DIY Doctor

Insulating old Cavity Wall Where Cavity is Open to Underfloor Void

Postby rtX » Sat Sep 29, 2018 9:16 am

I'm hoping to retro install cavity wall insulation into a cavity wall where the bottom of the cavity is open to the void below a suspended (concrete block and beam) floor. An added complication is that the cavity varies between 100mm and 200mm in size.

I'm struggling to find anyone who thinks this can be done. The options seem to be polystyrene bead (with resin), spray foam like Icynene or an alternative BASF product, and blown fibre. But these all have issues as they are likely to fall into the underfloor void.

I've been advised that it's not wise to mix insulation types, although it's okay to mix some spray foams and polystyrene beads.

I need to seal the bottom of the cavity somehow so that the cavity above the newly sealed base of the cavity can be filled. I'm looking for advice from anyone who might have ideas on how to do this.

I've suggested using a suitable rapid expanding spray foam insulation to do the sealing at the bottom of the cavity and then to fill the balance of the cavity with polystyrene beads/resin, but I've been told that the spray foam won't expand quick enough itself to fill the void and will itself drain into the underfloor void.

Thanks in advance.
rtX
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