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

Suspended timber floors are a common feature in older homes, and insulating them is often one of the most effective first steps in improving energy efficiency and occupant comfort. But while the basic process is straightforward, there are several critical details — particularly around moisture management and ventilation — that can make or break the performance of the upgrade.

If you’d prefer to watch the video, just click play below to watch Chris and Andy from our technical team talk through this question in Can I Just Ask? – Ep.16 👇

Upgrading Suspended Floors: Why It Matters

Suspended timber floors are notorious for letting in cold air. With just 20–30mm of timber separating your feet from the subfloor void, it’s no surprise that occupants often feel the chill, especially given how sensitive we are to heat loss through our feet.

Aside from comfort, floors represent a significant thermal bridge in older properties, especially where gaps between floorboards and draughty voids below allow warm air to escape. So addressing this layer of the building envelope is a logical and impactful early intervention.

A Common (and Effective) Approach

The most straightforward method involves installing insulation between the floor joists. The basic steps include:

• Breather membrane: Draped between the joists and stapled or taped to their lower edges, this provides a cradle for the insulation and limits wind-washing.
• Insulation: A semi-rigid, flexible batt (such as Steico Flex) is friction-fitted between joists to fully fill the void.
• Vapour control layer (VCL): Laid over the joists and taped around the perimeter to minimise air leakage and moisture ingress.

This approach is accessible, relatively low cost, and within the capability of competent DIYers or general builders. But it does come with important caveats.

Moisture Risks and Ventilation Requirements

Floor voids are naturally moist environments. The timber joists already operate close to the threshold for decay risk, and adding insulation – which reduces heat flow into the void – makes conditions more susceptible to condensation and moisture build-up.

To mitigate this, it’s essential to:

• Maintain cross-ventilation: Air should flow freely from one side of the building to the other. That means venting both front and back elevations, not just one side.
• Use hit-and-miss dwarf walls: These partial brick walls beneath joists allow air to pass between different sections of the void. Solid walls inhibit airflow and can create stagnant zones.
• Install an oversite membrane: Moisture from the ground can evaporate into the void. Laying a damp-proof membrane (DPM) or polythene sheet over the soil surface, ideally on a bed of sand, with more sand on top – helps suppress this evaporation without fully sealing the void.

When It Doesn’t Work: The Case for Solid Floors

In some properties – especially terraced houses with extensions, poor ventilation routes, and heavy clay subsoils – achieving reliable cross-ventilation is practically impossible. In these cases, the safer and more robust approach may be to remove the suspended floor entirely and replace it with a solid floor construction.

One solution is to fill the void with Foamglas aggregate, a recycled, lightweight, non-capillary material that provides both insulation and structural support. This eliminates the need for a concrete slab and creates a warm, dry base that can support underfloor heating and floor finishes.

Key benefits of this approach include:

• Moisture resilience: No organic material within the insulation zone means minimal risk of decay.
• Thermal continuity: Eliminates thermal bridging at joist ends and wall junctions.
• Simplified layering: Floor finishes can be laid directly over the insulation layer using dry construction methods.

Final Thoughts

Insulating a suspended floor can be a quick win for energy efficiency — but only when moisture and ventilation are properly accounted for. If those risks can’t be confidently managed, transitioning to a solid floor may offer a safer and more effective long-term solution.