Damp and blocked cavity

At one of our recent workshops we were alerted to a damp problem in an Edwardian house in North Shirley. The side (SW facing) wall had cavity insulation fitted approximately 20 years ago. The damp is just above the skirting  running the length of the hallway which is adjacent to the south west wall.

Damp wall by staircase

There is a cupboard further towards the back which had signs of mould as well as the damp:

Damp wall in cupboard with mould

We assume that the mould there is mostly due to lack of ventilation, along with possible condensation due to the warm air from the boiler. It would be worth putting some ventilation in the door or the side of the cupboard. Any residual heat from the boiler would also help in warming the house.

Beyond that cupboard is the kitchen, and we could see under the cabinets that at some time before the current occupier there appears to have been an injected damp course.

Recently, a damp specialist company had been to do a “free survey” and diagnosed rising damp due to failed damp course and proposed an injected chemical damp course and replastering over a waterproof membrane (“tanking”). However we were told that they did not even ask about the cavity let alone investigate it, nor did they investigate the kitchen wall. We believe that such a treatment could stop the damp inside for a while but is more of a sticking plaster remedy rather than a cure for the root cause of the damp.

Our concern was that the actual cause of the damp (as is often the case) was not a failed damp course but cavity blocked with rubble etc around the original slate damp course and so causing a bridge for damp to access the inner wall.

We removed a brick on the outside which was immediately above the damp course and found the cavity to be completely blocked by wet lime and brick rubble. The disintegrated lime was like clay and was blocking the cavity to such an extent we originally thought it was a solid wall. Normally when you remove the debris from behind a brick hole, anything above will fall down but this was quite wet and compacted so it was stuck up there just beyond reach.

This photo is looking upwards inside the cavity, all you can see is muddy lime, no insulation.

Up inside the cavity

This was about two courses above the hole we’d made, so we decided to take another brick out further up and after removing more muck we could finally see some insulation at the top of that hole:

Second hole

So in all we had five courses of debris above the damp course, much of which was just lime mortar that was so damp you could mould it like putty:

Damp lime

Part of the reason it was so damp is that the prevailing wind is south westerly bringing a lot of rain from the English Channel. This debris must have been there when the insulation was first injected so was either from damage to the wall during the war or (more likely) just lazy bricklayers in 1905 chucking all their spare muck down the cavity. Cavity walls were very new in 1905 so they would probably have no idea that they should be kept clear.

We believe this debris was probably the main cause of the damp and should be removed, allowing air into the cavity. We suggested that this lower part of the cavity should be cleared, and that this is worth doing before even thinking about expensive and probably unnecessary chemical damp course and tanking internally, which we think won’t be necessary as the damp will probably dry out once there is some air to the bottom part of the wall.

While we were there a neighbour who is an experienced professional builder living in a similar age house turned up and agreed that in the first instance the cavity should be cleared to allow air flow and that this would probably allow the wall inside to dry out.

We also noticed that the shingle pathway itself was quite high, and rose above the damp course at the back. We advised digging this out to at least two courses below the slate dpc.

The wall itself has been repointed with hard cement mortar, probably after war damage, and the repointing is now showing some signs of beginning to fail. This is a common issue when lime mortar has been repointed with a cement mortar. This would also be a likely culprit in making the wall damper.



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