Cavity Insulating Stone Barn Walls


I have a barn conversion. The majority of the living area has walls between 400mm and 1m thick. The outside is the original thick Cornish stone wall. There is a cavity and the internal wall. I’m not sure why there are different thicknesses to the walls, but each side is a different thickness; however, they all have unfilled cavities.

I am undecided about insulating the cavities. In principle I would like to do this, but we live in Cornwall and it is damp. I am worried that adding cavity wall insulation will make the house damp. I know this has been a problem with traditional houses, would having external stone walls make this risk larger and smaller? I would only need to insulate downstairs as upstairs was added later on and does not have uninsulated cavities.

Also, are there any particular insulating approaches that would be less risky for my house?

Many thanks in advance.

If you live within a few miles of the coast or an elevated position (I think that covers all of Cornwall) then I don’t recommend insulating the cavity. I remember Cornish rain to be near horizontal and even a waterproofing treatment (which I recommend anyway) would not stop all the leaks into the cavity.

Are you planning a serious retrofit? If so I suggest that you read about Shepherds Barn at

Thanks. Thats what I thought, but I had hoped there was a wayaround this. The house is finished, so there is no real scope to do anything fundamental.

I’m guessing the barn is made of granite but as it wasn’t originally built to be lived in a cheaper stone may have been used. An unweathered perfect piece of granite is weatherproof but what about the mortar? Has the stone developed a permeable weathered coating? Presumably not if it is granite. Weatherproofing the outside and allowing the fabric to dry out will help marginally.

Is the cavity vented? If so can you limit the amount of ventilation seasonally? In summer you want maximum ventilation to dry out but in winter you might want to limit it to reduce the chill effect of outside air circulating against the inner wall.