The floor is the part of the house people forget, because you cannot see heat leaving through it the way you watch it disappear out of a window. A ground floor sits against cold earth or, in older homes, a ventilated void with outside air blowing through it. In a draughty Victorian terrace the floor can account for a tenth of the whole heat loss, and the cold air rising between the boards is why a room can feel chilly however high you push the thermostat. This guide covers both kinds of floor, what the work costs, what it saves at current gas prices, and the cheap fixes worth doing first.
The short answer. Insulating a suspended timber ground floor typically cuts heat loss enough to save in the region of 30 to 50 a year on gas for an average home, and it kills the draughts that make a room feel cold. The cheap first step, sealing the gaps between boards and at the skirting, costs a few pounds and stops the worst of the draught immediately. The full job pays back over many years on saving alone, so do it when the floor is up for other reasons or when you want the comfort.
Two kinds of floor, two different jobs
Before you do anything, work out which floor you have, because the approach is completely different.
A suspended timber floor is the common one in homes built before about 1950. Floorboards sit on joists over a ventilated gap, with air bricks in the outside wall keeping that void dry. On a windy day you can often feel cold air moving up between the boards, and a tissue held near the skirting will flutter. This is the floor that benefits most from insulation, because there is genuine cold air circulating underneath it.
A solid floor is concrete laid straight onto the ground, normal in newer builds and in extensions. It does not have the draught problem, and it loses heat more slowly because it is in contact with earth rather than ventilated air. The downside is that a solid floor can usually only be insulated as part of a major job, when a new screed and floor covering go down anyway. Retrofitting insulation on top raises the floor level, which then fights with door heights and skirting, so it is rarely worth doing on its own.
If you are not sure which you have, lift a corner of carpet or look for air bricks low down on the outside wall. Air bricks almost always mean a suspended timber floor.
Do the cheap draught fixes first
The single most cost-effective thing you can do to a suspended floor is not insulation at all. It is sealing the gaps, and it costs the price of a tube of sealant.
Cold air comes up through the gaps between floorboards and through the larger gap where the boards meet the skirting. Fill those and you stop the draught, which is most of what makes a floor feel cold. Options that work:
- Flexible filler or sealant run into the gap between boards. Use a flexible decorator's caulk or a proprietary floor filler that moves with the timber rather than a rigid one that cracks out.
- Proprietary gap-filling strips pushed between wider boards, which flex as the wood expands and contracts with the seasons.
- Sealing the skirting gap with caulk or quadrant beading, which is often the biggest single draught path.
- A rug or underlay over bare boards, which adds comfort and a little insulation for the price of the rug.
These steps take an afternoon and take the edge off long before you contemplate lifting a floor. They sit alongside the rest of the draught-proofing jobs and are exactly the kind of thing in our quick wins under a tenner list.
The full job on a suspended floor
When you want to go further, the aim is to put a layer of insulation between the warm room and the cold void below, while leaving the void itself ventilated.
There are two routes:
- From below. If you have a cellar or a crawl space with access, insulation can be fitted between the joists from underneath, held up with netting or battens. This is the least disruptive route because the room above stays intact and you do not lift a single board. It is the easiest version of the job if you have the access.
- From above. Where there is no access underneath, the floorboards are lifted, insulation laid between the joists on a supporting net or membrane, and the boards relaid. It is more work and more mess, but it is the only option in most terraces without a cellar.
Either way the material is usually mineral wool or rigid foam board cut to fit between the joists. It follows the same logic as loft work, putting a warm layer between you and the cold, and is covered alongside loft insulation as one of the fabric-first measures. If the boards are up anyway for rewiring or replumbing, do the insulation at the same time and split the disruption.
What it costs and what it saves
Floor insulation is a slower payback than loft or cavity work, so be honest about the numbers. Here is a rough comparison for an average semi or terrace at current gas prices.
| Measure | Rough cost | Typical annual saving |
|---|---|---|
| Sealing gaps and skirting (DIY) | 5 to 30 | 15 to 40 |
| Suspended floor insulated from below | 400 to 1,200 | 30 to 50 |
| Suspended floor, boards lifted | 1,000 to 2,500 | 30 to 50 |
| Solid floor (only with full refurb) | part of a larger job | 20 to 40 |
At 7.33p per kWh gas, Ofgem cap July to September 2026. Savings are estimates for an average gas-heated home and vary widely with house size, exposure and how draughty the floor was to start with. Worked examples; check your own situation.
The headline is clear. The cheap draught-sealing pays for itself in a single winter. The full insulation job is a long payback on energy alone, often ten years or more, so most people do it for the comfort and the end of cold-floor draughts rather than the bill. It makes most sense bundled with other floor work, or in a genuinely cold, draughty old house where the saving is at the top of the range.
Never block the air bricks
This is the one mistake that turns a good job into an expensive one. A suspended timber floor depends on those underfloor air bricks to keep the void dry. Moving air under the boards is what stops the joists rotting.
It is tempting, when you are chasing draughts, to block the air bricks from outside. Do not. Seal the floor above the void, not the void itself. If you stop the ventilation, moisture builds up under the boards, the timbers stay damp, and you can end up with rot and even damp problems that cost far more than the heating you saved. The whole point is to insulate the warm side while the cold void keeps breathing.
Done properly, the timbers stay dry, the room above stops feeling like it has a cold breath rising through it, and you have closed off a real heat-loss path. Pair the warmer floor with sensible thermostat settings and you may find you can run the heating a touch lower for the same comfort.
Where floor insulation sits in the order of jobs
If you are working through the house, floor insulation is rarely the first thing to spend on. The order that usually makes sense, cheapest payback first:
- Draught-proofing everywhere, including the floor gaps. Days, not years, to pay back.
- Loft insulation topped up to current depth, one of the best returns going.
- Cavity wall insulation if you have a fillable cavity, often grant-supported.
- Floor insulation and double glazing, both slower paybacks bought largely for comfort.
- Solid wall insulation for older homes with no cavity, the biggest job of all.
For renters, the full floor job is out of reach, but the cheap draught-sealing and a good rug are fair game and covered in saving energy when renting. Whatever you do to the fabric, it works best with the heating set sensibly, so it is worth reading what temperature should I set my thermostat alongside.