Example of U value calculation
The example uses the following formulae.
Resistivity (R) = material thickness (d) Thermal conductivity coefficient (k)
Thermal transmittance (U value) = 1/R
For a cavity wall system, two elements R1 and R2 exist:
R1  The resistivity of the exterior wall plus cavity. This is made up from the following components:
Resistivity of exterior wall surface  Rso Resistivity of external render  Re Resistivity of external blockwork (with mortar joints)  Reb Resistivity of air gap in cavity  Ra
R2  Resistivity of the interior wall. This is made up from the following components:
Resistivity of expanded polystyrene insulation  Rix Resistivity of internal blockwork (with mortar joints)  Rib Resistivity of internal plaster  Rip Resistivity of interior wall surface  Rso
Sum the resistivities for each componant through which heat will pass to find R1 and R2. Add R1 and R2 to find R. Calculate the U value from the inverse of the total resistivity R.
Allowance for the mortar joints around the blockwork should be made by using the resistivity of the mortar. This adjustment is calculated from the relative surface area of the mortar to the area of the blocks within the wall.
Note: This is a simplified method. For a more comprehensive calculation method, see BS EN ISO 6946:2007 Building components and building elements  Thermal resistance and thermal transmittance  Calculation method (ISO 6946:2007).
For an online calculator see http://www.cbablocks.org.uk/tech/tech_uvalue.html
