I am converting an old stable. The project is still in its early stages; I’m about to lay the dpm and concrete slab.

When I bought the property there wasn’t any obvious evidence of damp. The building has stone walls (rubble fill) on three sides, and a double skinned brick wall which forms the party wall to the house next door. There is evidence of a slate-type original damp course in places on the exterior (stone) walls.

The floor was stone flagged, which I have taken up to fit a concrete slab and dpm (as per instructions of building control). When I took up the flags I was surprised to find a sough in the floor, with a shallow stream of water running approx 3.5m below the house. The stone on top of the sough had been ‘sealed’ beneath the flagstones with a 2nd slab and lime, which was not damp.

After taking up the floor the bare earth was left exposed for around 2 months and the sough covered but not sealed. The air inside has become very damp, resulting in a struggle to open and close the wooden door and (what I hope is) large amounts of condensation forming on the wooden floor above and the roof timbers (there’s no sign of a roof leak and the house was re-roofed approx 10 yrs ago, a roofer has checked and after a lot of thought we’ve come to the conclusion the timings work and the source of the moisture must be the open earth floor; we’ll re-assess once the dpm is installed and the concrete slab has dried out).

So that’s one source of damp we’ve identified as probably being down to condensation from the floor. However I also have a small area of damp, perhaps 50cm across by 80cm high on the brick party wall. It’s around half-way along. The property next door is in good repair, so there’s no obvious source of penetrating damp, and there’s no pipe work to have sprung a leak. Floor levels are similar to mine and their side is screeded, since it’s a garage. Which from what I can gather having read around the subject leaves me with either condensation or rising damp as the cause. There’s no “tide mark” though, and I hadn’t identified the issue when I first bought the property. The only other factor I can see is the party wall has a disused chimney on my neighbours side, though I’m not sure my damp patch exactly matches with this.

My plan is to insulate the walls using 50mm celotex fixed to 40mm batons, leaving a 40mm air gap. I really don’t want to go the route of lime plaster etc due to the cost, plus I have to meet building regs for insulation and want to lose minimum internal wall dimensions since the house is tiny. I hope this 40mm air gap goes some way to allowing the walls to breathe?

My builder initially suggested an injected damp proof course in the party wall to solve the damp problem. He’s since said they have mixed success and has suggested I use a self adhesive visqueen to “tank” the wall. My concern is that this will stop the wall breathing. Also I’m concerned the installation of the damp proof membrane to the floor will “push” damp to all the walls (although I’ve capped the sough the issue with condensation in the roof presumably indicates there’s a lot of water coming out of the ground). So I’ve also considered a tanking membrane attached with plugs (commonly used in basement conversions from what I gather) which I could install around the whole house, hopefully ensuring any increase in damp from the dpm would be dealt with. But ditto re losing the wall’s ability to breathe.

I’m happy to go with the best solution, but the more I read re the myth of rising damp, the possibility of habitation increasing damp, allowing old walls to breathe etc etc, the more confused I get re the possible source of the dampness on the wall and the best solution to solve it! Part of me says leave it be and the air gap will ensure interior decor isn’t affected, but if the dpm increases the problem this may not work. My apologies for the very long-winded question, but I wanted to paint the whole picture in the hope of getting the best informed advice.

So please help, what’s my best course of action to deal with the damp wall? Many thanks in advance.

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