Mystery internal efflorescence

[HouseOfCaz]

Hey all,
I bought a Victorian terraced house 3 years ago and a year ago efflorescence started occurring in a horizontal line in my hall. I am at a loss though about what's causing it The wall is a shared wall with my neighbour and is solid. I had got it covered in lining paper before but have since stripped it and it's still occuring. I've checked the roof inside and bedroom above and no signs of damp, I've asked the neighbours and they had a tiny amount of it but it's now gone, they have a radiator on their side so it's not damp their side. It's only in the centre on the wall so not rising damp and I've looked at the outside of the house and the pointing seems ok. I'm baffled, any suggestions most appreciated!!

 

[mrrusty]

The clue is "Victorian". You can guarantee that for a greater part of its life this house had coal fires. It would also have had lime plaster, and will have lime mortar. Combustion gases + water (either vapour or penetration) + lime results in various nitrates, sulphates and chlorides accumulating as salts within the bricks and mortar. These salts are hygroscopic and will suck water vapour out of the air and make the wall appear damp.

I expect the lower part of your wall up to the tide mark has been sealed with a cement-based render isolating the salts, whereas the upper part hasn't, exposing the salt contamination to the air.

In my experience the solution is to isolate the salts in the wall. On that piece of wall I think I would use 9mm plasterboard fixed with foam adhesive (effective isolation - don't use water based dab adhesive). Skim it up, and I doubt you'll see it again.

We have learned a lot about this phenomenon in our own 1902 refurb. "damp" spots half way up an otherwise dry internal wall where dabs are is a good example - the water in the dabs draws the salts out, and as the dabs dry through surface evaporation, the salts get left behind on the surface, creating "damp" patches.

 

[tomm12]

Can you show a photo of the outside of the door frame & the party wall division between your door & the neighbours front door? Where on the party wall Is the second photo showing? Is it an interior hall or a porch? Is that solid plaster or plasterboard?