Will insulating my cavity walls cause damp?
Posted by Tasha Kosviner on 29 September 2014 at 9:01 am
Q. I have a 1930s bungalow with brick outside skin and breeze block inner skin. Following a visual scoping by the prospective cavity insulation installers, I have removed some bricks and manually removed rubble. But I can’t easily get to the rubble at the very bottom of the cavity. The rubble is three or four courses below the damp proof course. As this is so far below damp proof, can I assume that it will not cause damp?
A. This is a good question.
As you know, any rubble that sits in a cavity wall, whether that’s mortar sitting at the bottom of a cavity, or mortar that sits on the ties in the wall – so called snots - can cause damp if a cavity is subsequently filled with insulation. This is because the insuation stops any air movement in the cavity that would otherwise evaporate any rain water penetration.
Snots are particularly susceptible to damp because water they provide a bridge or rainwater to travel across.
Before filling a cavity there are three vital things you need to do:
1. Make sure the outer skin of the cavity is completely sealed so that water cannot get into the cavity through cracks, holes or from the top of the cavity.
2. Make sure the cavity is completely dry so that you are not sealing damp in when you fill it.
3. Make sure a cavity is completely free of rubble and snots before you fill it.
However, from what you say, the rubble is all below the damp proof course, so there should not be a problem.
The bigger problem will be snots on the wall ties. You must be 100 per cent sure they are all dealt with before you have your cavity filled.
If the insulation installation company have done their scoping and are happy then you should be reassured. If you’re still worried, it’s worth getting another company to investigate as well. There are also businesses who specialise in cavity wall cleaning, using vacuum technology to remove old insulation, rubble and snots. They might be worth looking into.
If there is any doubt over the quality of the cavity then you could consider internal or external wall insulation which, although more expensive, is less likely to cause damp problems.
More informationDoes cavity wall insulation cause damp?
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