The overall rate of heat transmission through and external wall ('U1 valve) results from the combined values of internal surface resistance, type and thickness of the walling materials.
The contribution of brickwork to insulation depends upon the density of the bricks and upon thier moisture content.
It is now generally accepted that for heat loss calculation purpose standard moisture contents should be assumed. Table gives the values
Standard moisture content for producted and exposed brickwork
Situation Moisture content % by volume
Internal walls Inner leaf of cavity walls having
continuous air space Wall protected by tile 1.0
hanging or equivalent cladding
Walls exposed to rain, unrendered 5.0
or rendered with untreated mortar
To obtain the conductivity value ('K') for brickwork at varying moisture contents laboratory test result may be converted, using the values in Table (2) See Example
The absorption value of brickwork will not very significantly for most normal types of brick. Painting the surface reduces the absorption value. Plastering may increase or decrease the absorption depending upon the type used. Most common plasters with fairly hard and smooth finish will have a lower value than exposed brick. Table shows typical absorption values at three frequencies.
Absorption can be increased to some extent by filling some joints with a highly absorbent material or by having gaps in facing brickwork with an absorbent material behind.