Which floors
Suspended timber floors — boards on joists over a ventilated void — are the target. Solid concrete floors can only be insulated from above by raising the floor, which is a bigger job usually done during renovation.
How it is done
- From below, where the void is accessible through a hatch or cellar: mineral wool or multifoil between the joists, held with netting or battens.
- From above, lifting boards: PIR or wool between joists, then boards relaid. More disruptive, more thorough.
- Robotic spray applied under the floor from a hatch — inspectable materials only; we do not recommend closed-cell foam in floor voids for the same reasons as roofs.
Keep the void ventilated
Air bricks stay clear. A suspended floor relies on airflow beneath it; insulation must not block it.
Cold floor?
Check my homeTell us the property and we will show the likely route and cost before any contact details.
Cost
£800–£2,800 for a typical semi's ground floor. Multifoil between joists is a quick, thin option where joist depth is shallow. Insulation costs.
Questions homeowners ask
Can I use foil insulation under floorboards?
Yes. Multifoil draped between and stapled to the joists is a common, thin option. Keep the ventilated void beneath.
Sources and when they were last checked
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026