How to insulate a suspended timber floor

Inspired by a helpful YouTube guide. We based this walk-through on "Is Your Floor Insulation Up To Scratch?" by Skill Builder, a long-running UK building and renovation channel run by Roger Bisby whose videos take a practical, trade-level look at how homes actually get built and improved. The detail on joist depths and ventilation is particularly useful.

1. Confirm you have a suspended timber floor

Lift a floorboard near a wall and look underneath. If there is a void — air space below, with timber joists bridging across it — you have a suspended floor. Solid concrete floors cannot be insulated this way.

Most pre-1920s houses in the UK have suspended timber ground floors. Many 1930s and 1950s properties do too, though later builds tend to have solid slabs. If you are not sure, the void check is definitive.

2. Understand what is involved

The idea is simple: lift the floorboards, push insulation up between the joists, and support it so it stays in place. Then re-lay the boards. The tricky part is lifting boards without splitting them and working in a confined space.

The whole job on a single room typically takes a day. Two people is easier — one lifting and passing boards, one working below or reaching in to fit the insulation. That said, many people do it alone without too much bother.

3. Choose your insulation

Glass or mineral wool batts are the most common choice. They are cheap, widely available, and can be cut to width with a bread knife or sharp blade. Aim for a thickness of 100 mm or more — 150 mm if your joist depth allows it.

Rigid foam boards also work well, particularly PIR (polyisocyanurate) boards, which give you more performance per millimetre. They are more fiddly to cut but easier to support. To be fair, either approach works. The important thing is filling the full depth between joists without leaving gaps at the edges.

4. Lift the floorboards carefully

Start by drilling a small hole at one end of a board and using a bolster chisel to lever it free. Work along the board gently, moving the bolster forward as it lifts. Old floorboards are often nailed at every joist and the nails can be stubborn.

Label each board and its direction as you go. They are often slightly different widths and lengths. Fitting them back in the wrong order creates gaps and squeaks.

5. Fit the insulation between the joists

Cut your insulation to the width of the joist bays, allowing a slight compression fit — about 5 mm wider than the gap. This holds the batts without any extra support. If the batts are heavy or slippery, staple a net of garden mesh across the bottom edges of the joists to carry the weight.

Make sure the insulation sits directly below the floorboard with no air gap above it. Push it snug up against the underside of the boards. Fill the full length of each bay, cutting and butting pieces together where needed.

6. Keep ventilation intact

Suspended floors need ventilation to stop the joists rotting. Do not block the air bricks in the external walls. They are there to let air circulate through the void below, and blocking them is a serious mistake that causes damp and rot over time.

If you find any blocked air bricks while you are at it, clear them. You want steady airflow through the void — just not through the floor into the room.

7. Re-lay the floorboards

Nail or screw boards back in the same position and orientation they came from. Use ring-shank nails or screws rather than cut nails if possible — they hold better and are easier to remove next time. Punch nail heads just below the surface so they do not snag.

If any boards have split or warped badly, now is a sensible time to replace them. Structural plywood sheet can fill any awkward gaps neatly.

When to call a handyman

Call Richard if the joists show signs of rot or woodworm, if boards are badly damaged and need replacing in quantity, or if the void is very shallow and you cannot safely get the insulation in without lifting the whole floor. Also worth calling if there are pipes or cables in the floor that you would rather not disturb.