I’m losing heat through my damp cellar – what should I do?

A Ledbury homeowner who received a fully funded EPC+ report through Future Ready Homes asked for advice about their cellar.

Their home is a detached solid brick pre-1900 house which has been retrofitted with 100mm of internal wall insulation (IWI). They’re losing heat through their damp cellar (not wet) and have engaged a builder who doesn’t know what to do with it. He doesn’t know what materials to use because of the damp and the need for it to breathe.

Anne Netherwood

As it’s a suspended floor over the cellar, the easiest way to mitigate the heat loss would be to insulate between the joists with wood fibre or hemp-jute batts, sightly oversized so that they will friction fit, and then wood fibre boards below. It would be as well to staple some ProClima Solitex membrane to the underside of the joists before fixing the wood fibre boards; the membrane would need to have all joints taped as well as be taped to the wall.

These products are all available from Ecological Building Systems https://www.ecologicalbuildingsystems.com who will advise on how to install their products, and may also be able to give you calculations. The builder may need to paint the ends of the joists with a lime plaster skim coat as a preservative to help prevent them rotting if the joists are set into the damp wall.

Regarding the damp specifically, the main thing to find out is where the damp is coming from. Check the drains, incoming water main, ground conditions adjacent to the building etc. Once all those possibilities have been checked out, and if no obvious cause is found, it’s most likely that the damp is coming from the ground water. Check what the level of the water table is outside, and if it’s above the cellar floor level then you can either tank the whole basement or install a drainage membrane on the walls with a drain below to collect the water. The drain would have to discharge somewhere so either you’d have to pump the water up to connect to the household surface water system, or if there isn’t a lot of water coming through the walls, drain into a container which can be removed and emptied manually.

Or, you could accept that the cellar is damp and leave it as it is – maybe with a continuous mechanical extract fan with humidistat booster in order to stop the equipment rusting. The wood-fibre plus Solitex should stop any damp getting into the house.