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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 information

YouGen guide to cavity wall insulation

Does cavity wall insulation cause damp?

Find an insulation installer

Need help with any Jargon?


If you have a question about anything in the above blog, please ask it in the comments section below.

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4 comments - read them below or add one

Cathy Debenham

Cathy DebenhamComment left on: 17 November 2014 at 11:44 am

Sorry to hear that you are having problems with damp. Cavity wall insulation usually comes with 25 year insurance/warranty, so it's worth checking what you've got. You'll need expert advice on what's gone wrong, whether the installer was at fault, and what ways there are to remedy it. It may mean that you have to have the insulation removed. A good place to start would be the National Insulation Association where you can search for members near you.

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Diji85Comment left on: 13 November 2014 at 7:26 pm


We had our walls insulated a couple if years ago and now suffer from damp and mould especially along a certain stretch of wall. Reading the stuff on here it wouldn't surprise if the company who done it didn't dry out the walls or get rid of 'snots'. Is this something that would be easy to rectify now or a tough job? What would our next move in getting it sorted be? 


Thanks for any advice

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Gilly Jones

Gilly JonesComment left on: 1 October 2014 at 10:09 am

Hi Philip

Cavity wall insulation is a great idea if you have a clean cavity. Do check out information on our blog does cavity wall insulation cause damp?

YouGen team

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Philip Smith-Lawrence

Philip Smith-LawrenceComment left on: 1 October 2014 at 8:10 am

Not being a builder, I have always thought that was a very good reason why there are caivities in walls! I understand that buildings need to be air tight, but does this mean that if all the steps mentioned in the article are not followed that underataking cavity wall insulation will cause more problems than it will solve? 

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