Insulation -Suspended Timber Floor

Most of the traditional buildings in Cheltenham have suspended timber floors. These are relatively easy to insulate without affecting the look of the floor.

If your ground floor is of suspended timber construction, you will be losing heat in the winter through the fabric of the floor while being chilled by the draughts that whistle though the gaps in the boards. Usually suspended timber floors are ventilated to the outside to remove any moisture and prevent timber joists rotting, which means that the void under the floor can get very cold in the winter. Insulation will reduce the fabric heat loss but will not necessarily stop up the draughts, so draught proofing should be undertaken at the same time. Insulation is installed between the floor joists either from above or below.

In some houses it is possible to reach the underside of a suspended timber floor through a vault, cellar or crawl space without taking the floor boards up. This makes it possible to insulate the floor without the risk of damaging the boards. If you do not have access to the subfloor space, you need to assess the quality of your floorboards and consider whether any damage from lifting is acceptable. If you feel it is not, you should focus on draught-stripping and consider laying a carpet with an insulating underlay over the floor. However, with the help of an experienced carpenter, even high quality, long-established floor boards can be lifted and re-laid with minimum risk.

Whether you are able to install insulation from below or from above, it is important to keep the remaining void below the insulation ventilated. This space will be even colder once the insulation has been installed, so condensation of moisture will be more likely. If the void or basement is not currently ventilated and is at all damp, you should seek professional advice about how to ventilate the space.

The best way to insulate from below is to push flexible, moisture permeable insulation such as sheep’s wool into the gaps between the joists, pin a breather membrane (e.g. Tyvek) under the insulation to hold it in place and then install a breather board (such as a wood fibreboard) to complete the job and provide a new ceiling to the basement room. However this basement space will be outside the insulation and so will be cold. You will need to treat the door to this room as an external door and draught-strip it to keep the cold out of the rest of the house.

If you are insulating from above, lay the breather membrane over the joists, tape up the junctions and tape the membrane to the wall where the skirting boards will be reinstated (use the proprietary tape supplied with the breather membrane). This provides good draught-proofing as well as a strong support for the insulation which can now be laid in the channels between the joists.