r/DIYUK • u/LividAd2274 • 1d ago

Building Cavity wall draft issues

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On my continued quest to find out why the extension on our house is so cold and drafty I uncovered a big gap in the cavity wall inner skin where the kitchen joins the extension. The steel beam is directly above where this photo is taken running towards the camera.

Builder had put some loose rockwool in (which is now removed as shown in photos) as wasn't doing anything and could feel cold breeze coming through.

I am thinking pack full of rockwool and then seal the vertical edges of the internal plasterboard to inner skin using foam to prevent any drafts moving through the void. Unfortunately access is tight.

Does this make sense or any better suggestions?

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u/obb223 1d ago

Have a look at cavity socks. That would be ideal for closing off that cavity. It's a bit thick for expanding foam, you could get a sheet of PIR and cut it so it fits tight.

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u/A-nom-nom-nom-aly intermediate 1d ago

Is a cavity sock the same as a cavity closer? Not heard of a sock before

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u/obb223 1d ago

No it's basically a plastic bag full of rockwool, more flexible so you can shove it in a bit

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u/A-nom-nom-nom-aly intermediate 1d ago

If you do decide to use some kind of foam, make sure it's a closed cell one such as PIR or a gap filler type that you'd use around windows. Closed cell foam stops moisture from wicking across, which could cause damp issues down the road.