Calculating a Uvalue

'U' Value definition?

A 'U' value is the measure of heat flowing through a structure  W/sqm.K

Examples of materials used in a house built in 1920;

Single glazing = 5.6   Solid 9” thick wall 2.0

The Heat Loss through 10sqm of a 9” wall assuming

inside temperature of 21C and outside of 1C

 =  Area X Uvalue X Temperature diff =  10 X 2 X 20= 400W

Compare this with a better insulated wall.

Diagram showing the heatloss in a building

Each material has a resistance to heat flow and the overall U value = 1divided by the sum of resistances


Watch our Uvalue calculation video


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Calculation of resistances for an insulated wall

Item Thickness (m) Thermal Conductivity (W/mK) Thermal Resistance (m2.k/w)
Iside surface e.g. wall paper / paint - - 0.12
Plaster 0.015 0.170 0.09
Block 0.100 0.230 0.43
Insulation 0.050 0.035 1.43
Brick 0.100 0.860 0.12
Outside surface - - 0.06
Total - - 2.25

U Value = 1/2.25 = 0.44 W/m2K

Heat loss for 10sqm of wall = 10 x 0.44 x 20 = 88w

i.e. An insulated wall is 5 times more efficient as a 9 inch wall (Buy insulation products)

NB: Thermal conductivity factors can be obtained from the product manufacturers

To test a U Value, watch our video on testing a U value