Suspended timber floor insulation

Summary

Typically used until the 1950s, uninsulated suspended timber floors can be a significant source of heat loss. This type of construction involves timber joists that are ‘suspended’ above the damp ground below the floor with a ventilated space in between to ensure the joists stay dry.

If there is a large enough void below the floor then insulation can be placed between the joists, ensuring that the insulation is at least 150mm clear of the ground level.

Best practice involves draping a windtight breathable membrane over the joists which allows any residual moisture to disperse whilst creating a cradle to support the insulation material. The joists are then packed tightly with a flexible, breathable insulation such as wood fibre or hemp. The joists are then covered with an airtightness membrane with specialist airtightness tapes used to seal the interface between the floor and wall.

*How do I know if I have a suspended floor?

Clues that you have a suspended floor are a slightly bouncy feel to the floor and the presence of sub-floor vent bricks.

As floorboards wear over time gaps can appear between boards or around skirting boards which allows cold air to enter the room. It may be tempting to block up vent bricks to stop the draughts but this must be avoided to ensure the space under the floor stays ventilated or the joists could rot.

Cost/difficulty

MEDIUM COST – Estimated £3000-£5000 depending on house size – The Surrey Home Energy Improvement Guide (2025), page 40.

MEDIUM DIFFICULTY

Applicability in Letchworth

Applicable – No specific requirements.

Insights from ASBP

“Conventional foil-faced and impervious insulations such as PIR/PUR boards are commonly used in suspended floor upgrades. However this approach is not ideal due to the inflexibility of the insulation material and its inability to enable vapour to be absorbed and gradually dispersed.”

Insights from People Powered Retrofit

“If you are insulating a suspended timber floor, it might make sense to upgrade radiators and pipework at the same time. This would make use of the access provided by lifting floorboards.”

Case study

A Best Practice Approach To Insulating Suspended Timber Floors

A blog post from Ecological Building Systems discussing the thermal loss issues associated with suspended timber floors and outlines a best practice approach to tackling them.

Energy Efficiency Guide for Letchworth Garden City residents (2024), page 5
HCCSP Retrofitting Your House Guide (2024), page 21
Retrofitting Your Home – Cambridge Guide (2022), page 19

Image credits – Ecological Building Systems

This guidance is part of Floors

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