Roof and loft insulation

  How much could you save?

These are estimates based on insulating a gas-heated, semi-detached home with three bedrooms, showing savings when you insulate an uninsulated loft, and when you top up 100mm of insulation to 270mm. (The recommended depth for mineral wool insulation is 270mm but other materials need different depths). *Average unsubsidised professional installation costs, loft top up assumed to be up to £300 although these may vary. **DIY costs are based on average retailer costs for insulation up to 270mm or more, based on a 44m² loft.

If your loft is already insulated, it's worth checking that you've got enough insulation to get the maximum saving. If everyone in the UK installed 270mm loft insulation, we could save nearly £500 million – and 2.7 million tonnes of carbon dioxide every year, the equivalent of taking nearly 100,000 cars off the road.

  Choosing loft insulation

Easy access and regular joists

If your loft is easy to access and has no damp or condensation problems, it should be easy to insulate - you could even do it yourself.

If access is easy and your joists are regular, you can use rolls of mineral wool insulation. The first layer is laid between the joists - the horizontal beams that make up the 'floor' of the loft – then another layer is cross-laid at right angles to cover the joists and make the insulation up to the required depth. This can be done by a competent DIY-er or a professional installer.

Storage or living space

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:

• insulate between the joists with mineral wool and then lay rigid insulation boards on top, with wooden boarding on top of that.  You can buy insulation board pre-bonded to floor boarding to make the job easier.  Or:
• raise the level of the floor so you can fit enough mineral wool beneath the new floor level. 

Either way, make sure you don't squash the mineral wool when you fit the boards on top - this will reduce its insulation value. 

If you want to use your loft for living space, you can insulate the roof of the loft instead of the floor by fixing rigid insulation boards between the roof rafters. Boards must be carefully cut to the right width so that they fit snugly between the rafters. They can then be covered by plasterboard. Rafters aren't usually very deep, so to get the best performance you may have to insulate over them as well, using insulated plasterboard.  If there isn't room to do this, make sure you use the highest performance insulation board available.

Difficult access

If your loft is hard to access, you can have blown insulation installed by a professional, who will use specialist equipment to blow loose, fire-retardant insulation material made of cellulose fibre or mineral wool into the loft. This doesn't usually take more than a few hours.

Irregular joists

If your loft space is irregular, the joists are the wrong distance apart for rolls of mineral wool, or there are lots of obstructions that make laying matting tricky, you can use loose-fill insulation. This is sold in bags as cork granules, vermiculite, mineral wool or cellulose fibre, which can be poured between the joists to the right depth. This can be done by a competent DIY-er or a professional installer: you may need to increase the height of the joists to get the insulation deep enough.

Flat roofs  

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. In fact, if your flat roof needs to be replaced, you have to 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 done correctly. Either is generally a job for a professional.

Installing flat roof insulation could save you around £190 and 800kg of carbon dioxide a year if half your roof area is flat - the savings will vary depending on how much of the property has a flat roof.

Damp lofts

Insulation stops heat escaping from living spaces, so it will make your loft space cooler - which could make existing damp or condensation problems worse. Get professional advice before installing insulation to see if you can fix the damp problems first.

Installing loft insulation  

Find out about the materials used for loft insulation.

To get a list of possible installers:

• go to the National Insulation Association website
• call your local Energy Saving Trust advice centre free on 0800 512 012.

Pipes, water tank and loft hatch  

Insulating between the joists of your loft will keep your house warmer but make the roof space above colder. Pipes and water tanks will be more likely to freeze, so you will need to insulate them. If your water tanks are some distance from the loft hatch, you will also need something to walk on for safe access.

The cooler air in your insulated loft could mean that cold draughts come through the loft hatch. To prevent this, you can fit an insulated loft hatch and put strips of draught-excluding material around the edges of the frame.