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My cavity wall insulation is soaking wet. What can I do?

Posted by Tasha Kosviner on 13 June 2014 at 10:30 am

Q: We bought a 1930s house which we have been updating including installing french doors. Late last year serious damp patches started spreading on each side and across top of doors, as well as in many other places on this wall both upstairs and down. The house has cavity walls which have been filled with insulation (like shredded cotton wool) probably some years ago. We have used blow heaters and dehumidifier to try and dry it out with some limited success. We are now getting ready for boiler to be installed on this wall and have drilled a hole for the flue. The insulation exposed in the cavity is soaking wet so that grabbing a handful of it and squeezing it brings a stream of water. We have taken advice from various sources and are painting the outside of the wall with a waterproofing agent. Is there anything that can be done to dry out the cavity or remove the insulation while we have the opportunity to access in a few places? We hope soon to have central heating system running - will that be enough to keep the internal skin dry?

A: I am sorry to say that this is what sustainable homes expert Tim Pullen calls the 'ultimate nightmare situation'.

Now that you have water in your wall, it is no longer an insulator but a thermal conductor, actively carrying heat out of your home through the walls. This is quite apart from the damage and mould that is building up on the inside of your house. To prevent further damage and energy loss, you urgently need to identify where the water is getting in and stop it.

Tim says the cause of the water is probably rainwater penetrating the outer skin. Unless a wall is waterproofed, the weather will be penetrating the bricks and causing water to flow down the inside of the wall.

Once you have located and repaired any acutal leaks, then painting the outside with a waterproof agent is, as you have been advised, a good emergency measure. However, to be really sure that you are weatherproofing this wall, Tim advises that you should consider installing an external insulation and rendering it with a waterproof render. This is a big and expensive job but it is the best way of ensuring that your wall is truly protected from the elements.

Next you want to look at drying out the cavity. Realistically, it will be almost impossible to remove the insulation that is in there, so you need to try and dry it out. It sounds drastic, but to do this, Tim suggests you drill a lot of large, low level holes internally in order to try and draw the water out. 

"I'm talking four inches in diameter and every meter or so, all along the wall," he explains. "Pull out as much of the wet insulation as you can then leave the holes there for six months to a year. You need to get all that moisture out before you re-seal the holes. 

Assuming this is successful, the final question, is what to do with your chilly wall next. If, as suspected, your wall is very exposed, this is a classic example of a cavity that should never have been filled in the first place. Exposed cavity walls need ventilation in order to dry out after heavy weather and cavity wall insulation effectively deprives them of this opportunity.

A far better solution would be internal or external wall insulation but this must only be attemped once the wall is truly dry.

I can offer you one glimmer of hope here: I put in a call to the Cavity Insulation Gurantee Agency (CIGA) on your behalf and they very much want to hear from you. It is possible that your intial installation is still under their 25-year guarantee. If you have the original paperwork from when the insulation was put in, dig it out and see - 90 per cent of cavity wall installations these days are protected by CIGA. Even if you don't have the paperwork you can call CIGA up. If your installation is protected, they will have a record of it under your address. However, you should manage your expectations, it's possible that your subsequent works, such as the installation of the doors, could have rendered the guarantee invalid.

If this is the case, and you do manage to dry out your wall, then more help may be at hand to help you re-insulate. If your wall is, as expected, very exposed, then it will be classed as a 'hard to treat' cavity wall for the purposes of government subsidies. Have a look at our sections on the green deal, energy company obligation and the green deal home improvement fund to find out what grants you may be entitled to for improving your insulation.

Good luck.

More information

YouGen guide to insulation

Does cavity wall insulation cause damp?

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

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2 comments - read them below or add one

Nigel Donohue

Nigel DonohueComment left on: 26 March 2019 at 9:08 pm

Not good advice.

Mineal wool can hold upto 8 times its weight in water and holds it for an awful long time. The only solution to wet insulation is full extraction and to let the walls dry out - that means sucking it out with machines. It's not a DIY job.

Any sort of waterproof paint is a terrible idea as it will lock the mosture in causing irreprable damage to the brick work which will freeze and spall in winter.

1930s cavities were designed to breathe. 

As to the suggestion of external wall insulation to a cavity wall. it's an expensive solution - even on this site the suggestion is a 15 year payback.

Despite what the industry say. An empty cavity offers far better protection against penetrating damp getting into the inner leaf. The BBA have proved this in the experiments.

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ExtractUK Ltd

ExtractUK LtdComment left on: 29 October 2014 at 8:27 pm

This comment was removed: spam

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