Cavity wall damp problems

While we had floorboards lifted to carry out the underfloor insulation, we noticed that where the joists met the outside wall of the room, they were sitting on a timber plate that ran along between the brick courses immediately above the original slate damp course. Although the timber plate looked OK, we tested it with a damp meter and found it did have a fairly high reading. [EDIT: since then we have discovered that damp meters are notoriously inaccurate for plaster or masonry – even if they have a specific setting for it). It was obvious that the cavity was blocked with rubble, lime dust and bits of yellow spongy insulation which was breaching the damp course.

We had been aware of a very slight smell in summer months. Not a musty or moody smell but just a vague whiff of damp plaster.

We asked various damp surveyors and local builders who were all of the opinion that the main issue was probably due to blocked cavities which could be breaching the damp course. Any issues could possibly be made worse by an 80s retrofit cavity insulation, which has the potential to  drop into the lower part of the cavity and act like a sponge to retail moisture.

We then discovered something rather disturbing. Outside this room a patio had been built up. On the exterior wall all looked fine because the patio was not built above the damp course, but we then discovered that the internal damp course was actually four courses of bricks lower – so the patio was actually way higher than the internal damp course. This issue  was compounded by the fact that the cavity itself was blocked above the internal damp course with rubble, old lime dust and chunks of mortar (aka “snots”) and a few bits of the of cavity insulation.

Although the timber plate did not look too bad from inside and could be treated, when bricks were removed to inspect from outside and the debris removed from the cavity it was a different story. It was obvious all the stuff filling the cavity had created a bridge for damp to accumulate and not be able to dry out due to the breaching of the damp courses. Here you can see the joist sitting on the timber plate, and just how rotten the plate was. Luckily none of the joists were so bad that they need fully replacing.

Damp timber

All of this timber needed to be removed along that wall and replaced with bricks.

Here you can see the internal view showing the clear cavity all nice and dry,  brick mortared in on top of the original slate damp course. We added some plastic damp proofing above the brick course. This was not really necessary as the original slate course was still fine, but we happened to have some and there’s never any harm in a “belt and braces.”

Internal view with brick

The other side of the house had a concrete footpath built up above the damp course. Both the patio and the footpath were cut using a disk cutter and a trench dug out so that bricks could be removed (every two or three bricks) and the cavity cleared. Here you can see the footpath side with debris removed (bits of rubble, mortar snots and disintegrated damp Urea Formaldehyde insulation). Note home made cavity clearing tools. The damp rubble filled the cavity to about 100mm above the slate damp course:clearing the cavity


It occurred to me that the ideal solution would be some kind of brick sealant that stopped moisture getting in, but would also allow moisture already within the fabric of the wall to dry out. Impossible? That’s what I thought. Even if such a product exists then surely it just be a scam.

But after searching the web for breathable brick sealant, I did find products which claims to do just that. One of them is Emperor Masonry Creme which has a “trust certificate” and statement about being scientifically tested. Being a profound sceptic I was about to poopoo the whole thing until I actually looked into the tests which were validated and certified by the Energy Savings Trust. As this is an organisation which is 100% reliable and trustworthy I have decided to try this product on a couple of the walls, a chimney and will report back. Watch this space…

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