On a suspended timber ground floor, insulation is normally fitted between the floor joists while the ventilated void beneath remains able to move air through the building. Do not block air bricks or underfloor ventilation to stop draughts.
Check the floor before covering it with insulation
Government best-practice guidance recommends assessing the existing floor first. Look for damp, decay, blocked ventilation, damaged joists, plumbing leaks and other defects. Fix the cause before insulating.
Keep the underfloor ventilation path open
Cadw specifically advises maintaining a good through-flow of air beneath suspended timber floors. Insulation should not bridge across or obstruct the ventilated void or block external air bricks.
Fit the insulation continuously between the joists
The insulation needs continuous support so it remains in contact with the floor above without sagging into the void. The support method depends on the insulation system: netting, battens or proprietary supports are common options. Follow the insulation manufacturer's instructions for fit, support and any required membrane layers.
Can PIR insulation be used between floor joists?
Yes, rigid PIR can be used in suitable suspended timber floor build-ups. The boards need to be cut accurately to fit the joist bays and supported as required by the chosen system while keeping the underfloor ventilation path clear.
Shop our PIR insulation board, or see How to Cut PIR Insulation Board before fitting boards between joists.
Do not guess the insulation thickness from the joist depth alone
The required thermal performance depends on the floor build-up and the project requirements. The joist depth tells you how much physical space is available, not automatically the insulation specification.
For PIR specifically, see our PIR U-value and thickness guide.
Upper floors are a different problem
Insulation between an intermediate floor is often being installed for sound reduction rather than heat loss, so the correct material and build-up can be different.
Read the UK government suspended timber floor guidance, then browse our insulation range.
