100 year old house. Wall 12 ft X 12 ft newly plastered. I had originally taken off the old plaster because of damp. Could not see any obvious sign of where coming from, actual brickwork was dry ( maybe as we had a dry spell) and thought was condensation as I had foil behind radiator. Wall has dried out completely except for an area 2 ft x 2 ft in the bottom near a corner. Have done the wall with dpc injecto cream (Sikamur) last week. Nothing against the wall outside. Went round the window outside anywhere vulnerable with mastic. Any bad pointing was raked out and repointed. Where can it be coming from? Is it coming through the bricks from the outside (the bricks do look a bit porous in places). Would painting the wall with a damp proof solution outside help at all or just seal it in. I am going nuts!

needawin, November 2011

Hi,
Are you sure it's not condensation? Any damp will always find the coldest spot on the wall. Is this wall with the damp on a north face? Have you got plenty of ventilation in the house? I assume it's a cavity wall. Could be a wall tie be bridged (mortar on it allowing damp to track across)? Easy way to check this is call a chap in who does wall ties and he can check it easily with his boroscope looking into the cavity. The painted on silicone stuff on the brickwork may work if the brickwork's sound but even then it won't last very long. If it's porous, you're wasting your time. Even put on with a pressure spray, it's not effective. But you shouldn't have any sign on the inside wall even if the outer skin is saturated if the wall ties are clear. May sound daft, but have you got a leak from your heating system under the floor? We had that years ago when we had a new system installed. Seems you've done most things apart from the suggestions here. Not much else left! My guess would be condensation, particularly if it's a north facing wall. That or a bridged wall tie. If all else fails, then it's a call to a builder but even a good builder sometimes is stumped by damp.
Best of luck

Denis (ex wall tie installer and pointer), December 2011