How do I insulate a loft?

The most cost-effective insulation job in most houses — what depth to aim for, where not to insulate, and how to do it without wrecking the ceiling.

Difficulty: intermediate
Time: 5 hr
Read: 2 min
Safety: warning

Short answer

Aim for around 270mm total depth, laid in two layers with the second at right angles to the first. Never insulate under the cold water tank, and never block the ventilation gaps at the eaves.

A quarter of a house's heat can go through the roof, and loft insulation is unusually cheap for what it saves. It is also one of the few jobs where doing it badly causes new problems — condensation, damp and, with downlights, fire risk.

Safety

Only stand on joists or boards, never on the ceiling between them — you will go through it. Wear a mask rated for particles, goggles, gloves and long sleeves; mineral wool irritates skin, eyes and lungs. Never cover the eaves ventilation gaps, and never insulate over or under recessed downlights unless they are rated for it, which is a fire risk. If the loft has old, unidentified insulation, do not disturb it until you know it is not asbestos.

What you'll need

Mineral wool or equivalent insulation
Particle mask, goggles, gloves
Boards to kneel on
Sharp knife
Head torch
Loft ladder

Step by step

Check what is already there and how deep.Measure it. Many houses have 100mm from decades ago; the current recommendation is around 270mm. Topping up is cheaper and easier than starting fresh.
Check for asbestos before disturbing old material.Older loose-fill insulation and some old boards can contain asbestos. If you do not know what it is, do not disturb it — get it identified.
Set out boards to kneel and stand on.Two boards across the joists that you move as you go. Standing between joists puts you through the ceiling — this is the most common loft accident.
Deal with the electrics first.Lift cables above the insulation where you can rather than burying them. Cables covered by insulation can overheat. Recessed downlights need proper fire-rated covers or a clear gap.
Lay the first layer between the joists.Cut to fit snugly without compressing it — compressed insulation loses much of its value. Push it gently to the edges without blocking the eaves.
Lay the second layer across the joists at right angles.This covers the joists themselves, which conduct heat straight through. It is what takes you from adequate to good.
Leave the eaves ventilation clear.There must be an air gap at the edges where the roof meets the walls. Blocking it causes condensation and eventually rot. Use eaves vents if the insulation reaches that far.
Do not insulate under the cold water tank.It needs heat from below to stop it freezing. Insulate the sides and lid of the tank instead, and insulate any pipes above the insulation line.

Tips

The loft hatch itself is a significant hole. Insulate its back and fit draught seals around it — a small job with a noticeable effect.
If you want to board part of the loft for storage, you need raised boarding above the insulation. Boarding directly onto the joists compresses it and wastes the job.
Check for grants. Loft insulation is frequently subsidised, and sometimes free depending on circumstances.

Common mistakes

Compressing the insulation to fit — Its value comes from the trapped air. Squashed into a gap it can lose half its effectiveness.
Blocking the eaves — The roof void needs ventilation. Without it, moisture condenses on the timbers and causes rot.

Questions people ask

How thick should loft insulation be?

Around 270mm of mineral wool is the usual recommendation. Two layers, the second laid at right angles across the joists.

Can I insulate under the water tank?

No. The tank needs some heat from the room below to avoid freezing. Insulate the tank itself instead, and lag the pipes.