Loft Insulation: Why It Matters and How Thick It Should Be

Warm air rises, and in a poorly insulated home a significant share of heat escapes through the roof. Loft insulation is one of the most cost-effective ways to keep a home warm, because the material is relatively cheap and the work is straightforward in many lofts.

How much do you need?

The Energy Saving Trust recommends a depth of around 270 mm of mineral wool insulation in a typical loft. Many older homes have much less, and some have none. If you can see the tops of the joists, you probably need more. Where you already have insulation, topping it up to the recommended depth is usually worthwhile.

How to check what you have

Use a torch and look into the loft from the hatch, if it is safe to do so. Do not step between the joists, because the ceiling will not support your weight.
Look at the type of material: rolls of mineral wool, loose-fill, blown fibre or boards.
Estimate the depth in millimetres. Insulation that is level with the top of the joists is typically about 100 mm.
Check for damp, stains or signs of leaks, which should be fixed first.

Types of loft insulation

Mineral wool rolls: the most common choice for DIY. Lay a first layer between the joists and a second layer across them.
Loose-fill: poured between joists, useful for awkward spaces, but it can be disturbed by draughts.
Blown insulation: installed by professionals using a machine, good for hard-to-reach areas.
Rigid boards: useful where you want a boarded floor for storage, because they can support boarding without being squashed.

Things to watch

Ventilation: do not block eaves vents, because the roof space needs airflow to prevent condensation. Use eaves trays where needed.
Cold water tanks and pipes: insulate the tank and pipes themselves, but leave the area underneath the tank uninsulated so that heat from the house keeps it from freezing. Take advice if unsure.
Electrical cables: do not bury old or heavily loaded cables under insulation without advice. Cables that run warm need space.
Recessed lights: use covers rated for use with insulation.
Safety: wear gloves, a mask and long sleeves for mineral wool, and make sure you have good lighting and a safe platform.

Loft storage

If you want to use your loft for storage, raise the boards on a frame so you do not compress the insulation, because compressed insulation works less well.

DIY or professional?

Laying mineral wool in a clear, accessible loft is a reasonable DIY job. A professional is a better choice for awkward lofts, for blown insulation or if you find damp or wiring concerns. Ask for the type, depth and a written quote and check whether any support is available. At the time of writing, large national insulation schemes have changed, so check GOV.UK and your local council for current help.

Once the loft is insulated, look for other weak points, such as walls and draughts.