If your home was built before about 1950, the chances are it sits on a suspended timber floor: floorboards laid across joists, with a ventilated void of cold air underneath. That void is deliberate, because it stops the timber rotting, but it also means your floorboards are exposed to outside air on their underside. On a windy day you can feel it as a steady chill rising through the boards and around the skirting. Insulating that floor is one of the better-value jobs in an older house, and in many homes you can do most of it yourself for the price of the materials alone.
The short answer. Insulating a suspended timber floor typically saves around £75 a year in an average house and up to about £110 in a detached home (Energy Saving Trust). If you have access from a cellar or crawl space the job can cost as little as £500 to £1,500, and a confident DIYer can do it for the cost of materials. Mineral wool or wood-fibre batts held up between the joists are the usual approach.
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How a suspended timber floor loses heat
A suspended floor is a deck of timber that spans across brick walls (called sleeper walls) with an air gap below. You can usually spot one from outside: look for small grilles or air bricks low down in the wall, often just above ground level. Those vents keep the void dry, but they also let cold outside air wash continuously under your floor.
Heat leaves through two routes. The first is straight conduction down through the boards into that cold air. The second, and often the bigger one, is draughts: gaps between the floorboards and around the edges let cold air be pulled up into the room. In an old house the floor can account for roughly a tenth of total heat loss, which is enough to make a noticeable difference to comfort.
Insulating the floor tackles the conduction. Sealing the gaps tackles the draughts. The best results come from doing both together, which is why this job pairs naturally with draught-proofing the gaps between boards and around the skirting.
How the job is actually done
There are two ways to reach the underside of the floor, and which one you have changes the cost dramatically.
Access from below
If you have a cellar, a basement, or a crawl space big enough to get into, you work from underneath. You push insulation up between the joists and hold it in place with netting, battens, or breathable membrane stapled to the joist sides. Nothing in the room above is disturbed, so this is the cheaper and far less disruptive option.
Lifting the floorboards
Most homes have no crawl space, so the floorboards have to come up. The boards are lifted carefully, insulation is laid between the joists on netting slung below them, and the boards go back down. This is more work, it means clearing the room, and it raises the cost, but it is still very doable as a weekend project for one room at a time.
The usual materials are mineral wool batts, sheep's wool, wood-fibre board, or rigid PIR boards cut to fit snugly between the joists. Whatever you use, leave the air bricks clear. Blocking the under-floor ventilation to stop draughts is a common mistake that traps moisture and rots the timber. The aim is an insulated floor with a still-ventilated void beneath it.
A breathable approach matters here. The void needs to stay dry, so a vapour-open insulation and membrane let any moisture escape rather than building up against the joists.
What it costs and what you save
Material cost is modest. Mineral wool batts cover a floor for a few pounds per square metre, and a roll covering roughly six square metres costs around £25 to £30. The big variable is labour, and that comes down entirely to access.
| Job | Typical cost | Notes |
|---|---|---|
| Materials only (DIY) | £3 to £8 per m² | Mineral wool, netting, membrane |
| Professional, access from below | £500 to £1,500 | Cellar or crawl space |
| Professional, boards lifted | £1,500 to £3,000 | More labour and disruption |
| Annual saving, average house | around £75/yr | Energy Saving Trust |
| Annual saving, detached house | up to about £110/yr | Larger floor area |
Savings figures from the Energy Saving Trust; gas at 7.97p per kWh, Ofgem cap October to December 2026. Costs are mid-2026 trade estimates and vary by region and access.
The economics are simplest if you do it yourself with access from below. At a few pounds a square metre for materials and a saving of £75 or more a year, a typical front room pays for itself inside a couple of winters. A full professional job with boards lifted is a longer payback, but it buys you the comfort and the draught reduction in one go, and it lifts your home's EPC rating at the same time.
One point on the savings: most homes still run on gas central heating, so the saving is set by the gas rate, not the electricity rate. Because gas is cheap per unit, floor insulation saves less in pounds than the same job would if you heated with electricity. If you do heat electrically, the saving is roughly three to four times larger.
Grants that may pay for it
Floor insulation is one of the measures government energy schemes will fund for eligible households, so it is worth checking before you pay out of pocket. The schemes change names and rules often, so treat the following as how it works rather than a fixed promise.
- The ECO4 scheme funds underfloor insulation among other measures for lower-income households and those on certain benefits, and is currently due to run into late 2026.
- The Great British Insulation Scheme, which ran on council-tax band and EPC criteria, closed to new referrals at the end of March 2026.
- The Warm Homes Plan is administered through local authorities rather than energy suppliers, so eligibility and what is offered varies by where you live.
Because these schemes open, close, and change their amounts, check the current rules and what is on offer on the official GOV.UK and Ofgem pages, or with your local council, before assuming you do or do not qualify.
Doing it yourself: a sensible order of work
If you are tackling one room from above, the job is well within reach of a competent DIYer. A workable order:
- Clear the room and lift the skirting if it traps the board ends, then carefully lift the floorboards, numbering them so they go back in the same place.
- Check the joists and the void for damp, rot, or signs of woodworm before you cover anything. Fix problems first.
- Staple breathable netting or membrane to the joist sides to form a cradle, then drop in mineral wool or wood-fibre batts cut about 10mm wider than the gap so they grip.
- Keep the air bricks completely clear so the void still breathes.
- Relay the boards, then seal the gaps between them and around the skirting. A flexible sealant or proprietary gap filler stops the draughts the insulation alone will not catch.
Sealing the surface gaps is the step people skip, and it is the one that makes the room feel warm rather than merely less cold. For the detail on that part, see the guide to draught-proofing. While the boards are up is also the ideal moment to lag any exposed pipework in the void so it does not freeze; see pipe lagging and frozen pipes.
The bottom line
A suspended timber floor is one of the few insulation jobs in an old house that a homeowner can largely do themselves, for the price of the materials, and feel the benefit the same winter. If you have access from below, it is close to a no-brainer. If the boards have to come up, weigh the disruption against doing the whole room at once: insulate, seal the gaps, lag the pipes, and relay.
It will not transform your bills on its own, because the gas saving in pounds is real but modest. Its value is in comfort and in being one piece of a warmer house. Done alongside loft insulation and, where relevant, cavity wall insulation, it helps the heat you have already paid for stay in the room rather than draining away under your feet. Just never block those air bricks, and keep the whole thing breathable so the timber stays dry for the next hundred years.