Closing space between roof joists at solar overhang

[phatboy]

Hi all,

 

Sorry for the long confusing topic name.  I'm hoping for a little expertise on a design detail.

 

The extension I am part way through building has the flat roof overhanging the building by 1m on the south side.  It will all be externally insulated (no cavity), and the overhang will be clad on the underside and down the wall.  It is likely to be cladding with gaps.

 

This leaves me unsure how I will close between the joists above the walls, so that the inside is airtight and remains a warm roof.

 

Will cutting the EWI boards tight around the joists and foaming, then rendering be enough?  I worry it may end up with gaps as the seasons change?

 

Not really sure what other options I have.  Please see photos to clarify what I'm trying to do.

 

Thanks!  

 

 

 

Edited November 24, 2022 by phatboy

[Russell griffiths]

Isn’t your airtight layer on the inside?

seal up the inside then bring your external insulation up to soffit height, then do separate insulation between joists. 

Soffit on and render bead under soffit for a neat finish. 

[Andehh]

Yep, exactly that. I would cut and fit rigid insulation tightly between those joists, and expanding foam to seal tight. Then external insulation/cladding right up over it 

[phatboy]

4 hours ago, Russell griffiths said:

Isn’t your airtight layer on the inside?

seal up the inside then bring your external insulation up to soffit height, then do separate insulation between joists. 

Soffit on and render bead under soffit for a neat finish. 

 

Well that gap currently goes into what will be the ceiling void.  Thanks, I wonder if there is benefit to doing separate insulation, or just letting the EWI boards all the way to the top?  It won't be a closed soffit, it will be some kind of cement cladding, Millboard etc

 

3 hours ago, Andehh said:

Yep, exactly that. I would cut and fit rigid insulation tightly between those joists, and expanding foam to seal tight. Then external insulation/cladding right up over it 

 

Thanks, nice that someone else would do what I was thinking!  Just wondering if the seasonal changes will open gaps between the timber and insulation.....

[ProDave]

I would have closed them with noggins where they crossed the outside wall, then insulate up to the noggins on the inside.

[Andehh]

Rigid insulation will be very thermally stable, even the wood will be providing it doesn't get wet & is dry when installed. A thick bead of silicone or expanding foam to seal the insulation in will be flexible enough to cope none the less.

[phatboy]

Thanks all,

 

I'll go with cutting and pushing the insulation all the way up, then foam it well, and also silicone it after it's rendered.

 

Thanks for the help :)

[JohnMo]

If you zoom in you will see how I did mine.  Basically lots of PIR off cuts, tight fitted into the gaps, plus spray foam.  Did mine before the roof was plywood sheeting over, so it was quite easy to do and get at.

 

 

[phatboy]

Excellent @JohnMo, I will do basically the same.

 

Thanks for all the help