How do I insulate under a floor?

What can be done from above and below, and why floor insulation is more involved than loft or walls.

Difficulty: advanced
Time: 10 hr
Read: 2 min
Safety: warning

Short answer

For a suspended timber floor, insulation goes between the joists supported on netting — from below if there is a cellar, or from above by lifting boards. Never block the airbricks that ventilate the void.

Around a tenth of a house's heat can go through the ground floor. It is worth doing, but it is disruptive, and it is the insulation job most likely to cause damage if the ventilation is not respected.

Safety

Do not block underfloor ventilation — suspended timber floors need air moving beneath them or the joists will rot. Check for cables and pipes before lifting boards or cutting. Confined underfloor spaces can contain poor air; do not work in one alone. If the floor has old materials you cannot identify, have them checked for asbestos before disturbing anything.

What you'll need

Insulation suitable for floors
Netting or battens
Floorboard lifter
Dust sheets
Torch

Step by step

Identify the floor type.Suspended timber, with a void underneath and airbricks in the outside wall, or solid concrete. The approach is completely different and only one is a realistic DIY job.
For a suspended floor, check access.A cellar or crawl space means working from below, which is far less disruptive. Otherwise the floorboards have to come up.
Never block the airbricks.The void must stay ventilated or the joists rot. This is the single most important rule, and blocking them is a common and expensive mistake.
Lift boards carefully if working from above.A bolster and floorboard lifter. Check for cables and pipes before cutting anything. Number the boards as you lift them.
Fix netting between the joists.Draped between the joists to form a cradle. It supports the insulation and stops it sagging into the void over time.
Lay insulation between the joists.Cut slightly oversize for a friction fit with no gaps. Gaps at the edges undo a large part of the benefit.
Consider a breathable membrane.Over the insulation and under the boards. Ask advice for your specific construction — getting the vapour control wrong causes condensation within the floor.
Seal the gaps between the floorboards as well.Often as significant as the insulation itself in an old house, and far easier. Filler strips or a flexible sealant.

Tips

If the full job is not feasible, sealing the gaps between boards and around skirting removes most of the draught for a fraction of the effort.
A solid concrete floor can only be insulated by building up on top, which raises the floor level and affects doors and skirting. It is a renovation, not a weekend.
This is a good candidate for professional work. The consequences of getting the ventilation or vapour control wrong are rot, which is far more expensive than the job.

Common mistakes

Blocking the underfloor airbricks — The void needs constant ventilation. Without it, moisture builds and the joists rot — a structural repair costing many times the insulation.
Leaving gaps at the joist edges — Air moves through the gaps and around the insulation, which loses much of the benefit for the same amount of work.

Questions people ask

Is floor insulation worth it?

Around a tenth of heat loss in a typical house, so yes in principle. It is disruptive though — loft insulation and draught-proofing are better value first.

Can I insulate a concrete floor?

Only by building up on top with insulation board and a new surface. It raises the floor level, so doors and skirting need adjusting. It is a renovation job.