Dormers for the loft conversion

The walls and roofs of dormers should be insulated to Best Practice standards (i.e. to achieve maximum U-values of 0.25W/m2K for walls and 0.13W/m2K for roofs) – see Table 1. Since dormers are almost always of timber-framed construction, this will involve a combination of flexible insulation quilt within the frame and thermal lining board on the inside.

• Where felt is used, under tiles or as the finished surface, an internal vapour barrier is critical. A high interstitial condensation risk (even with a vapour barrier) means that lead coverings should only be used for small areas.

• If ordinary plasterboard linings are used (rather than thermal board), polythene vapour barriers should be included on the warmer side of the insulation, behind the linings, in order to reduce the risk of interstitial condensation.

• All edges, joints and breaks in the vapour barriers (e.g. at electrical switches and sockets) should be sealed with tape.