Best Methods Of Installing Loft Insulation

You can save up to 17 pence in every £1 spent on home heating by having effective insulation in the loft, which means having 100mm thick insulation. Usually the insulation will be placed directly between the joists over the bedroom ceilings. Where the loft space is used, as a workroom or darkroom, say, it will be necessary to fix the insulation to the underside of the roof rafters.

Fitting insulating matting

Glass fibre or mineral wool matting are the best materials to use in unlined lofts because they are not blown about by draughts in the same way that loose granules can be.

Start by cleaning the loft, at the same time checking the roof timbers for dampness and woodworm damage which should be remedied before the insulation material is laid. Where the house has plasterboard ceilings it is a simple, if somewhat dirty, job to sweep between the joists with a dust pan and brush. It is better to use a powerful industrial vacuum cleaner for this job, and this is the only way to clean a loft where the ceilings are the old lath and plaster type. However, great care must be taken here not to knock off the plaster nibs which hook over the laths and hold up the ceiling.

To keep the loft reasonably clean, and to prevent snow from being blown in and settling on the insulation, it is a good idea to pin building paper to the rafters.

Avoid putting a foot through the ceiling by working from boards laid on the joists.

The insulating matting is simply unrolled between the joists, starting at the eaves and finishing in the middle of the loft area. The end of each strip of matting should rest over the outer walls of the house. With an unlined roof (where you can see the under sides of the tiles or slates) the matting can be tucked into the eaves as far as you can reach in order to block up some of the draughts, but where the roof is lined with felt or boards, it is best to pull back the matting to leave the soffit boards in the eaves uncovered. Then from the outside, a series of 18mm holes can be drilled through the soffits to ensure adequate ventilation in the loft space.

The matting can lie flat between the joists or, where it is a little wider than the joist spacings, tucked in between them. The matting is easily cut with a trimming knife, sharp breadsaw, or scissors. The ends between one strip and another are simply closely butted together.

Remember to fix insulation above the loft hatch, but do not take any under the cold water storage cistern as it will stop warm air from following under the cistern and preventing freezing.

Installing loose-fill materials

Loose-fill materials are quick and easy to lay, especially in awkward corners, around obstructions, and where the joist spacings are uneven. However, they can be blown about if the roof is very draughty, so it can be a good idea to line the loft with building paper when loose-fill insulation is used, paying particular attention to seal the eaves.

A big advantage of loose-fill insulation lies in the fact that it can be poured, making it ideal for insulating attic rooms where the sloping ceilings are fixed direct to the rafters. As long as there is access to the roof space above the attic room, the granules can

simply be poured into the voids between the rafters. But first the lower outlets from the voids should be sealed with wads of insulating matting.

In a loft, measure the depth of the ceiling joists before installing loose-fill. The joists should be at least 100mm deep so they are not completely buried by the insulation. If they are covered it will be virtually impossible to walk about in the loft in the future. In the case of shallow joists, either use insulating matting, or nail timber blocks to the joist tops to which permanent timber walkways can be fixed.

Loose-fill materials are difficult to clear away to give access to pipes and cables, so make a careful check of these before installing the insulation, and if necessary carry out any plumbing or electrical work beforehand. Also check over the roof timbers and clean the loft area as described for insulating matting.

Before installing loose-fill, drape building paper over any water pipes which run between the joists so that the loose-fill insulation does not run under them and insulate them from heat rising from below. Then simply pour out the loose-fill between the joists and rake it level. The ideal tool for this is a 500mm length of wood, notched at each end to rest over a pair of joists so that when the wood is drawn along the insulation is levelled to the required depth. Gaps at the eaves of an unlined loft should be sealed with lengths of building paper.

Insulate a loft hatch with glass fibre matting, or use loose-fill by nailing a 10cm deep box-like frame to the top of the hatch, pour in the loose-fill granules, and then keep them in place with a lid, such as a piece of hardboard. Another method of insulating the hatch is to put the loose-fill, . material in a polythene bag and attach this to the top of the hatch. It is always easier, though, to use insulating matting for this job.

Lining under rafters

There are basically two types of under-rafter linings. The most common is a simple lining used as an adjunct to loft floor insulation to keep the loft-space clean, draught-free and weatherproof. The second type of lining involves fixing effective insulation under the rafters so that the entire loft space becomes a useful heated area, and water pipes and tanks within the

area do not need to be individually protected from frost. However, this is a more costly and time-consuming way to insulate than laying insulation directly on the loft floor.

Lining with building paper

Use ordinary building paper, or foil-backed building paper. Both will keep out draughts, but the foil-backed type gives additional insulation of radiated heat which keeps the loft space warmer in winter and cooler in summer.

The paper should be fixed to the undersides of the rafters in horizontal strips, working from the apex of the roof and finishing at the eaves. A staple tacker is the ideal tool for fixing the paper, using a small square of scrap card to reinforce each fixing. Overlap each strip by about 50mm so that any in-blown rain or snow will be channelled down to the eaves. Fix foil-backed paper facing inwards.

Lining with roofing felt

Roofing felt is a more durable material for lining a loft to keep it draught-free. Fix it in horizontal strips as building paper, or cut the felt into strips about 200mm wider than the rafter spacing and fix the strips between the rafters running from the apex of the roof down to the eaves. Use thin timber battens and 25mm galvanised nails to fix the felt to the sides of the rafters.

Insulating lining

For entire loft insulation it is necessary to fit insulation between the rafters and then cover this with boards to give a smooth finish and perhaps add a further degree of insulation.

For insulating between the rafters, insulating matting is ideal and this can be pushed into place and temporarily held with lengths of springy bamboo canes wedged between the rafters until the surfacing boards are fitted. Alternatively, use slabs of thick expanded polystyrene cut to fit between the rafters. These can be temporarily held with tacks until the surface covering is fixed.


For surfacing there is a wide choice of wallboards. Thermal board and fibre insulating boards are insulating in their own right, or you can rely solely on the inter-rafter insulation and use ordinary lining boards, such as vapour-check plasterboard, tempered hardboard, plywood, and so on.