Which specialist do I need?

[Straysocks]

Bathroom renovation. The room was originally a reception room that was changed to a bathroom before I purchased it. The floor was taken back to the ground, joists replaced and membrane put under them. Chimney breast (capped years ago) had plaster removed half way up as previously drawing salts from the ground. That area re-plastered and whole room skimmed. Mist coat on, Lino down, new bath suite in and there are crystals forming against on chimney breast but also on opposite wall. Under sink cupboard is damp when doors kept closed.

No extraction fitted yet, though a space prepared for it. Bathroom has been used about 15-20 times for showers (some long) and baths, ventilated with window and door open. I don't think this explains the scale and type of problem alone.

I'm aware that remedies have moved away from 'damp-proofing'. It seems likely everything will need to come out again but whose expert advice should I seek on this? I've used tradespeople rather than a building company, great work elsewhere in the house but the bathroom v problematic.

Thanks for reading, appreciate advice.

Is the bathroom ground floor? Which walls are external? Can you take photos of the outside walls, focusing on the guttering, the bit at the bottom of the walls and any pipework which goes through the walls.

It is ground floor. The chimney wall in internal (neighbour) as is the opposite wall where the crystals are newly occurring (dining room). The only external wall doesn't seem to be problematic. I'll take some photos later today when home. Thanks for responding.

So you are mid terrace? Sounds like it might be an issue with sub floor ventilation. Do you have air bricks or a cellar?

Yes, mid-terrace. Air brick only, maybe need to add more? Guttering is just at edge of roof atop first floor, nothing nearby and no stains suggesting leaks. Think you're correct about sub-floor ventilation?

Given where the air brick is, which should be below floor level, the external render is breaching your damp proof course. But I would expect that to cause issue ls on that wall, not the walls shared with your neighbours.

Adding another air brick could help but might be difficult given where the soil stack is because you want to encourage flow through the space and so want a hole far away from the existing one.

Just reread your original post. The membrane they put under the joists, could that be totally blocking the airflow? Or was it just under the ends of the joists?