Cavities not immune from damp

Cavity walls were designed to keep damp out of homes

Cavity walls have been the standard way of building for the past century. They are designed to have two walls (bound together by wall ties) with one acting as the external face that can get a little wet and the inner one that is protected by the cavity from this potential moisture ingress.

The cavities were vented to allow air to circulate in them to help dry out any moisture that did make it through to the cavity.

All sounds grand.

However, the system has some issues with it on a number of levels.

1. Initial Build - many cavities were not well formed as cement mortar etc would fall down the cavity during the construction period. This can cause a bridge to be formed across the cavity from the outer wall to the inner.

2. Maintenance - the wall ties that were used to bind the two walls together were mild steel and this rusts. So slowly the failure of the ties can cause cracks to appear in the walls and this requires the home owner to replace the ties when needed. Cracks in render also tend to go un-repaired and re-pointing is not undertaken when needed. These issues mean that water gets trapped in the outer wall, thus making the chances of water ingress across the structure more likely.

3. Improvements - many cavities have been insulated over the past couple of decades. This fills the cavity with material that can cause a bridge to form between the two walls. Some materials that have been used in the past have also failed (mostly foams) and these have slumped to the bottom of the cavity in a bit of a mess. It should also be noted that many houses are not fully filled - companies do not like filling around doors, windows, pipes etc as their products can escape through cracks etc and they don't want the hassle of tidying up, so many cavity walls are not very well insulated.

I have come across a number of houses recently where the wall ties and the use of cement renders has caused damp to find its way into what should be a damp free house. Basically what has happened is that the cracks associated with ties and lack of regular maintenance have let water into the outer wall and trapped it there. On mainly westward facing walls this moisture has then either tracked across ties, through or across the top of the insulation, thus reaching the inner wall. The lack of ventilation in the cavity means that the inner wall then gets wet and stays wet (only drying to the internal space).

So just be aware that cavity walls can be damp and that the fundamental remedies to problem might involve:

Re-rendering or at least repairing the 'blown' render

Re-pointing where required

Removal of old cavity wall insulation and potential insulation replacement once outer wall has been repaired

Replacement wall ties (with new stainless steel ties)

So if you are looking to buy a house make sure that you test all the walls (including upper stories) for damp.

There are of course other issues like insulation at wall plate levels, tying in ceiling and wall insulation etc.

Please remember that we can provide an independent damp report for you that will highlight areas of damp, their root cause and appropriate remedies rather than the standard tick box approach that is taken by the more mainstream industry.