How To Insulate Existing Suspended Floors – Can I Just Ask? | Ep.16

Suspended timber floors are one of the most accessible entry points for improving a home’s energy performance — and one of the most overlooked. Human physiology makes us particularly sensitive to heat loss through our feet, and in many older properties there’s as little as 20–30mm of timber between the occupant and effectively the outside. Draughts through floorboards compound the problem further. The good news is that the basic approach — draping a breather membrane between the joists, friction-fitting a semi-rigid batt such as Steico Flex to fully fill the void, adding a vapour control layer over the top and taping it around the perimeter — is relatively straightforward and accessible to a competent DIYer.

The caveats, however, are significant. Suspended floor voids sit in a naturally moist environment, and the timber within them is often already operating close to the threshold where decay becomes a risk. Insulating the floor reduces heat flow into the void, making conditions more onerous, so cross ventilation from opposite faces of the building must be maintained — including through hit-and-miss brickwork in the dwarf walls supporting the joists. A ground vapour barrier is also essential: a layer of polythene blinded with sand prevents moisture evaporating out of the soil into the void. Without it, adding insulation above simply concentrates that moisture in the very place where it can do the most damage.

Where those ventilation requirements can’t be met — in extended terraced houses, properties on heavy clay soils, or anywhere that genuine cross-ventilation from front to back is blocked — the only reliable solution is to remove the suspended floor entirely and convert it to a solid floor. Foam glass is the preferred material for this: it can bulk-fill the void, provides the necessary insulation, and has no capillary rise, completely removing the moisture risk. The timber joists come out of the insulation zone altogether, underfloor heating can be integrated cleanly, and the screed requirement is eliminated by laying a dry resilient layer directly over the foam glass. It’s more disruptive and more expensive, but in the right circumstances it’s the only approach that genuinely resolves the problem.