Insulating ceilings

In most houses, the cheapest and most efficient insulation is that installed in the roof cavity or loft. One way is to fill the gaps between the ceiling joists with lightweight loosefill insulation, which is simply poured in from a bag. It may be made of vermiculite, cork or polystyrene foam granules. Loosefill insulation is not suitable under draughty roofs with open eaves.

The wind blows it away. But in calm air it insulates as well as anything else. A 50mm layer gives a U value of 0.15; a 75mm layer gives a U value of 0.10.
A 76mm layer is an ideal amount, except in very cold or very hot climates. 50mm is reasonable, but less than that is simply not worthwhile. In Britain, the material comes in standard-sized 13kg bags. One bag covers 2.3sq m to a depth of 51mm or 1.6sq m to a depth of 76mm. These measurements take the ceiling joists into account, so when you are measuring the floor area of your loft or roof cavity, measure the total area and do not subtract anything for the area taken up by the joists. When you have filled one space between a pair of joists, level the granules off to the correct depth with a piece of cardboard cut to fit between the joists and slid along them to act as a rake.

This will show you whether you have poured in the right amount, and provide a guide for all the other spaces. Make sure that you cover every part of the area with an even layer of granules. Even quite a small gap will lose a surprising amount of heat. The only gap should be the trapdoor leading into the loft from the room below. In most houses, the joists form a box shape around the trapdoor, so you will not have any trouble with granules falling through.

If not, you can box off an area between the neighbouring joists. If your loft or roof cavity is too draughty for loosetill insulation, the right type to use is fibreglass matting, which comes in rolls in several standard widths. Choose a width slightly wider than the space between the joists, so that it fits tightly without gaps. If you cannot get the width you need, it is easy to cut pieces to shape with a handyman’s knife. Cut the strips too long as well as too wide, and tuck the ends up under the edge of the roof for a better seal. You can glue a small piece of matting to the top of the trapdoor to seal that off as well.

Fibreglass matting comes in several thicknesses from 25mm; U value 0.16 to l00 U value 0.08. There is also a wider version that goes over the top of the joists. It is easier to lay, but you cannot see the joists alter it is laid, so next time you go up there, there is a danger of putting your foot through the ceiling.