Insulating a suspended timber floor

My next task was to insulate the suspended timber ground floor.

I calculated that the gaps between my floor boards was equivalent to a hole 800mm by 800mm, no wonder it felt drafty. The floor boards on the ground floor are the original ‘pine’ boards, a beautiful golden colour. I wanted to retain them as a feature of the house so the only option was to insulate underneath them. The two ways to achieve this were either to lift all the boards and insert the insulation from above or get under the floor and insert the insulation from below. The floor was fixed firmly, it had been sanded and varnished already and lifting all the floorboards would have caused a lot of breakages and require re-sanding. The only option was to insulate and draft proof from underneath.

I removed a couple of boards in the hall, donned a boiler suit, goggles and face mask and squeezed under the floor – I could just move around, so it may be possible.

I now had to determine exactly what I needed to do. The Web has a number of good links covering insulating under a timber floor.–insulating-suspended-timber-floors.html

The aim is maximum insulation between the floor joists, using a fibrous insulation material that takes up the imperfections in the timber and that minimises the air movement around the insulation. The other objective is to stop the movement of air through the insulation and into the room but allow water vapour that may build up to escape. In addition; the insulation should not come into contact with any masonry walls but the gap between the joists and the foundation walls (usually about 50mm) needs to insulated and made draft proof, the air bricks from the outside need to be kept clear, cold water and central heating pipes below the insulation level need to be insulated (the void below the insulation will be much colder) and any electrical cables need to be prevented from overheating and kept free of insulation.

I needed to establish categorically if I could do all of this from underneath. There is a 400mm space between the bottom of the joists and the ground level but the joists rest on 100mm timbers so the working height is 300mm. I had to be able to crawl around underneath the floor for several hours completing the insulation and draft proofing work. I donned my boiler suit, goggles and face mask and after ten minutes established that 300mm was not sufficient room to do what was required – back to square one.

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