Cavity Wall Insulation and Damp.

Cavity Wall Insulation Blamed Again

The potential damp problems allegedly caused by cavity wall insulation continues to be very topical at the moment. With this in mind it may be worth relaying details of another defect we were were asked to investigate this week.

We were called to London to investigate an isolated patch of penetration  damp to a ground floor bedroom.  My client had been unable to resolve this problem and was somewhat mystified as to what was causing the problem.

Leaking downpipe

Leaking downpipe

On the surface, it appeared to be a straightforward case of penetrating damp and an external check to the rear of the property immediately confirmed a longstanding problem with gutter leakage that had saturated the rear wall to such an extent that a dark patch of moss and lichen growth could be seen behind the gutter down pipe, extending from floor to roof level. The roof was parapet walled with a lead box gutter outlet feeding into the RWDP hopper. A simple problem, easily diagnosed and fixed, with the internal penetrating damp easily explained if we were dealing with a solid wall.


Brick Bonds

However, this was a mock Georgian townhouse, constructed of 320mm cavity walls with bricks laid in stretcher bond. Indeed, the brick bond was the first clue that the building was not as old as it initially appeared.

Signs of leakage to hopper head.

Signs of leakage to hopper head.

There were obvious signs of decorative spoiling and penetrating damp to the right hand side of the bedroom window and we immediately tested the area that gave highest electronic readings with calcium carbide, which returned a total moisture content of 2%. Technically, BRE 245 tells us that this would not require remedial action since it was below 5% threshold. In fact, we believe that 5% is too high and generally recommend remedial action at figures of 3% or more. Two per-cent is still significant and we were of the opinion that the masonry has dried out to some degree over the summer months and would likely revisit with increased severity as we move into the autumn and winter months.

How does moisture cross the cavity?

Since we have proved penetrating damp to the inner leaf of blockwork, and since we have a cavity wall, the key question has to be, ‘what caused the moisture transfer from the outer to the inner leaf?’

The proponents of the theory that all CWI is bad may well have liked to blame the blown fibre CWI that was installed. We consistently advise our clients against installing blown fibre CWI on the grounds that we have never yet found it properly installed and it is not inherently waterproof. It may be worth checking out our previous blog on this issue… Slumping in CWI

Thermal imaging confirmed that CWI fill appeared to be sporadic, which gave us our first clue that blown fibre was installed. We  then drilled through the wall and inspected the internal CWI fill in two locations. Even insertion of the borescope confirmed a problem with voids in the fill material. Inserting through one hole hit soft material, whilst insertion through the second hole immediately hit the hard outer leaf of brick, which of course it would not have done had there been soft CWI material present to block its path.

Borescope inspection of Cavity

Borescope inspection of Cavity

We inspected the cavity with a borescope, which confirmed that blown fibre was installed in extremely limited quantities and that, as usual,  there were voids in the material. However, it was clearly not responsible for moisture transfer across the cavity since it was incredibly loosely packed and would have slumped away from the damp area had it got wet.




Defective cavity closer

This left only one potential cause, and an issue we have encountered many times in the past… a construction defect in the cavity closer. bespoke cavity closers are inherently waterproof and insulated but we have seen cavities closed off with a number of materials that are inappropriate for circumstance. Indeed, it may have simply been closed off with the inner leaf blockwork. Whatever material was used, we believe that this is transferring moisture across the cavity, which also explains the close proximity of damp staining to the window reveal.

Poorly detailed cavity closer proved to be the cause

Poorly detailed cavity closer proved to be the cause

We have recommended repair of the external rainwater goods and further recommended that the UPVc window be temporarily removed to inspect and remedy this problem.

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