Building Construction Handbook, Eighth Edition

Thermal resistances (R) are a combination of the different

structural, surface and air space components which make up an

element of construction. Typically:

U=R^1

so+R 1 +R 2 +Ra+R 3 +R 4 etc...+Rsi(m^2 K=W)

Where: Rso= Outside or external surface resistance.

R 1 ,R 2 , etc. = Thermal resistance of structural components.

Ra= Air space resistance, eg. wall cavity.

Rsi= Internal surface resistance.

The thermal resistance of a structural component (R 1 ,R 2 , etc.) is

calculated by dividing its thickness (L) by its thermal conductivity

(), i.e.

R(m^2 K=W) =

L(m)

(W=mK)

eg. 1. A 102 mm brick with a conductivity of 0„84 W/mK has a

thermal resistance (R) of: 0„102...0„84 = 0„121 m^2 K/W.

eg. 2.

Note: the effect of mortar joints in the brickwork can be ignored,

as both components have similar density and insulative properties.

U=R^1

so+R 1 +R 2 +Rsi

R 1 = 0„215...0„84 = 0„256

R 2 = 0„013...0„50 = 0„026

U= 0 : 055 1

+^0 :^256 +^0 :^026 +^0 :^123

=2:17W=m^2 K

Thermal Insulation, U-Value Calculations---2