Roof and loft insulation

If you plan to use the loft or attic for storage, you will want to lay boards over the joists. Unfortunately, if you only insulate between the joists before doing this, the insulation won’t be thick enough.

To get enough insulation, you can raise the level of the floor so you can fit enough mineral wool beneath the new floor level. You can do this by fitting timber battens across the joists, or you can buy purpose-built plastic legs that fit on the joists and support the new floor. It’s important to leave a ventilated air gap between the insulation and the boards to prevent condensation on the underside of the boards.

Make sure you don’t squash the mineral wool when you fit the boards on top, as this will reduce its insulation value.

Insulation stops heat escaping from living spaces, so it will make your loft space cooler, which could introduce or worsen existing damp or condensation problems. If you are installing loft insulation yourself, please keep in mind that you may need to increase ventilation.

Get professional advice before installing insulation to see if you can fix any damp problems first.

An alternative way to insulate your loft is to fit the insulation between and over the rafters – these are the sloping timbers that make up the roof itself. You can use rigid insulation boards, carefully cut to size, or you can have foam insulation sprayed between the rafters.

Whichever approach you use, you will need a specialist professional to insulate your roof – this is not a DIY job.

Advantages:

• The roof space will be warmer than with standard loft insulation, so you won’t need to worry so much about freezing tanks and pipes.
• You can board the floor for storage without having to raise it to create extra depth.

Disadvantages:

• Insulating at rafter level is considerably more expensive than most standard loft insulation.
• As well as insulating the roof, you will have to insulate any gable walls, party walls and chimneys in the loft space. If you leave these uninsulated, then the heat will bypass your new insulation making it ineffective.

Some companies may offer to fix your damaged or leaking roof by spraying foam insulation directly onto the underside of the roof without first fixing the problem. We do not recommend that you do this. As with any type of insulation, you need to make sure that the roof is dry and in good condition before any insulation is added.

If you want to use your loft as a heated room, then you need to take a slightly different approach, and create a proper room in the roof.

If you want to use your loft as a living space, or it is already being used as a living space, then you need to make sure that all the walls and ceilings between a heated room and an unheated space are insulated.

• Sloping ceilings can be insulated in the same way as for a warm roof, but with a layer of plasterboard on the inside of the insulation.
• Vertical walls can be insulated in the same way.
• Flat ceilings can be insulated like a standard loft.

Make sure you insulate all the areas of wall and ceiling round any dormer window, and that you use high performance glazing for the window, or for any skylights.

As with warm loft insulation, this is not a DIY job. You will need a professional installer to ensure that the insulation is appropriate and complete, and that adequate ventilation is provided where needed.

Air needs to flow in and out of your house so it stays fresh, dry and healthy. A good installer will be sure not to block or seal any intentional ventilation. If you’re doing any DIY insulation, be careful that you’re not covering any vents, grilles or airbricks.

In Northern Ireland, building regulations require adequate ventilation when installing loft insulation. This usually means installing more vents if there are not enough. Check with your local building control office to find out what you need to do for your home.

If your loft is hard to access, you can have blown insulation installed by a professional, who will use specialist equipment to blow appropriate insulation material into any awkward space. They may use mineral wool fibre, treated cellulose or polyurethane foam.

A flat roof should preferably be insulated from above. A layer of rigid insulation board can be added either on top of the roof’s weatherproof layer or directly on top of the timber roof surface, with a new weatherproof layer on top of the insulation. This is best done when the roof covering needs replacing anyway. If your flat roof needs to be replaced anyway, you must now insulate it to comply with building regulations.

It is possible to insulate a flat roof from underneath, but this can lead to condensation problems if not completed correctly.

Installing flat roof insulation could save you similar amounts on your heating bills to loft insulation. The savings will vary depending on how much of the property has a flat roof.