m 2 K/W)Building surfaceEmissivity0.95Emissivity0.05Outside (R so )walls 0.06 ±roots 0.04 ±floors 0.04 ±Inside (R si )walls (horizontal) 0.12 ±roofs (to above) 0.10 ±ceilings (to above) 0.10 ±floors (to below) 0.14 ±Unventilated air gap (R a )horizontal 0.18 0.35above 0.17 0.35below 0.22 1.06Ventilated air gap (R a )cavity wall (horizontal) 0.18 0.35behind tiles on hung tile wall 0.12 0.30loft space over flat ceiling 0.14 0.40void under unsealed pitched roof 0.16 0.40void under sealed pitched roof 0.18 0.35void within flat roof 0.14 0.40