Airtight Warm Roof - How?

[Mr Blobby]

We are finalising the construction details for our block cavity passive house and I would really like some feedabck from the group on the roof detail. 

The warm cut roof will be suspended on a ridge beam, so no trusses or complicated structure in the loft. 

 

The architect has suggested open cell foam to provide the air tight layer and insulation.  Is this the best approach?  What other options are there?

 

For completeness, here is the roof detail:

 

 

 

My concern with open cell foam as the airtight barrier is the risk of it not being done well, what then? 

I dont want to use PIR as insulation between the rafters.

 

Open Cell Foam for air tightness - Yes or No?  And if not, why not?

 

 

 

 

 

[ProDave]

Fit a proper air tight membrane over the inside of the rafters with all joints taped, and taped to the air tight layer on the walls.

[Mr Blobby]

2 minutes ago, ProDave said:

Fit a proper air tight membrane over the inside of the rafters with all joints taped, and taped to the air tight layer on the walls.

 

Thanks @ProDave , that makes a lot of sense to me, and ensures continuity with the membrane that is plastered into the wall.

What about insulation between the rafters?  Still use OCF between rafters and then membrane under rafters?

[JohnMo]

Overall seems a complicated roof to build.

 

Why not.

 

Metal roof.

Suitable membrane

Ply

Rafter

Under draw with counter battens to suitable thickness 

Spray foam to circa 350mm

Airtightness vapour control layer

 

Comments on your build up

 

As said above.  I used Passive House Systems, reflective air tight vapour barrier.  Their primer on the parge coat, then the airtight adhesive.  All joints and staples taped.

 

Photo of roof and wall prior to battens.

 

I don't believe the open cell foam is that airtight, as I missed some taping on the ceiling and had an air leak at that location. (air test prior to plasterboard). To

 

If you are doing a parge coat add some lime to the mix, give a better mix for brushing on and stays more flexible after drying.

[redtop]

Only comment is you have the OSB in the middle of the insulation, surely this would be a magnet fir damp to eventually rot the boards. Or am I being daft again....

[Dave Jones]

you have a trussed roof, whats the point in trying to make it a warm roof ?

[Mr Blobby]

4 minutes ago, Dave Jones said:

you have a trussed roof, whats the point in trying to make it a warm roof ?

 

No its a cut roof with ridge beam, no trusses.

[Dave Jones]

ok its an expensive trussed roof, way you trying to make it a warm woof ?

[Mr Blobby]

1 hour ago, redtop said:

Only comment is you have the OSB in the middle of the insulation, surely this would be a magnet fir damp to eventually rot the boards. Or am I being daft again....

I'm hoping the VCL under the ply on top of the rafters will stop condensation.  Will it not?

[Mr Blobby]

Just now, Dave Jones said:

ok its an expensive trussed roof, way you trying to make it a warm woof ?

 

Its part vaulted ceiling upstairs and I like the MVHR and storage in the warm lofted bit.

 

Also running services and light fittings from loft through ceiling is challenging if that is the airtight boundary, is it not?

 

[redtop]

2 minutes ago, Mr Blobby said:

I'm hoping the VCL under the ply on top of the rafters will stop condensation.  Will it not?

I don't think it does. We have a warm roof and have osb fixed to roof joists, then plastic sheet (well a posh vapour proof layer) then insulation which is glued to the sheet, and then the waterproof bit. So condensation forms in the insulation whilst the OSB stays warm and dry. 

[Mr Blobby]

15 minutes ago, redtop said:

I don't think it does. We have a warm roof and have osb fixed to roof joists, then plastic sheet (well a posh vapour proof layer) then insulation which is glued to the sheet, and then the waterproof bit. So condensation forms in the insulation whilst the OSB stays warm and dry. 

 

Do you think the VCL in my build-up should be above the ply then? 

 

I thought the warm moist air would come from inside the building and so a VCL underneath would potect the ply from condensation.  Is that not how it works?

 

Edited January 19, 2022 by Mr Blobby
spelling

[redtop]

11 minutes ago, Mr Blobby said:

 

Do you think the VCL in my build-up should be above the ply then? 

 

I thought the warm moist air would come from inside the building and so a VCL underneath would potect the ply from condensation.  Is that not how it works?

 

Others know this better than me, and my warm roof is a flat roof so somewhat different. I was just of the impression that condensation happens anyway when you go from warm to cold so you make sure nothing that can rot is in that zone. The vapour proof layer on top of the OSB significantly reduces it as it stops moist air from the house getting into the insulation but it wouldnt stop it entirely as you will always get some moisture migrating down from the final waterproof covering

[Mr Blobby]

8 minutes ago, redtop said:

Others know this better than me, and my warm roof is a flat roof so somewhat different. I was just of the impression that condensation happens anyway when you go from warm to cold so you make sure nothing that can rot is in that zone. The vapour proof layer on top of the OSB significantly reduces it as it stops moist air from the house getting into the insulation but it wouldnt stop it entirely as you will always get some moisture migrating down from the final waterproof covering

 

I think you're right, and thank you for pointing this out.  Googling around, what you say appears to be bang on, the air flow can move both ways and the ply should be inside the VCL.

 

Looking again at my build-up, however, I'm not sure why there is a need to have the ply on top of the rafters, when the PIR could be attached directly.  Maybe architect thinks the OCF between the rafters can't be sprayed against the underside of the insulation.  I think I need to chat with my architect about this.