How to buy loft insulation

Roof insulation

  • Find out if you should be insulating your roof or your loft
  • How to insulate a flat roof and a pitched roof and what options are available for both
  • Warm loft insulation and cold loft insulation explained - and which one to choose

Roof and loft insulation

The options available to you for insulating your roof will depend on the type of roof you have.

  • Pitched roofs - 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 loft-insulation solution is the classic 'cold loft' option. This is often referred to as loft insulation

Loft insulation involves insulating between and over the wooden joists immediately above the ceiling of your home's top floor. 

It is called 'cold loft' as it will result in your loft feeling rather cold in the winter, as the roof itself might not be insulated. But your living space underneath will be insulated.

This is generally the only grant-funded option and is also fairly easy to do as a DIY project. You can buy loft insulation at all good high street DIY stores.

Roof insulation

Insulate your roof especially if you can't insulate your loft

Warm loft insulation for pitched roofs

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

For a warm roof, you will need to install insulation directly under the roof in the plane of the pitch. This will also result in your roof space not becoming excessively hot in summer or cold in winter, and will reduce heat loss. It's not as easy as laying rolls of insulation, but it can still be done as a DIY job. Otherwise, professional help is at hand. Go to Which? Local to find an insulation installer recommended by Which? members.

One important point to remember with a warm-roof solution is the need for ventilation immediately below the roof tiles. This is to prevent any condensation build up or water getting in through or around the tiles, otherwise the roof structure may begin to rot.

Types of insulation for warm loft

Spray foam roof insulation

Spray foam insulation can help strengthen your roof (image: courtesy of Cdpweb 161)

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, which are sometimes supplied foil-covered, can be fitted. These usually have to be cut to size. Another option is EPS (expanded polystyrene) 'squeeze' products; these are manufactured with a 'concertina' shape and spring-like effect and can be pushed into place.

There is an exception to the ventilation rule, and that's when applying polyurethane spray foams. These solutions, mainly professionally installed, but also now available as a DIY measure, can generally only be applied where the underside of the tiles is bare and there is no roofing felt. 

Spray foam may also provide additional physical strength where a roof is not in the best condition, by holding the structure together. However, missing or slipped tiles must be fixed before applying the foam insulation. 

Flat roof 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. 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.

There are three types of insulation for flat roofs and a professional should help you to decide which one is best for you. Find a recommended installer on Which? Local.

  • Warm deck or warm roof refers to a situation where the 'deck' of the roof, which is usually made of wood, is below the insulation.
  • Cold deck or cold roof is where the insulation is below the roof deck (and the associated joists). Typically a gap will have to be left for ventilation; because this is a 'cold' area, condensation may form which can lead to rot. In both these cases, the weather membrane, typically formed of roofing felt and bitumen, will be the topmost layer, protecting against rain.
  • Inverted roof is where the insulation goes above the weather membrane, effectively protecting it from heat and cold which 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.

More on this...