Cold feet in the living room, a draught along the skirting and a gap between the boards you could post a coin through: that is a suspended timber floor doing what it was built to do in 1910. If you are pricing suspended floor insulation in the UK, the good news is that a typical living room and hall can be done for a few hundred pounds in materials if you do it yourself, and a four-figure sum if you hire it in.
Insulating between the joists from above is the cheapest route and usually pays back within about ten years, but only if you keep the underfloor void ventilated. Block the air bricks and you trade a cold floor for damp joists.
What a suspended timber floor is
A suspended floor has timber boards fixed to joists that span across a void, with air bricks in the outside walls letting air move through the gap. It is common in houses built before the 1950s. The alternative is a solid concrete floor, which needs a different approach and costs more to upgrade.
You can tell which you have in two minutes. Look for air bricks near ground level on the outside walls, and tap the floor. A hollow sound, a bounce underfoot or visible boards point to a suspended floor. If you can lift a board or open a hatch, you will see the void and the earth or concrete below it.
How much heat the floor loses
An uninsulated suspended floor lets heat escape downwards and lets cold air rise through gaps in the boards. The share of a typical home’s heat loss that goes through the ground floor is modest compared with the roof and walls. The comfort gain is often bigger than the bill saving, because draughts at ankle height are what you notice most.
Three ways to insulate it, and what each costs
Between the joists from above
You lift the floorboards, fix breathable netting or boards to the sides of the joists, and lay mineral wool or rigid board between them. This is the standard method and the one most owners use.
Disruption: high in the room being done, because furniture and carpets come up for a day or two.
Cost: materials are modest, and professional installation adds labour.
Skill: a competent DIYer can manage it, but lifting old boards without splitting them takes patience. Seal gaps between boards and along the skirting at the same time, using the methods in our guide to draught-proofing your home.
From below, through the void
If you have a crawl space or cellar, an installer can work from underneath and leave your floor untouched. This is the least disruptive method indoors, but access must be good, and working in a cramped void is hard and slow. Specialist firms may spray foam onto the underside of the boards. Take advice first, because some foams and sealed voids can trap moisture in older timbers.
Taking up the floor and replacing it
This is the full job, usually done as part of a wider renovation. You lift the whole floor, insulate properly, add airtightness measures and relay the boards or fit a new deck. It costs the most and gives the best result, and it makes sense when you are already replacing floor coverings or rewiring.
Cost summary: what to budget
For a typical 15 to 20 square metre living room, expect a spread of costs depending on method, access and the insulation you choose. Add the cost of refitting skirting boards, redecorating and any carpet or flooring you need to replace. If a joiner has to repair rot or replace damaged boards, that comes on top of the insulation price.
Ask any installer to quote the whole job, including making good, in writing, so you can compare like with like.
Our view: do it from above, and do it with a draught-proofing plan
If you can lift your own boards, insulate between the joists from above and seal the gaps in the same weekend. A thick layer of insulation under leaky boards still leaves cold air racing across the top of it, so the savings never arrive. Pair the job with proper draught-proofing and your results will be far better than either job alone.
The mistake that causes damp: blocking the ventilation
A suspended floor needs air moving through the void to keep the timbers dry. That is what the air bricks are for. When owners add insulation, extensions, patios or raised flower beds, they often block those air bricks without realising.
Keep every air brick clear of soil, paving, leaves and paint.
Never fill the void completely with insulation that touches the ground.
Leave a gap between the insulation and the ground, and leave the air path open.
Do not cover the underside of the joists with a sealed membrane unless a specialist has designed the whole system.
Rot and woodworm need damp, still air. If the void smells musty or the joists feel soft when you press them with a screwdriver, get a survey before insulating. The cost of repair is far greater than the cost of the insulation.
Which material to choose
Mineral wool is cheap, widely available and easy to fit between joists, but it needs a breathable support so it does not sag. Rigid boards cost more, fit tightly and give a better result per millimetre of thickness. Natural fibres such as sheep’s wool or wood fibre handle moisture well and suit older houses, at a higher price. Our guide to types of insulation compared sets out how they stack up on cost, thermal performance and moisture tolerance.
Building regulations and your EPC
Replacing a floor or upgrading insulation as part of building work can trigger Part L, which sets minimum standards for thermal performance. Simply fitting insulation between joists in a single room is often exempt, but it depends on the scale of the work. Check with your local building control team before you start, and keep any certificate they issue.
Insulating the floor can raise your EPC score, although the effect is modest compared with loft or wall insulation. If you plan to sell, add the receipts and photographs to your records so the upgrade is on record when the next assessor visits. For the wider picture, see our insulation and energy guides.
What to do in the next 30 minutes
Go outside and count the air bricks on each wall, and check that none is blocked.
Lift a board or open a hatch and photograph the void with your phone torch.
Press a screwdriver into a joist end to check for soft timber.
Measure the room and work out the floor area in square metres.
Ask two installers for written quotes, including making good and skirting refits.
Log the upgrade in Home+: open your free Home+ logbook and store the receipts, the product spec and the before-and-after photos, so the work is on record when you sell or reassess your EPC.
FAQ
How much does it cost to insulate a suspended timber floor? It depends on the room size, the method and whether you do it yourself. A small room from above is the cheapest. Get written quotes that include making good.
Is suspended floor insulation worth it? For a draughty older house, usually yes. The bill saving is modest, but the comfort gain is large, and it pays back more quickly if you do the work yourself.
Can I insulate under the floor without lifting the boards? Yes, if you have a crawl space or cellar with good access. Specialist installers work from below. Ask them how they manage moisture in the void.
Do I need to keep the air bricks? Yes. Air bricks ventilate the void and keep the timbers dry. Blocking them risks damp and rot.
Does floor insulation need building regulations approval? It depends on the scale of the work. Minor upgrades in one room are often exempt. Ask your local building control team before you start.
Will insulating the floor improve my EPC? It can, by a small amount. Loft and wall insulation usually make a bigger difference, so treat floor insulation as part of a wider package.
