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PostPosted: Wed Sep 20, 2017 9:52 am 
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Is there any downside to using mastic and/or plaster to fill the gaps between wooden joists and the joist sockets in a brick wall?

These joists form the ceiling for the ground floor so would have warm rooms above and below. Plasterboard ceiling underneath, standard wooden floorboards on top, the void between the joists will be filled to half-depth with rockwool acoustic insulation. The brick walls the joists are set into are the inside leaf of a (poorly) insulated cavity wall.

I'm planning on soundproofing the party wall between my house and next door which should address the direct path the noise is taking. The small gaps around the joists would provide a path for flanking noise from the cavity to enter my house though. I'd like to block that path but I'm a little worried that if I reduce airflow too much within the voids that might lead to moisture getting trapped and could lead to rot.

Then again the whole ceiling/floor should be at roughly the same temperature as the rest of the house, so perhaps condensation in there wouldn't be an issue? Any opinions?

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PostPosted: Wed Sep 20, 2017 11:10 am 
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In the normal course of a new build, those joists would usually be built in with a sand & cement mixture the same as used for laying the blocks/bricks.

That would be a better option than either mastic or plaster.

One day it will all be firewood.

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PostPosted: Wed Sep 20, 2017 10:48 pm 
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joist tails in wall pockets have to be well built in solid so that thres no movement

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