Insulating exposed walls of a house extension

Exposed walls should be insulated to the Best Practice standards.

The external walls of domestic extensions are usually constructed by one of two methods:

• masonry cavity construction;

• timber-framed construction.

Masonry cavity construction consists of an outer leaf of brickwork, a ‘cavity’ that is fully or partially filled with insulation, an inner leaf of concrete blockwork, and a plasterboard lining (see Figure 7). The thermal performance of this type of construction varies with the thicknesses of the cavity and of the insulation, the type of blockwork used for the inner leaf, and the type of lining board. In order to meet the Best Practice standard it is necessary to:

• make the insulated cavity at least 100mm wide;

• use lightweight ‘thermal’ blockwork for the inner leaf; and

• use a thermal board instead of ordinary plasterboard.

Timber-framed construction usually consists of a structural timber frame with insulation placed between the framing members, lined internally with plasterboard and externally with a sheathing board and a waterproof breather membrane. The timber frame is often clad externally with a skin of brickwork, separated from the frame by a cavity (see Figure 8).

Timber frames are usually either 90mm or 140mm thick. In order to meet the Best Practice standard it is necessary to:

• use at least 140mm thick framing;

• use high-performance insulation within the frame; and

• use a thermal board instead of ordinary plasterboard.