Use of vapour barrier in separating timber floor?

Lumen

I'm replacing upstairs subfloors today in my 1980s masonry cavity house.

What is there (top down):

- carpet & underlay
- plank floorboards
- 9" joists going into masonry cavity wall at one end and masonry dividing wall at other other
- ceiling plasterboard

What will be there when I'm done:

- 18mm T&G OSB, glued and screwed, 5mm gap to masonry walls sealed with proclima tape
- acoustic joist strips (foam)
- 9" joists with 6" acoustic rockwool between them
- ceiling plasterboard

The ground floor under these rooms is currently unheated and leaky, a sort of unfinished half-basement. I'm going to sort that eventually.

My main concern is uncontrolled airflow between the joist space, the ground floor and also the unsealed joist junctions into the external masonry walls.

I have a couple of rolls of proclima membrane that was intended for my attic, but is it worth putting that down over the ceiling plasterboard and joists and taping it to the masonry walls?

I guess I care less about air movement between the floors and more about leakage into the cavity. So I guess I could attempt to tape up those joist-wall junctions?

Thoughts?

Lumen

This is the kind of detail I was expecting. Sigh.

Fill these gaps with airtight foam ?

BryanF

VB warm side of insulation. In you case not at ceiling, should be under ply.

Lumen

Thanks Bryan. In the end I burned a load of time trying to seal the bare block of the external wall with PU foam, VB and tape to stop air leakage from the cavity, which is probably better done as a plastering job.

And then for the floor I just PVA'd the T&G joints and taped the edges to the messy block/plaster junction under the skirting. Not entirely satisfactory but it's much better than what I had before.

Prob would have been better to bring a sheet of VB up from under the OSB and tape to the wall and then tape the edge of the OSB to that.

Is glued OSB air tight?

I need to learn rough plastering.

Wartburg

Lumen wrote: »

Is glued OSB air tight?

I need to learn rough plastering.

No 1: It all depends what make and type of OSB you use. The air tightness of the OSB is created by the glue they use during the manufacturing process of the boards. OSB 3 in a certain thickness will be good for air tightness.

No 2: Consider air tight paint instead of sticking tapes and membranes to the rough blocks. No issues with bonding performance and no risk of wear.

Lumen

Ah, paint looks much easier!

There's a supplier of that stuff just down the road.

Thanks Wartburg.

re the OSB, it's 18mm OSB3 so I should be OK.

Wartburg

Lumen wrote: »

re the OSB, it's 18mm OSB3 so I should be OK.

Should be allright. One of the German researches about the air tightness of OSB describes that the performance depends to the sd-value of the board. The higher this value, the more air tight the board is. I´d be interested in someones experience regarding the air tightness of plywood, because her sd-value is all the time higher in comparison to OSB.

Lumen

Wartburg wrote: »

I´d be interested in someones experience regarding the air tightness of plywood, because her sd-value is all the time higher in comparison to OSB.

Is plywood used much any more for sheathing and flooring? It seems to have no real advantage over OSB and several disadvantages.