Suspended wooden floor insulation - vapour barrier?

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Gomez999
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Joined: Fri 26th Oct, 2018 11:07 pm

Suspended wooden floor insulation - vapour barrier?

Post by Gomez999 » Thu 17th Sep, 2020 11:01 pm

Hi all. Just about to insulate between the joists in cellar which has dining room above. Earlier in the year I took down the old cellar lath and plaster ceiling. The cellar is pretty damp which is controlled through decent ventilation. The dining room above which is open plan with lounge next to it (lounge has solid original flag floor) suffers from high humidity levels and I can't work out whether this is from flags or from cellar (or both). Is it wise to install some sort of vapour barrier when I do the insulation? I am favouring cutting PIR boards to fit between joists, securing with batons where necessary and ensuring any gaps are filled with foam . The bottom of joists will still be visible so I can check their condition over time.

paulc
Posts: 344
Joined: Sat 22nd Oct, 2016 7:05 pm

Re: Suspended wooden floor insulation - vapour barrier?

Post by paulc » Fri 18th Sep, 2020 10:17 am

If you are using PIR/PUR foam boards with a foil facing, you shouldn't need an additional vapour control membrane. The foil will be impervious to moisture, and the foam boards will be pretty resistant too.
Recently finished insulating a wall with two layers of Kingspan boards. I taped the joints on both layers with foil tape and didn't bother with a VCM. Didn't see much point with the foil doing the job for me.

Gomez999
Posts: 9
Joined: Fri 26th Oct, 2018 11:07 pm

Re: Suspended wooden floor insulation - vapour barrier?

Post by Gomez999 » Fri 18th Sep, 2020 10:50 am

Great - that's what I was thinking. Thanks for your reply.

paulc
Posts: 344
Joined: Sat 22nd Oct, 2016 7:05 pm

Re: Suspended wooden floor insulation - vapour barrier?

Post by paulc » Fri 18th Sep, 2020 12:21 pm

Tip with the foam: Do use a gun with a screw on canister. It is much easier to lay down a thin bead of foam compared to the hand held cans. Working overhead, you'll want to use a pit of flexible tube on the end of the gun too.

Grab a cheap gun off ebay, and don't forget to give it a real good clean with acetone after use.

Gomez999
Posts: 9
Joined: Fri 26th Oct, 2018 11:07 pm

Re: Suspended wooden floor insulation - vapour barrier?

Post by Gomez999 » Fri 18th Sep, 2020 11:59 pm

Awesome. Thanks.
paulc wrote:
Fri 18th Sep, 2020 12:21 pm
Tip with the foam: Do use a gun with a screw on canister. It is much easier to lay down a thin bead of foam compared to the hand held cans. Working overhead, you'll want to use a pit of flexible tube on the end of the gun too.

Grab a cheap gun off ebay, and don't forget to give it a real good clean with acetone after use.

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