Can I Use Leftover Rockwool RWA45 Insulation for Cavity Wall Filling? Comparing with Knauf Dritherm Cavity Slab

fandyman · 7 Apr 2025

I have a leftover 50mm insulation Rockwool sound insulation slab RWA45 that I used in floor insulation like this one:

Rockwool Sound Insulation Slab - 50 x 400 x 1200mm | Wickes.co.uk

Rockwool Sound Insulation Slab - 50 x 400 x 1200mm

www.wickes.co.uk

Can I use it to fill in 50mm cavity wall or it is not a good idea? I cannot use anything rigid due to the fact I am replacing some bricks and there is a wall ties in place. There used to be polysterene beads that had fallen out in a section so I need to replace it with sth else.

I have also found dedicated Knauf cavity slab insulation here: https://www.selcobw.com/knauf-insulation-dritherm-cavity-slab-37

Is this comparable to Rockwool sound insulaiton slab above or do I have to buy this one specifically?

Deluks · 7 Apr 2025

Yes, there's very little in it.

fandyman · 7 Apr 2025

Do you mean I should follow on and get the knauf one instead of the leftover Rockwool for cavity wall insulation?

cdbe · 7 Apr 2025

I would use it provided I was happy that the wall won't be affected by penetrating damp. The proper cavity stuff is coated with something to repel water. Otherwise it's probably slightly less thermally efficient but that won't be an issue.

fandyman · 7 Apr 2025

I guess that is the thing - I assume if it is a standard brick & block wall with no render that would stop water penetrating it would allow the water to pass through therefore the insulation may get moist etc. Would it still work with these non rigid insulation products that are advertised as 'full fill'?

23vc · 7 Apr 2025

I wouldnt use it. The full fill cavity slabs are designed not to wick moisture etc. Building control wouldn’t pass it.

Deluks · 7 Apr 2025

The description on Wickes states that it repels water, maybe contact Rockwool for confirmation.

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Can I Use Leftover Rockwool RWA45 Insulation for Cavity Wall Filling? Comparing with Knauf Dritherm Cavity Slab

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