2 years ago I had cavity wall insulation installed in my property (built 1911), it now appears I have underfloor condensation.
The ground is bone dry, but the floor boards and joists in direct line with the air brick are suffering badly with condensation.

It only affects the area directly in line with an air brick, could this be caused because the cavity wall insulation is preventing the cold air to rise up the cavity, and is forcing excessive cold air under the warmth of the house.

It was brought to light when dropletts of water had been dropping on a junction box causing fusing problems.

Any suggestions would be greatly appreciated

I have already blocked up what I consider to be the worst affected air-brick, how many more can I cover up without causing other problems?

At ground level there is 3 at the rear (blocked 1 up), 4 down the side (end terrace) and 2 at the front.

Thank You

I'm no expert, but when it comes to condensation the answer is always More ventilation, not less.

I would be looking at checking that all the air bricks are free of blockages as a starter.

Chances are the cavity wall insulation caused some blocking in the first place....

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Verwood Handyman
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Hi bob,

Pre 1930's properties do not usually have a cavity - not saying it doesn't but its unlikely. Does the property have a DPC (Damp proof course) at least 6" above ground level? Again being built in 1911 I doubt it even has a DPC!
If your property does not have DPC but does have a cavity chances are the injected wool (assuming it is mineral wool) is sitting directly on the ground and acting like a sponge. Air bricks should be sleeved from outer to inner leaf of bricks so they can't be blocked by insulation. Air bricks supplying ventilation under the floor boards are also usually below the level of the DPC meaning they shouldn't have insulation anywhere near them!
Who surveyed/fitted your insulation were they regulated by the BBA and CIGA?
Cavity wall insulation does not cause damp contrary to some belief. Any damp should be identified at point of survey and if it is present needs to be either resolved before the fit or omitted using a cavity brush.
In addition the air bricks should have also been checked post fit to ensure no insulation is blocking them.

Also on a personal safety note. If you have any fuel burning appliances in the room/rooms where you have blocked vents, get this sorted ASAP or you run the risk of carbon monoxide poisoning.
If you were issued a guarantee and it is as mentioned above this is null and void as the property should have never been insulated in the first place.

Hope this helps
http://www.ciga.co.uk http://www.bbacerts.co.uk/