Hello,

We did a big renovation last year and have moved back in for around 9 months. On the problematic areas we had prior (mould on external walls), we made a big effort to use lime and/or lime/cork insulating plaster on all external walls. Thankfully, we've experienced no mould this year, and a much cosier living space.

However we have just noticed one internal wall has efflorescence and is reading damp on the cheap Amazon meter we have (around 30%). This is the under stairs storage room so we don't use it often and I'm not sure how long it's been like this. This is a solid brick wall that goes into the ground, however it is in the middle of the property (terrace property) so I don't think it can be water in the ground from rain. We have neighbours on the other side, so I did wonder if it could be something coming from their side, however the worst of it seems to really be internal on our side. None of our external walls have any issues. On our internal walls, we used some sand/cement, some plasterboard. I *think* this wall has sand/cement render on it, over brick. The paint is a breathable Lakeland paint.

There are also no water pipes under this wall. We completely re-did all the pipework/plumbing, with brand new pipes from the street supply, and they don't run under this wall.

We do have underfloor heating within the polished concrete floor, however there are no damp marks on the concrete (and also, I think we'd have boiler pressure issues if there was a leak in the UFH?)

The other side of the wall looks and seems dry, however the other side is waterproof tadelakt, and we had waterproofed the wall (it is a bathroom with just a single bathtub in it, but very seldom used - probably used it four times since we moved back).

Our neighbour had mentioned to us that prior to us buying the property, there had been a leak from a water pipe under that wall. But that would have been 2.5+ years ago... I can't imagine the wall could still be damp from then, and only now just drying?!

Completely at a loss.. we are thinking to hack off the affected part of the wall and assess the brick below, but I really don't know how we can locate the source of the water (it is a polished concrete floor, so we can't just break that apart).

Any ideas?

Thanks for the reply.

The concrete floor does have a proper DPM, yes. The original solid brick wall is Victorian so I don’t think that has any DPC (but nor do the external walls, and those are fine).
If it’s the old salts, could that reasonably be attracting enough moisture to get it to 30% damp?

I don’t know the humidity in the air in rest of the property but I think it’s below 60%RH as we have dMEV & decent ventilation.

1) take the plaster off the wall

2) worth having a listen to the wall/floor as a leaking pipe can be audible

3) let the wall dry out for 6 months


I wory about condensation in cupboards, if it is cold in there then moisture will migrate there from places where partial vapour pressure is higher

Thanks, I have a hygrometer but I haven't calibrated it yet, so I'll go ahead and do this today as a priority. The storage area is heated (we did install UFH in there). We are obtaining a thermal imaging camera to help us detect if it's a leak or not. If not, then we'll take off the plaster and let it dry, with keeping the door to the cupboard open to let air circulate. Thanks all!

how high does the water table get?
to me efflorescence is more likely to be salts from groundwater than dampness from lack of ventilation (that would more likely show up as mould)

Posted By: jfbhow high does the water table get?
to me efflorescence is more likely to be salts from groundwater than dampness from lack of ventilation (that would more likely show up as mould)

This is what I would have thought, I'm not sure it can be condensation / lack of ventilation.

Lime is not a cure-all anyway. I have some internal walls i my basement which I plastered with lime rather than giving them the full drainage membrane treatment which I gave the external ('buried') walls. The lime discoloured and fell off. I have 'naked' brick now, as a 'feature'.

Back to the beginning. Beware damp meters as an accurate indicator of damp in masonry. Tony has a way of telling what is really going on.

And back to Kristeva's comment re slate dpcs, I think it's a bit of a regional thing. You find very few late Victorian houses with slate DPCs where I live in S.Yorks.

Shellac primer used on a friend's wall having problems 10 years ago. That was an internal wall.

Hi PinH - shellac primer worked on the problem at the time and still working. Zinsser stuff.
I think we thought it would block the salts leaching out.