Can unvented roof assemblies be insulated with fiberglass if I use rigid foam on the exterior of the sheathing?
I have read the Aug 21st post titled “Can Unvented Roof Assemblies Be Insulated with Fiberglass? https://www.greenbuildingadvisor.com/articles/dept/musings/can-unvented-roof-assemblies-be-insulated-fiberglass I think the answer is “Yes, but…”. And here is the but: You need to add the appropriate thickness of rigid foam to the exterior side of the OSB sheathing. I’ve read so many different roof profile scenarios…I know I ought to be able to figure this out…but I have a few questions…I’m hoping to get some confirmation of my lessons learned as a rookie here on GBA.
For the roof of a Zone 3A-Simple gabled building. 10/12 pitch roof with an unvented attic (attic is that is proposed mostly for living space-think room in roof trusses) in order of construction/installation:
1. Engineered Trusses 24″ OC. (lumber unknown at this point) (likely a 2×8)
2. OSB Sheathing 5/8″?
3. Any Thickness of XPS or EPS (my Climate Zone 3A-Oklahoma City Area.)(Considering it doesn’t drop and stay below freezing for more than 3-4 days every 3rd or 4th year- I could even use Polyiso, right?)
4. #30 roofing felt (or would you put this on top of the sheathing/before the rigid foam layer?)
5. 1×3 or 1×4 horizontal “purlins”
6. Standing Seam metal roofing installed directly on to the purlins?
I’m not sure where the roofing felt goes…
But…now…lets get back to fiberglass insulation between rafters on unvented attics–I like the finished looks of that ProPink High Performance Conditioned Attic System….and with the Rigid foam over my sheathing I should be good if I’ve learned anything here on GBA. Do I need one of Lstiburek’s diffusion vents at the ridge? —I better stop there.
Last thought: Not all of this attic is going to be lived in….only the center 20 feet…the 8 feet on either side of the 20 feet is proposed to be within the building envelope-within the insulated space…but not actively heated and cooled for comfort….I will have interior walls framed and will only use the 8 foot space for storage or other mechanicals….no harm there right?
Thanks in advance for keeping me on track…
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Replies
To meet IRC code prescriptives it can't just be any thickness of any foam above the roof deck- it needs to be at least R5 (of any foam.) An inch of polyiso would be fine, but a half inch of polyiso would not meet code.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm
The #30 felt would be on top of the roof deck, with the foam atop the felt.
1x furring wouldn't likely to meet the fastener retention requirements under wind loads for metal roofing in all areas, and maybe not yours. If it does, 1x4 would be less prone to splitting than 1x3. The purlins need to be secured to the rafter elements of the trusses with pancake head timber screws that penetrate the rafter by at least 1.5". (You can't just nail the purlins to the roof deck.)
With rigid insulation above the roof deck and fiber below the fiber needs to be snug up to the underside of the roof deck, and there would be no vents, ergo "unvented attic". If you vent under the roof deck with rigid foam above, the R-value of the rigid foam doesn't count from a code compliance point of view.
If you skipped the rigid foam above the roof deck and used the ProPink High Performance Conditioned Attic System, you would need soffit to ridge venting.
It's fine to have kneewalls partitioning off attic spaces and partition walls sectioning off unfinished rooms etc. fully inside the pressure and thermal boundary of the house. It's considered conditioned space even if those areas don't have directly controlled conditioning.
Inger,
Dana Dorsett gave you good advice. Here is a link to an article that should answer almost all of your questions: How to Install Rigid Foam On Top of Roof Sheathing.
Thats perfect Dana! Thank you. So I will plan on roofing felt on top of sheathing. At least R-5 on top of that...then 1x4 (or possibly 2x4) purlins attached to trusses with pancake head timber screws long enough to penetrate the truss by 1.5 inches.
One more twist...For any moisture (not air) that needs to escape....Would anyone recommend a vapor diffusion vent in or very near the ridge..(remember I'm in zone 3A and the attic will be conditioned living space with a bath and kitchen--and a minisplit ducted or ductless to keep it warm and cool.) . If yes to vapor vent I'm gonna have to figure out how big- how long? 1:600? Not sure what that means. Will start digging that out...My thoughts: Figure out how big a gap to leave in the osb sheathing. Put down the barrier material. Add some osb height spacers to the top of the trusses and then let my asphalt felt and R-5 rigid foam continue to the peak. Add a metal roof ridge vent and metal roofing panels. Still air tight. Thoughts? Thanks in advance!
Inger,
If you want to create an insulated roof assembly with rigid foam above your roof sheathing, and fiberglass insulation underneath the sheathing (and in direct contact with the sheathing), you are creating an unvented assembly. So no -- you don't want a vapor diffusion vent for this type of assembly. Your rigid foam needs to cover the ridge in an airtight manner. The seam between the rigid foam on one side of the ridge and the rigid foam on the other side of the ridge needs to be taped.
Whew.....That's a relief! I greatly appreciate the responses. I'm off to detail the footer/foundation/slab/wall assembly.... Knowledge is power....but I sometimes get overwhelmed with options....Glad your forum/Q.A. is here to keep me on track!