Suspended floor insulation: how to insulate a timber floor

What is a suspended timber floor?

Suspended timber floors have floorboards or chipboard sitting on timber joists, with a void (air gap) underneath. The void is usually 400mm to 600mm deep and ventilated via air bricks in the external walls. Common in pre-1950s UK homes.²

Insulating from below (if you have access)

If you have a cellar or crawl space, you can insulate from below by fitting insulation boards or batts between the joists from underneath. This is the easiest method because you don't have to lift floorboards.

Materials:

• 100mm PIR boards cut to fit between joists, supported by netting or timber battens.
• 150mm mineral wool batts (flexible, fits snugly between joists).

Cost: £700 to £1,500 per room (professional install). DIY materials: £300 to £600.

Steps:

1. Measure joist spacing (usually 400mm or 600mm centres).
2. Cut insulation boards or batts to fit.
3. Fix netting or battens under the joists.
4. Push insulation up into place, leaving a 50mm air gap to the floorboards.
5. Check air bricks are not blocked.

Insulating from above (lifting floorboards)

If you don't have access to the void, you have to lift the floorboards, fit insulation between the joists, and re-lay the floor.

Cost: £1,200 to £2,500 per room (more expensive due to lifting and re-laying floorboards).

Steps:

1. Lift floorboards (carefully, to avoid breaking tongue-and-groove joints).
2. Fit netting under the joists to support the insulation.
3. Lay insulation batts or boards between the joists.
4. Re-lay floorboards or replace with chipboard.

Only worth it if you're already lifting the floor for another reason (rewiring, replumbing, damp treatment).

Important: leave ventilation

You must leave a 50mm air gap between the insulation and the underside of the floorboards. The void needs to stay ventilated to prevent condensation and rot. Don't block the air bricks.²

Related reading

• Is floor insulation worth it? (full cost and savings guide)
• Concrete floor insulation guide
• Underfloor insulation cost breakdown