Airtight/VCL ideas for cantilever posijoist roof

[Adam2]

Hurray - building the roof! It's a warm roof construction

 

plasterboard

posi-joists (225 deep)

18mm ply vapour control layer

150mm PIR (avg)

single ply membrane

edge detail will be aluminium (Tag Fascia and ali soffit with vents for any moisture to drain out)

 

The roof - see pic, projects over the steel beam and I'm trying to work out the best way to stop this being a route for cold air beneath the insulation. 

 

Am thinking I could cut blocks of PIR inside the clear between the top and bottom timbers of each joist directly above the wall EPS + a thicker block of PIR or EPS in the gap between the joists.

 

Is there a better way?

 

Under the joists I was thinking to use an airtight VCL stapled to underside of joists with tape over the staples and with double sided tape to adhere to the steel and the EPS interior walls. To help with accommodating light fittings I plan to secure the VCL up the inside of specific joists to provide extra fitting space.

 

I guess it is either this kind of approach or we deal with air tightness by really carefully detailing around the joists and taping this to the underside of the ply and down on the back of the steel beam (or insulation we'll install on the back face of this. Maybe this is a lot more economical than this additional airtight VCL running under the whole of the ceiling on this floor - about 80m2

 

Would be great to see pics of any similar installations.

 

 

 

 

 

 

 

 

[Iceverge]

You have your work cut out there.

 

I would use the 18mm ply as my airtight layer. Tape all joints as it's installed. 

 

The tricky detail is joining it to the wall continuously and insulating between those joists. You can painstaking cut PIR and fit it between the joists with minimal gaps. Then use a closed cell spray foam inside to join the underside of the ply to the cut PIR infills to the wall. It should provide a continuous air barrier and insulating. It is expensive though. 

 

To present a different option entirely. I suspect cheaper too.

 

Revise your buildup to. 

 

Waterproof Membrane.

OSB

Fitting strips to create ventilation gap.

Breather membrane taped at joints. Wrapped around the cantilever and returned to the wall externally.(for windtightness)

OSB. 

225mm POSI joists filled with blown in cellulose or batt insulation.

Airtightness membrane returned to interior wall.

45mm service cavity with batt insulation. 

Plasterboard. 

Edited October 15, 2021 by Iceverge

[Adam2]

Thanks @Iceverge we're sticking with warm roof I think as bit late to change that now.

 

The joists are all a precise width and the space between top/bottom timbers is of course the same so I was thinking we could make a production line - getting there cut on a jig could be relatively quick and accurate to make them precisely to size.

In our warm roof the issue of course with the ply being the VCL is that it is above the joists so we would have this issue (the waterproofing/insulation co) will be installing a vcl over the timbers in their own scope (price agreed so can't really remove and may be needed for warranty anyway)

 

Maybe I could install a long roll of earthwool running through  the joists across the width of the building maybe in 3 layers and thick enough so it is pushing into all the open space with PIR fixed inbetween the joists as per oriignal plan - may be a big quicker that fitting teh smaller pieces 

 

 

 

 

[Mr Punter]

1 hour ago, Adam2 said:

Would be great to see pics of any similar installations.

 

We did it by fixing glulams to the side of the web joists as I wanted the beams exposed externally.  Birch ply on the exposed bit, normal ply elsewhere.

 

 

 

[ProDave]

What I would have done is lay a sheet of air tight membrane over the wall plate before the joists went on and applied the Tony Tray principle.

[Adam2]

55 minutes ago, ProDave said:

What I would have done is lay a sheet of air tight membrane over the wall plate before the joists went on and applied the Tony Tray principle.

The joists are not yet strapped onto the wall plate so could be lifted (not easily mind) to allow this but I'm not sure how that would help in this scenario. On the outside face would the membrane do out under the joists and be then secured onto the ply deck? If so the membrane could also be secured onto the outside of the steel beam and achieve the same, though I wanted to avoid wrapping the joists like this due to possibility of any condensation (or other moisture even if from an installation defect) within this space then being trapped rather than dropping into the soffit/soffit vents. But maybe I misunderstood the Tony Tray application here - I did look it up and see how it applies in "regular" scenarios 

[Adam2]

Thanks @Mr Punter looks very nice. From the pics it looks like the posi-joists stop at the wall and the extended beam is what continues meaning you had both a nice detail and one which was easier to block in. I guess I could install exterior ply to the sides of the posi-joists then in-fill - bit of work there mind with I think 17 joists at 1.2m projection.

 

[Iceverge]

4 hours ago, Adam2 said:

Thanks @Iceverge we're sticking with warm roof I think as bit late to change that now.

 

It doesn't look like it's beyond the point of changing your mind from the photos. Is there something else prohibiting it not in the pictures?

 

4 hours ago, Adam2 said:

In our warm roof the issue of course with the ply being the VCL is that it is above the joists so we would have this issue (the waterproofing/insulation co) will be installing a vcl over the timbers in their own scope (price agreed so can't really remove and may be needed for warranty anyway)

I'm sorry but I don't understand what you mean here. 

 

4 hours ago, Adam2 said:

Maybe I could install a long roll of earthwool running through  the joists across the width of the building maybe in 3 layers and thick enough so it is pushing into all the open space with PIR fixed inbetween the joists as per oriignal plan - may be a big quicker that fitting teh smaller pieces 

 

Would you care to share a drawing of your PIR plan please? I can't figure out where you intend to put the earthwool.