Building Construction Handbook, Eighth Edition

Domestic buildings (England and Wales) ~

The area weighted average U-value for an element of construction

depends on the individual U-values of all components and the area

they occupy within that element. E.g. The part of a wall with a meter

cupboard built in will have less resistance to thermal transmittance

than the rest of the wall (max. U-value at cupboard, 0.45).

Note: Maximum area of windows, doors, rooflights and roof

windows, are not specifically defined.

An alternative to the area weighted average U-value for

windows, etc., may be a window energy rating of not less

than minus 30 (see page 480).

Energy source ~ gas or oil fired central heating boiler with a

minimum SEDBUK efficiency rating of 86% (band rating A or B, A

only from Oct. 2010). There are transitional and exceptional

circumstances that permit lower band rated boilers. Where this

occurs, the construction of the building envelope should

compensate with very low U-values.

SEDBUK = Seasonal Efficiency of a Domestic Boiler in the United

Kingdom. SEDBUK values are defined in the Government's Standard

Assessment Procedure for Energy Rating of Dwellings. There is also

a SEDBUK website, http://www.sedbuk.com.

Note: SEDBUK band A = > 90% efficiency

band B = 86†90% ..

band C = 82†86% ..

band D = 78†82% ..

Element of

construction

Area

weighted ave. U-value (W/m^2 K)

Limiting individual

component U-value

Roof 0.25 0.35

Wall 0.35 0.70

Floor 0.25 0.70

Windows, doors, rooflights

and roof windows

2.20 3.30

Element of construction U-value targets (W/m^2 K)

Pitched roof (insulation between rafters) 0.15

Pitched roof (insulation between joists) 0.15

Flat roof 0.15

Wall 0.28

Floor 0.20

Windows, doors, rooflights

and roof windows

1.80 (area weighted ave.)

Thermal Insulation, Elements of Construction