Gable walls and chimney breasts

When a loft is converted, exposed gable walls and any attached chimney breasts become the external walls of the new living space. Insulation should be installed to achieve Best Practice.

Gable walls and chimney breasts are usually insulated internally, behind new plasterboard linings. However, if the existing wall is of cavity construction and is not already insulated, this can be achieved by blowing mineral fibre, plastic bead or foam insulation into the cavity. This is a specialist operation that involves drilling holes in the brickwork at approximately one metre spacing (horizontally and vertically), then blowing insulation material into the void. It can be carried out from inside, before any plaster or linings are fixed, so the external appearance of the dwelling is not affected.

For solid walls, there are two methods of insulating internally:

• line the wall with a thermal board;

• fix a metal or timber framing system to the wall, place insulation between the frame members and then line the wall internally with plasterboard.

The easiest method of dry-lining is to fix a proprietary insulating board to the inside of the wall.This consists of a layer of plasterboard bonded to a layer of rigid insulation.This can be fixed to the wall mechanically or, more usually, by plaster dabs.

An alternative method involves fixing galvanised steel laths or timber battens to the wall, placing insulation between them, and then lining with plasterboard.This can achieve a better standard of insulation than proprietary lining boards, but at the expense of room space. Insulation is usually 50mm thick, but up to 100mm of insulation can be included. Note that the insulation is 'bridged' by the framing.This problem can be dealt with by combining the two methods of dry-lining which will also deliver better thermal performance, in line with Best Practice standards.

Whichever dry-lining method is used, it is important to seal the edges and joints in the lining board (and any penetrations for services such as electrical switches and sockets).This is done to prevent warm, moist air getting behind the lining, resulting in hidden condensation on the cold masonry surface. Sealing should be carried out with tape, and/or a skim of wet plaster, before the linings are painted.

Other risks of thermal bridging and internal surface condensation need to be considered.

• Where the walls have chimneys or piers attached, the insulated lining should be taken around them – not just butted up to each side.

• Insulation should also be installed in the reveals and soffits of window openings, and beneath window sill boards.