Sophie Bennett
Start with the floor you actually have
Before you buy a single board or roll, find out what is under your feet. In UK homes, ground floors are usually either a solid concrete or beam-and-block slab, or a suspended timber floor with joists spanning over a ventilated void. The two need very different strategies; what works for one can waste money or cause damp on the other.
Lift a floorboard in a corner, or look for air bricks on the outside wall. If you can see joists, a dark crawl space and a steady breeze, you have a suspended timber floor. If you meet concrete, screed or blockwork, it is solid. Note the joist depth, the direction they run, and whether the void below is clear and ventilated.
- Suspended timber: joists over a void, usually with air bricks providing cross-ventilation.
- Solid ground floor: concrete slab, beam-and-block, or old quarry tiles on earth.
- Mixed construction: extensions and older houses often have both, so check each room.
Insulating a solid ground floor
If your floor is solid and you are not digging it up, the practical route is to insulate above the slab and lay a new floor surface over it. You lose ceiling height, so check door thresholds, skirting and radiators before you commit.
Rigid insulation boards — usually polyisocyanurate (PIR) or expanded polystyrene (EPS) — are laid directly on a suitable damp-proof membrane. The boards must be butted tightly together, with joints taped or foamed, and a continuous upstand should run around the perimeter where the floor meets the wall. That upstand reduces thermal bridging at the floor-to-wall junction, one of the most overlooked details in a renovation.
Thickness matters. If headroom is tight, 25–50mm of PIR may be the maximum you can fit, which helps but is not a miracle. If you can afford 70–100mm, you will feel a real difference underfoot. Finish with tongue-and-groove chipboard or plywood, glued and screwed, to keep the floor stiff and airtight.
- Clear the slab, repair any uneven areas, then lay a damp-proof membrane.
- Fit PIR or EPS boards tightly, taping all joints.
- Add a perimeter upstand before the final floor surface goes down.
- Glue and screw the deck; movement here creates draughts and squeaks.
Suspended timber floors: work with the void
Suspended timber floors are insulated between the joists, not over the top, so you keep your floorboards and your ceiling height. The golden rule: never block the ventilation path. Air must still move through the void and out of the opposite air brick. Seal it up and timber can rot.
For typical UK joists of 100–150mm, mineral wool or wood fibre is pushed up to the floorboards and supported with netting, a breather membrane or battens. Leave a clear gap beneath the insulation for air to move. Rigid PIR boards cut to fit between joists perform better per millimetre, but must be sealed at the edges with foam or airtight tape.
If the void is shallow or access is difficult, use a professional with the right equipment. A poor installation can undo the benefit entirely.
- Use netting stapled to the sides of joists to hold mineral wool in place.
- Keep insulation tight to the floorboards, with no gaps.
- Leave the void below open and maintain cross-ventilation.
- Seal gaps between boards, around pipes and at the joist ends.
Choosing materials without overcomplicating it
There is no single best insulation. The right choice depends on joist depth, budget, floor type and spare height.
- Mineral wool — cheap, breathable and easy to fit between joists, but needs depth to perform well.
- PIR rigid boards — the best performance per millimetre, ideal for solid floor overlays or shallow voids, but more expensive and fussy to seal.
- Wood fibre — breathable, renewable and good for older buildings, but usually pricier and thicker.
- EPS — inexpensive rigid foam, decent for solid floors where thickness is not critical.
For a suspended floor, aim for at least 100mm of mineral wool, or 50–75mm of PIR if space is tight. For a solid overlay, 50–100mm of PIR is a sensible target. Check with building control if the work is subject to Building Regulations.
Draughts, edges and ventilation
Insulation slows heat loss, but draughts make a floor feel cold whatever the thickness. The biggest culprits are gaps between floorboards, under skirting, around pipes and at the wall-floor junction. Deal with these before you lay a final surface.
Use flexible sealant or airtight tape, and fit a continuous upstand where a solid floor meets an external wall. For suspended floors, seal between boards and skirting, and fill gaps around services. Never seal air bricks. If unsure whether the void is ventilated correctly, ask a surveyor or experienced installer.
- Seal between floorboards and skirting with a flexible sealant.
- Fill gaps around pipes, cables and ducts.
- Fit a perimeter upstand on solid floor overlays.
- Keep air bricks and void vents completely clear.
What it costs and when to call in help
As a rough guide, mineral wool for a suspended floor costs £5–£12 per square metre in materials; PIR for a solid overlay costs £20–£45. Add floorboards, chipboard, fixings, membrane, sealant and labour. The payback is not just lower bills: a warmer floor makes a room usable in winter and reduces damp.
If this is part of a deeper retrofit, consider the floor alongside walls, windows and ventilation. Insulating one element in isolation can shift moisture elsewhere. A qualified installer or surveyor can help plan the whole assembly. And if the crawl space is tight, damp or full of debris, bring in someone who works there every day — your knees and timber will thank you.
