We use cookies to allow us and selected partners to improve your experience and our advertising. By continuing to browse you consent to our use of cookies. You can understand more and change your cookies preferences here.

Home & garden.

Updated: 1 Jul 2021

Roof insulation

All you need to know about insulating your roof or loft. We tell you the difference between insulating a pitched roof and a flat roof, and whether you should install cold or warm loft insulation.
Which?Editorial team
Roofinsulation1 450376

We'll help you work out what type of roof or loft insulation to go for, how much it will cost, if you can get it for free, and how much you could save by insulating your roof.

Do you have a flat or pitched roof? 

The type of insulation you should choose will depend on the type of roof you have.

  • Pitched roof – there are two options: warm or cold loft insulation.
  • Flat roof – there are three options available: warm deck, cold deck or inverted roof.

Cold loft insulation for pitched roofs

The simplest and cheapest roof insulation is the classic cold loft option. Insulation is placed over and between the wooden joists, above the ceiling of your home's top floor. This is often just called loft insulation.

It will stop the heat escaping from the living area of your home, but leave the loft space uninsulated. This means it will remain cold in winter and warm up in summer. 

It's fairly easy to do as a DIY project, and you can buy loft insulation from most high street DIY stores.

Cold loft insulation is the only type of roof insulation you can apply to have the costs covered by a grant. Click to find out more about insulation grants. 

Warm loft insulation for pitched roofs

If you can't install cold loft insulation because, for example, you're using your loft as a living space, you will need to consider warm loft insulation.

For a warm loft, you install insulation directly under the roof in the plane of the roof's pitch (slope). Insulation will reduce heat loss and your loft space won't become excessively hot in summer or cold in winter. 

It's not as easy as laying rolls of insulation, but it can still be done as a DIY job if you'd rather not pay an installer. 

One important point to remember with a warm loft solution is the need for ventilation immediately below the roof tiles. This prevents condensation build up or water getting in through or around the tiles, which could cause the roof structure to rot.

Types of warm loft insulation 

The materials used to insulate under the roof include batts of mineral or glass-wool insulation, held in place by battens of wood attached to and across the rafters. Alternatively, polystyrene slabs (that are sometimes foil-covered) can be fitted. These usually have to be cut to size.

Another option is EPS (expanded polystyrene) squeeze products. These are manufactured in a concertina shape that springs open when they are pushed into place.

Spray foams are mainly professionally installed, but also available as a DIY measure. Spray foam can alternatively be used to provide additional physical strength where a roof is not in the best condition – as it helps hold the structure together. Missing or slipped tiles must be fixed before applying the foam insulation. Find out how much spray foam insulation costs and if it's suitable for your home.

Flat roof insulation

There are three types of insulation for flat roofs and a professional should help you to decide which one is best for you.

  • Warm deck or warm roof has insulation above the roof deck (the panel underneath the roofing material that's usually made of wood). It's recommended in damp and cold areas, such as the UK.
  • Cold deck or cold roof has insulation below the roof deck and the associated joists. Typically, you leave a gap for ventilation, as condensation may form that can lead to rot. In both warm deck and cold deck options, the weather membrane (typically formed of roofing felt and bitumen) will be the topmost layer, protecting against rain.
  • Inverted roof has insulation that goes above the weather membrane, effectively protecting it from heat and cold that can shorten its life and that of the roof deck. It can even protect against wear and tear if there is access to the roof. With an inverted roof, the top-most layer is generally gravel or a similar material.

A good time to insulate a roof is when it's being replaced, although in many cases a roof that is in good condition can be retro-fitted with insulation.

You'll almost always need to use a professional installer to fit your flat roof insulation, and there are no energy grants available to help with the cost. You can use Which? Trusted trader to help you find a recommended, local insulation installer.

You can also use our Trusted Traders search tool below.