flat roof and thermal bridging abatement purlins and foam
In a previous post about a flattish roof on th 7a 6a HVAC zone board (super cold and moist), I discussed ideas on how to handle a 10-inch total depth roofing system, and it seems we are settling on a full cavity fill of spray foam (closed cell)
Before spending that kind of chemical cash we tried to think what else we should do while its open, what can we do for thermal bridging through th 2×10 rafters, a thought was to put purlins every x inch on center
Given the expense of the spray foam we are thinking of replacing all the roof decking with new wood as a preventative ain’t, and if the decking is off maybe the purlins would be an option to thermal break the 2x10s vs poly iso outside the roof deck. there would still be some thermal transfer (where purlins meet rafters) or if we are going that far in, maybe there are better cost-effective measures.
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Replies
I recommend this article from GBA.
https://www.greenbuildingadvisor.com/article/insulating-flat-roof
You indicated:
"it seems we are settling on a full cavity fill of spray foam (closed cell) "
Comment:
CAUTION! This is from the article in the link:
What happens if the roof leaks?
If Jordan were to go the spray-foam route, he wonders what would happen in the event of a roof leak. With a layer of impermeable foam coating the under side of the sheathing, where does the water go?
Answer in the article: ***********“You’re pretty much screwed if you get a roof leak,” ***********
You mention
"we are thinking of replacing all the roof decking with new wood"
Comment:
You will find that insulating from the exterior on top of the roof deck with Polyiso is pretty much the most recommended solution. This is how commercial flat roofs are done. There is lots of literature out there to back this up.
You mention
"a flattish roof on th 7a 6a HVAC zone board (super cold and moist),"
Comment:
I know someone who builds an interesting low slope roof assemblies and maybe you could do this on your roof???
From exterior to interior:
1. Roof covering material for low slow roof
2. Synthetic ice shield over the full deck
3. T&G Roof decking 1/2 or 5/8 (screwed down for greater wind resistance - see FEMA recommendations)
4. 2x4 on the vertical (not the flat) screwed to the roof trusses below with venting at both ends.
5. High quality european type vapour-open weather barrier taped at all the seams under the 2x4.
6. T&G roof decking over the roof trusses and tape the seams with a high quality tape (does not work with H clips for this assembly)
7. Insulate as much as you can below with loose fill or batt insulation (not spray foam because of the note from the the GBA article "You are screwed.")
8. Rigid vapour barier (ZIP osb) (and air barrier) below the trusses fully air sealed with a product that is designed to work with ZIP
9. 2.x4 on the flat below the vapour barrier to allow space for electrical.
10. Drywall screwed to the 2x4's on the flat.