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Damp Problems and Cavity Wall Insulation 

So you think you have cavity wall insulation?
I did - but all was not what it seemed....

Damp caused by voids and damp caused by condensation within the cavity wall.
Here's what happened and how I cured it.

Feb 2009
In the Winter of 2009, I decided to have my cavity walls insulated and contacted British Gas, they were offering a subsidised installation at a few hundred pounds. British Gas passed on the work to a company, Mark Group Ltd, who sent out a surveyor and a few weeks later the installation went ahead.

Now the job of a cavity wall installer is probably not the best paying and it must be pretty boring work, but the guy who turned up to do the job had as much enthusiasm as a vegan in an abattoir, he hardly spoke, never smiled and really looked as if he could not be bothered.

However, he set off to work and not wishing to breathe down his neck whilst he was working, I kept a reasonable distance, paying him a visit occasionally to see how he was getting on.

Half way through the job his miserable face appeared at the door and said, "Can you check something for me?",  "Yes" I replied, "Can you go upstairs and see if there is any insulation going into the upstairs, I have gone through a whole bag of insulation on this section and it's still not full."

I went upstairs and found nothing. Obviously, there was something else wrong. I went outside and took a look at what he was doing. The area in question was almost at the top of the gable end and the hole he had drilled was right into a chimney stack. If he'd just raised his head by just a few degrees he would have seen the chimney stack.

At this point I suddenly realised that the guy doing the work was incompetent and this threw into question the quality of the whole job.

As the chimney had been blocked off inside and was never to be used, I suggested that this might be OK (not wanting him messing around in my rooms trying to get it out, soot and all).

He then progressed through the rest of the house only stopping at the front bay windows, he informed me that he could not do over the bays as there were not enough bricks between the top of the bay window and the roof (we have a Dorma Bungalow) and as the bricks were rendered, he could not guarantee not pushing in a brick whilst drilling. I told him to "leave it then", he finished the rest of the building, packed up his gear and took his miserable face away.

This was in the February, the difference in the house was immediately noticeable, it felt warmer and the warmth was evenly distributed, no cold areas as we had before. I was happy that the job had gone well after all.

Boy was I wrong!
Another Winter went by.....

June 2010
By the end of the Winter of 2009/2010, a large damp patch had appeared in an upstairs room on the gable end. This was next to the chimney, but not the chimney itself. I therefore concluded that this was a cold patch and that the insulation behind was inadequate and all the moisture in the room was condensing in this one area.
I contacted Mark Group Ltd and they said they would send out a surveyor, the surveyor, a sub-contractor for Mark Group said any void within the cavity would cause a cold patch and possibly result in condensation. He reported back and it was arranged for someone to come out and sort it.

On the day he was due, I was having all the windows replaced and one of the downstairs windows on the gable end was removed. I peered into the cavity at the side of the window. To my horror, an area of approximately one square metre next to the window, was completely void of cavity insulation, how could this be?  There were drill holes next to the window, perhaps it had been just drilled but not injected.

I again contacted Mark Group Ltd, they said that when their man arrived that day to fix the other problem, I was to ask him to re-do any area I thought I had a problem with.

An hour later, to my surprise and chagrin, the same "miserable guy" who had made a bodge of the initial insulation work, arrived to make good.

He was not happy....
I was not happy....
He asked me where else I was having problems with. I told him to do just the one job he had been sent to do and to leave it at that.

He did and left.

I contacted Mark Group again and expressed my concerns about them sending the same incompetent person to do the remedial work. They immediately agreed to send another team that had just finished a cavity wall insulation job nearby.

That afternoon a van drew up outside and a pair of lads jumped out, all full of beans and enthusiasm, they said that they would re-do the whole house. The two, a pair of Lithuanians, were efficient and friendly and kept me informed of their progress throughout the job. They had no problem doing over the bay windows, they re-drilled and filled all the cavities all around the house and when they finally left, I have to say, I felt at last, a proper job had been done.

November 2011
A spare bedroom on the ground floor at the back of the house suddenly went damp along one wall. This had not been there the year before, something had changed.

The damp was along the whole length of the wall to a height of approximately 800mm.
When I say damp, I mean sodden, the wall was ringing wet and mould was getting a grip with a vengeance. What was happening? I checked for roof leaks over the cavity, ground water penetration and through wall penetration. I found no obvious cause.

I pondered the possibilities....

Could there be a void in the cavity wall caused by the lack of cavity insulation material?  If so, is it likely to be for the full length of the wall?  I could not see this being possible, I had seen the wall being filled, twice already.

Could there be water under the floor on the inside of the building that was rising up the inside brickwork?

I decided to take a look under the floor close to the wall and removed several floorboards. The space below was maybe 400mm onto a concrete base, a little damp but not soaking wet.

On closer inspection, I noticed a brick missing at a position that was directly opposite an outside cavity vent. I put my hand in and felt the insulation in the cavity. It was sodden, there was also an area where I could feel some insulation was missing and, in that area, the outside cavity wall was dripping wet.

After sitting and thinking about it for the afternoon, I concluded that humid air from under the floor was making its way into the cavity and condensing on the outside wall of the cavity, thereby soaking the cavity insulation and feeding the spread of dampness throughout the wall.

I decided to dry out the cavity and seal everything on the inside to prevent any moist air getting into the cavity.

A hair dryer poked into the cavity and a 2kW electric fan heater trained on the wall for a week (nights excluded for the hair dryer, didn't want a house fire) raised the room temperature to 35C, the cavity within the wall to over 60C and this eventually dried it out, as far as I could tell.

I then fitted a vent with a duct that passed from the outside wall through to the inside wall and sealed its edges so that no air from inside the underfloor area, or outside air, could get into the cavity. To be absolutely sure, I injected expanding foam around all the joist ends that were resting on the internal wall and any other tiny hole I could find that would allow the passage of moist air into the cavity.

December 2011
It is now one month later and the damp has subsided. The wall still feels dry apart from one small moist patch. I will have to wait and see how it dries out. There may be still a lot of moisture in the cavity and this will not clear until after the Summer.

In conclusion.
Apart from a very small patch that looks slightly damp, there has been a major improvement in the situation.

Initially, the damp started when the outside temperatures dropped below 12 degrees C and the wall became absolutely wringing wet. Since the rectification work the outside temperatures have dropped to freezing and the wall is still very dry.

I am sure that the slight damp patch remaining is just the last of the moisture held by the insulation and penetrating the wall. This is not surprising as the cavity wall insulation was sodden and even though I tried to dry it out, I am sure that some areas were left wet. The damp patch that remains is located at a point furthest point away from where I was heating the cavity and so could be still wet behind. 

This patch is not low down and is approximately 500mm from floor level covering approx 1 sq ft. Considering that the damp covered almost 18 sq ft at the start, I would say that this is a major improvement.

I fully expect this to improve even more over the coming months.

I will update the situation as soon as changes occur.

Chris Heaton
18th Dec 2011


Update: 7th Jan 2012
All dry at the moment - even remaining small patch has dried out.

Update: 23rd Jan 2012
Still Dry.

Update: 16 June 2012
Still Dry.

Update: 11 March 2016
Still Dry.


See also: Fix your squeaking creaking laminate floor



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