Hi Adrian, I guess you know there are some here who are worried about fully filling cavities on existing buildings [me included] Can you tell me what type of bricks you have on the outside wall? Also what mix you used for the pointing?

There is also more discussion about this here

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18&page=1#Item_26

The trouble with problems like this is that they are virually impossible to diagnose via forum. It could be any of the things mentioned or none of them. I think you need to get someone on site to have a good look.

I have no time for silicone based products on walls.

I think that you ought to get it out of there, use an wet/dry industrial hoover with a long piece of waste pipe attached having taken a brick out from the bottom corner of the wall.

Something is not right and the company that installed it may have some liability.

Foam may not be any good but polystyrene beads may be.

Cross posted with Tony and I agree with him

Hi Nick, You may be right there, like I said its practically impossible to diagnose via forum. Maybe the best thing is to get a Surveyor out to check. I'd be happy to go and have a look if the house is anywhere near me [South Wales]

Could it be that rain water is getting in under the tiles higher up the roof then running down the roofing felt, finding a crack in the latter just above the fascia board and then dropping down into the top of the cavity? I only mention this because in my last house the roofing felt sagged between the rafters and was prone to cracking (it was the old tarry based stuff) but although I also had a cavity full of Rockwool I didn't have a damp problem.

Following my own experiences, I generally advise against cavity wall insulation now, unless the house seems relatively newish (maybe 1980s onwards) as older houses were not designed to have the cavities filled.

My own situation which I am am having to rectify is this:

1920s house with loosefill rockwool cavity insulation.

Problem 1: Cavity is open right down to clay level, hence rockwool is soacking up moisture (extremely wet). As we read elsewhere, wet rockwool can become a good heat conductor, and where I have removed the wet insulation the wall warms up to the touch. We live in quite a damp area.

Problem 2: Insulation blocked all underfloor vents. This caused severe dry rot in one area.

Problem 3: Some water being carried over cavity higher up through water ingress through the outer leaf. I have repainted the render to try to waterproof it better, but it seems to have little effect. As the eaves overhang is good, and we are in sheltered area, this is not due to the exposed location / driven rain etc.

Problem 4: Old tarred paper DPC is breaking down, not helped by lack of ventilation to help dry the brickwork out above and below the DPC (in fact it is carrying the damp up above the DPC anyway.

Thus I have one option only, as far as I see it: to remove the insulation (not too bad as it is a bungalow undergoing renovation anyway), and cut in a new DPC on the inner leaf. I feel that is the only way to cure the problems.

I would urge anyone with an older house to think very carefully before putting in CWI, despite what the CWI sales people may claim.

Tough problem. Lots of useful speculation from previous commentators. Thoughts: you say it is a south-facing wall. Is it badly exposed to driving rain? where is the damp: top, bottom, near window etc? One possibility might be that the rockfwool has compromised the DPC over the window heads for example. Also, an easy way to rule out the roof as a source is to run a test on the affected wall. Batten some polythene on place, about 2m wide, running ground floor to eaves, and see if damp disappears. If it does, it's definitely rain penetration through the wall, and almost certainly cavity-bridging. In which case, removal is the only answer, coupled with a law suit.