Insulating the roof

Insulating the roof

About a quarter of the heat lost from an uninsulated house goes through the roof, and so some kind of roof insulation should be your first priority for saving money on heating bills.

Houses leak heat like a sieve. Up to 75 per cent of the warmth generated within the house finds its way to the outside world through the roof, walls, windows and doors, and this represents an enormous waste of energy and money. Insulation reduces the rate at which heat passes through the various parts of your house’s structure, by trapping ‘still’ air within the insulating material itself.

Still air doesn’t conduct heat much and so wrapping your house in suitable insulation serves the same purpose as putting a tea-cosy on a tea pot: the tea stays hot for far longer. The converse is true during the summer: the insulation bars the heat of the sun. About a quarter of all the heat lost from an average house goes through the roof, and this is a particularly easy area to insulate effectively. What you do is to put your insulation in one of three places: on the highest ceiling; on the loft floor; or on the inside of the roof itself. Insulating the loft floor is the simplest and most effective of these: you should insulate the ceiling where there’s no loft or the roof slope where the loft space has been converted into living accommodation.

Why you should insulate

The savings you can make by insulating your loft depend on whether there’s any insulation there already (it’s the first layer that’s most effective), how much you put in, and whether you can control or alter your heating system to take advantage of the insulation. The last point is particularly important: if you install loft insulation and leave central heating controls as they are with a thermostat in, say, the living room, the most noticeable effect will be warmer rooms upstairs rather than dramatically decreased fuel bills. If, however, you can lower the tempertures in upstairs rooms, or keep them the same as before, by fitting thermostatic radiator valves or by turning radiators down (or off), your house will lose less heat and your fuel bills will be lower. Types of insulation There are four main types of loft insulation you can use: blanket, loose-fill, sheet and blown fibre.

You can install any of the first three yourself but blown fibre must be installed by a roofing contractor. The most extensively used loft insulation material is rolls of glass fibre or mineral fibre blanket, which you lay between the joists of the loft floor. You can choose from either of two thicknesses: 80mm (just over 3 in) and 100mm (4in).

Although you’ll benefit in terms of warmer rooms by installing thicker insulation, the more you put in, the less cost effective it becomes. From a practical point of view thicknesses greater than 125mm (5in) will probably take the insulation over the top of the joists, making walking about or storing things in the loft rather difficult. To save storage and transportation space the material is compressed when rolled up and packaged but it regains its original thickness quite quickly when unwrapped. The most common width of roll is 400mm (16in). Lengths vary from brand to brand, but they’re usually about 6 to 8m (20 to 25ft) long. Some glass fibre insulation is available in 600mm (2ft) wide rolls for use in lofts with wider-than-usual joist spacings.

The 400mm (16in) width is the most suitable size for most houses and allows a little to turn up where it meets the joists. Joists are usually 400mm (16in) apart but they might be as much as 450mm (18in) or as little as 300mm (1ft). If you have narrow-spaced joists the 600mm (2ft) width is probably the best to use: you can cut a roll in half with a panel saw while it’s still in its wrapper. Working out how much insulation you’ll need is simply a matter of multiplying the length of the loft floor by the width to calculate how many square metres there are.

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