Insulation of Historic Suspended Timber Floors

Find out more about how we approach this building scenario.

In many Victorian houses a suspended timber floor was formed as the ground floor of the house. This was often built over bare earth and relies on good ventilation to keep moisture levels down and the timber floor structure dry. Without adequate ventilation the humidity can rise causing the rot in the timbers. Simply adding insulation can exacerbate the situation by sealing up one of the ventilation paths and by reducing heat flow through the floor, preventing the timber from drying. With this in mind it is important to treat this element of the building carefully.

The main source of water in the void under suspended floors comes from moisture evaporating from the bare earth, which can provide the equivalent amount of moisture as standing water. But, it can also come from the drying of building materials as well as leaks and rain penetration from bridged damp courses or cavities.

Moisture in this void is a problem because as the relative humidity in the air rises up to 80%, the moisture content of the timber also rises and when it reaches 20% and above, this can allow fungus and wood loving insects to grow in the timber, rotting the structure and causing it’s eventual collapse. Even treated timbers will not resist this problem indefinitely and should not be relied upon to perform in environments with moisture levels this high.

To refurbish and insulate these structures there are two main options. The first is to reduce the amount of moisture evaporating from the ground below by applying a moisture impervious layer. This would usually be in the form of a layer of compacted hardcore followed by a polythene DPM with a concrete covering, much as is formed when constructing a new solid floor structure. However, this cn also be achieved by flatting off the ground, laying a sand blinding layer, polythene DPM followed by another sand blinding layer. Whilst this is a little less permanent, it does avoid the use of concrete.

In addition to reducing moisture flow up from the ground it is important to ensure that the ventilation rates are correct to ensure that moisture levels are kept low. Currently the UK building regulations requires ventilation on two opposing external walls of not less than 1500mm² per metre run of external wall or 500mm² per metre² of floor area, whichever gives the greater amount of ventilation. Correct ventilation also ensures that any moisture penetrating through faulty damp proof courses or bridged cavities is kept to a minimum

Finally, installing insulation between the floor joists to give the required levels of insulation should be done with flexible insulation batts and should include a vapour control layer to ensure that moisture flow is restricted down in to the sub-floor void and also that air leakage is avoided, both of which add to the overall moisture content within this area. See our article on how to insulate a suspended floor.

The second option is to completely remove the suspended floor and install a solid floor. This may seem extreme but in some areas of the country the atmospheric humidity is at or above 80% for many months of the year. Consequently the moisture levels in the timbers within the floor regularly reach and exceed the critical moisture content threshold of 20%, above which rot can occur, making it impossible to insulate the floor structure without risk to the timber structure.

In these areas it can often be the heat loss form the floor that keeps the timbers dry enough not to rot and so insulating the floor actually makes the situation worse. since there is much less heat available to dry the timbers.

If a solid floor structure is to be installed then the standard concrete slab with insulation above and hardcore and damp proof membrane below can be used. Alternatively, Foamglas aggregate can be used as the insulant and also to fill the existing floor void. The Foamglas doesn’t allow the passage of liquid water and it’s thermal conductivity is not affected by moisture and so it can be laid directly on to compacted earth. A damp proof membrane can be installed over the top and then, along with a levelling layer, a 22mm wood fibre insulation board and Lithotherm underfloor heating tiles, the floor finish can be laid.

If you’d like advice on how to insulate your suspended floors please contact us at or alternatively give us a call on 01392 861763.

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