Airtight membrane on joists instead of under them?

springer111

After a lot of confusion in a new build it seems we have put up some ceiling slabs where there should maybe have been an airtight membrane. It's a bungalow with good height usable attic space. The area of particular concern has a lower piece of roof above if. It's unusable space as its only 1m high approx at the highest. I guess we should have ran the membrane under the joists before slab.

So am I left with 3 options

1 Take down slabs and put up membrane below joists

2 Run membrane up and down a d over each joist then lay 400 earthwool between joists and run membrane in to the main part of the roof to link in with the rest of the airtightness (main roof will be airtight up on rafters etc)

3 Insulate this small cubby hole area in the rafters and put membrane over that (like main roof)

Although it was a mistake I can't help but wonder is the membrane better off up over the joists. (Option 2) No penetration holes from putting up slabs. Also any wiring and recessed lights can be below the membrane and the wires will be ran in to main part of roof also. Is this a viable option. Of course it's the most akward way to do it but I wouldn't mind that

MicktheMan

1

rampantbunny

springer111 wrote: »

...
So am I left with 3 options

1 Take down slabs and put up membrane below joists

2 Run membrane up and down a d over each joist then lay 400 earthwool between joists and run membrane in to the main part of the roof to link in with the rest of the airtightness (main roof will be airtight up on rafters etc)

3 Insulate this small cubby hole area in the rafters and put membrane over that (like main roof)

Although it was a mistake I can't help but wonder is the membrane better off up over the joists. (Option 2) No penetration holes from putting up slabs. Also any wiring and recessed lights can be below the membrane and the wires will be ran in to main part of roof also. Is this a viable option. Of course it's the most akward way to do it but I wouldn't mind that

1

Re. 2, how would you propose accessing the existing AT line which I assume is right at the edge of your ceiling out at the eaves? Are you considering joining onto this AT line and bringing it up between the joists, then across the attic floor?
On my build, I ran my AT line between joists and up along my rafters as I am using my attic space. It was a very time consuming job and awkward working between the joists from below when no plasterboard was in place. Can't imagine how you'd accomplish anything like this with plasterboard already installed.

springer111

Ya it seems 1 is the only viable option. Thanks everyone