Diagnosing dampness in a Victorian-era cavity wall

Dave N · 2019-11-30T16:44:55+00:00

I'm trying to get to the root cause of dampness in the front wall of a small Victorian mid-terraced house, built around 1870. I've owned this house for many years, but the damp problem is relatively recent. Here's the history:Bought this house in 1991. The house is unusual for the era in having a cavity wall at the front, and the cast iron wall ties were failing- these were replaced with stainless steel ties. I believe that particular area had unusual local building regulations, and wall tie replacement has been big business round there. There is no evidence of a DPC, but the bottom 25cm or so of the wall has been rendered outside. The surveyor's report didn't point out any damp on the front wall at the time.In 1993 the council raised the pavement level outside the house so that the pavement now slopes towards the front of the house (it has no front garden). The level where the pavement abuts the house is the same as before but with the addition of a drainage channel along the front wall. The street has a gentle slope and water from the channel can drain away across the pavement further down. There is some water splash from the pavement sloping the wrong way but this didn't cause any noticeable internal damp while I lived there.Moved to a different area in 2002 and let the house. My last tenant (who generally looked after the house very well) started pointing out internal damp at low level on the front wall in 2014. Eventually I had the front wall repointed (it needed doing), along with raking out the cavity and fitting a couple of extra air bricks. I also checked that the window sill was properly sealed. However, we're now 4 years on since that work was done and the damp hasn't gone away. It is evident as small blemished areas of plaster and a little mould, and the wall feels damp to touch.I've gone over the wall with a cheap moisture meter (I know it isn't accurate but gives an idea at least) and am getting meter readings around 30-35% near ground level and...

Nov 30, 2019 Last reply: 6 years ago 42 Replies

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Brian Reay 6 years ago

Eldest had a problem in a flat they rented. The damp was mainly low level on an outside wall.

Landlord, who was pretty good, got someone in to do a damp survey and, as eldest and fiancée were at work, I attended to let them in etc.

The chap told me the damp was caused by condensation, which I found hard to believe, but he explained if there was mould on the surface in was condensation. Damp from the ground ?carried? salts etc which didn?t support mould.

The landlord had one of those fancy heat exchanging ventilators fitted and the problem soon went away.

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Dave N 6 years ago

Condensation has to be considered as a likely cause, of course, but the history of the house and the fact that the damp is clearly emanating from one corner would suggest that this probably isn't the main reason for the damp.

Three things suggest that it probably isn't condensation: 1. The problem has only occurred since about 2014 (possibly the exceptionally wet winter of 2013-4) and the same tenant lived in the house for ten years from 2009. He was pretty assiduous at reporting potential problems and would almost certainly have reported the damp earlier if it had been noticeable 2. There is evidence of damp on the outer skin (moss growth) and in the cavity (the sand I scooped out at the bottom was damp) and 3. The internal damp was clearly emanating from one corner and there's very little evidence of condensation even in the bathroom so I think the tenant was pretty attentive.

So I suspect it's penetrating damp - caused by an overflowing gutter, poorly sealed cable inlet or pooling in the drainage channel abutting the front wall. I've hopefully eliminated the first two now, and if the problem persists I'll need to take this up with the council as they constructed the channel.

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Brian Reay 6 years ago

All good points.

As for the approaching the Council etc, good luck with that. While I've never been in that position, having heard of people who have found themselves adversely impacted by Council work, instances of their 'winning' are not high.

One other point, and not made to suggest your are wrong, in the example I quoted, the damp wasn't in the bathroom (were the ventilator was fitted) or even in an adjacent room. The damp problem was on a 'cold wall', low down, in a bedroom.

Youngest daughter lived in a (student) house which was also plagued with damp. That was traced to an gutter problem. In her case, the landlord wasn't so quick to act. Once the gutter was cleared, the problem cleared quickly but, I suspect, damage had been done to the 'fabric' of the building which would need remedial action in time.

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