Roof Insulation

DIY Roof Insulation Tips

In many cases roofs are the easiest part of the house to insulate, and it is even possible to get a government grant to put in roof insulation. Before you do anything else, go to the local council office and ask about insulation grants; you will not be eligible for one if there is already some form of insulation in the roof.

Pitched roofs


Starting with pitched roofs, there are two main types, those where the slates or tiles are laid on roofing felt and those where they are not. By going up into the attic you can see at a glance what type you are dealing with: if there is no felt you can see the backs of the tiles and usually daylight as well.

It is easy to insulate a roof of this type, as there is no problem with ventilation of the roof space. If you intend to use a material like glass fibre which comes in rolls, you can cut the roll with a normal wood saw into widths that suit the spacing of the joists. Unroll the material and push it lightly into place between the ceiling joists. You will need to stand on a plank placed across the joists to save having to worry about balancing. Do not compress the material or you will reduce its thickness and therefore its insulating value. If the insulation is wider than the joist spacings you can turn up the edges to make it fit.

If you are handling glass fibre or mineral wool it is as well to wear gloves, a boiler suit and wellington boots to keep the itchy fibres from settling into your clothes and on your skin, and if possible a dust mask over your mouth and nose to keep fibres out of your lungs.

The insulation need not be laid right up to the eaves as long as it covers the top of the walls. The thickness is up to you: in the UK, the official recommendation is 100mm in houses, and 150mm is not excessive. But do some calculations before you make your decision. If you want to use the attic for storage the thickness will have to be the same as the depth of the ceiling joists so that you can lay boards across the joists to provide a platform for things to stand on. Many materials are suitable for loft insulation, the main deciding factor being cost and availability. However, if your roof has no felt under the tiles, it is inadvisable to use a loose material like vermiculite or expanded polystyrene granules. On a windy day the draughts that come under the tiles will blow the stuff into drifts and you will find it deep in one place and non-existent in others.


If there is roofing felt under the slates the insulation of the roof is a bit more difficult because the loft must be ventilated to prevent the dreaded dry rot, which thrives in damp air-less conditions. Under a felted pitched roof there may be no ventilation and moisture vapour will rise up into the roof from the rooms below; this creates the conditions in which dry rot spores, which are always present in the air, take root and grow. For this reason most felted roofs have small holes or a gap at the eaves to let in enough air for ventilation. The holes are often in the soffit board which covers the underside of the ends of the rafters where they stick out of the wall to form the eaves. The holes may also be in the fascia board to which the rainwater gutter is fixed.

If you have a felted roof and cannot see any holes, go up into the loft on a windy day and see if you can feel any draughts up there. Providing you feel a draught the roof is likely to be well enough ventilated. If there are no holes, or if you relay the tiles or slates of an old roof on felt to provide better rain resistance, you will have to make ventilation holes about 25mm diameter every 600mm along the eaves. It is a good idea to fix pieces of expanded metal mesh over the holes to keep out birds and mice, which will otherwise make their nests in your insulation. An alternative, if the house is detached, is to put a 225mm x 225mm air brick into each gable end to provide the necessary cross ventilation.

When laying the insulation under a felted roof it is essential that the ventilation holes are not blocked up by the insulation. If the ventilation comes through the eaves make sure that the insulation goes as far as the top of the walls but is not pushed into the eaves where it may stop the flow of air. If there are cold water tanks in the roof — and this applies to any roof space, not just to those under roofs with felt — the insulation must not be put under them: this would prevent heat from the house from reaching them and they could freeze. The insulation should be carried over the top of the tank. The easiest way to achieve this is to build a box of 100mm thick expanded polystyrene held together with 150mm nails pushed into the edges. The top of the box can be lifted off to give access to the tank.

Insulated ceilings

In some houses the upper rooms are built into the roof space and have sloping ceilings, so there is no loft in which to put the insulation. Here, you will probably have to replace the ceiling by one which provides a vapour barrier, such as foil-backed plasterboard. Alternatively, you can remove the roof covering to put in the insulation, and paint the existing ceiling with three coats of gloss paint (covered with emulsion if you do not like the gloss) to provide a vapour barrier.

The problem is that if there is no felt under the tiles, there is every chance that rain and snow will blow under them; so an insulating material must be used which will not be damaged by moisture. The best materials are expanded or extruded polystyrene or the resin bonded glass fibre used for filling cavity walls during construction. Once you have taken down the ceiling, you can cut the insulation to fit between the rafters and push it into place. It is important to leave no cavities between the back of the plasterboard and the insulation as these could provide a place for dry rot to grow. The use of a vapour barrier in the ceiling will reduce the chance of damp occurring in the roof structure.

If the tiles are laid on felt the insulation need not be waterproof, but it is then essential to fill the whole space between the ceiling and the felt with insulation to prevent the creation of any pockets of stagnant air, and again a vapour barrier on the ceiling is a must. In all cases, as a safeguard against rot, you should cover the roof timbers with three liberal coats of timber preservative once the ceilings or roof coverings have been removed.

Flat roofs

Internal insulation

Hat roofs present a problem because they cannot be entered as can pitched roofs. The only easy way to insulate them — assuming they are made of timber — is to remove the ceiling, fill the space between the underside of the roof and the ceiling with insulation, again ensuring there are no gaps, and then put up a new ceiling of foil-backed plasterboard to form the essential vapour barrier. The problem with flat roofs is that the waterproof surface acts as a further vapour barrier and any moisture which penetrates the roof cannot escape; so it is again a good idea to give all timbers three thorough coats of wood preservative while you have the ceiling down.

External insulation

If you do not want to remove the ceiling it is possible to lay slabs of extruded polystyrene on top of the flat roof, assuming that the waterproofing layer is in good condition. The roof waterproofing then acts as a vapour barrier and is protected against the damaging effect of the hot sun by the insulation. The problem with this method is that the insulation must be weighed down either with paving slabs or with 50mm of gravel to prevent it being blown away. This will add a load of about 100kg/m2 to the roof joists; most joists will not be strong enough to carry the extra load without contravening the structural requirements of the building regulations. If you plan to insulate a roof this way you should first consult the local building inspector. The manufacturers of extruded polystyrene recommend that this ‘upside down’ roof construction be carried out only by ‘reputable roofing contractors who are members of roofing trade associations’ as the processes involved are not of a do-it-yourself nature.

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