Cathedral – Diffusion – Airgap
Hello,
I have cathedral ceiling in two bedrooms and I want to make it unvented with diffusion port, I don’t want to do spray foam. Would the following arrangement work? I can’t match is with any of the code provisions. I’m concerned with moisture management.
Climate zone 3C mixed marine (San Jose)
Unvented vaulted with diffusion port
7.25” deep rafter cavity
-Roof profile:
-Asphalt shingles,
-felt underlayment
-Roof sheeting (plank tight skip sheeting)
-1.75″ air gap
-3.5″ mineral wool
-2” rigid foam between rafters
-2″ rigid foam (installed under the rafters to avoid thermal bridging as a continuous control layer, class 2 vapor retarder).
Initially I wanted to make it vented with intake and ridge vents but now I’m thinking to block the intakes and make it unvented with diffusion port.
Reason is I have solar panels which covers the cooling bill in the summer thus the heating cost in winter is where I can save and I don’t want to allow cold humid air on top of the insulation.
Also looking at the interesting data published in September 2024, in the summer, hot air in the vented roof makes the house warmer throughout the night.
Ref: (Kaufman, Z., et. all. (2024). Moisture Performance of Unvented Attics With Vapor Diffusion Ports and Buried Ducts in Hot, Building America Solution Center. https://basc.pnnl.gov/library/moisture-performance-unvented-attics-vapor-diffusion-ports-and-buried-ducts-hot-humid)
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Replies
There is no vapor diffusion port option in code with vaulted ceilings.
If you're worried about summertime heat gains from the ceiling assembly, then go with as much insulation as you can in the ceiling.
Perhaps just go unvented (code option-- eschew the air gap gap for more air impermeable insulation under the deck).
Yes, I’m confused after reading this code provision, which seems to prohibit cathedral ceilings with a diffusion port:
2021 R806.5
5.2.10. Where air-permeable insulation is used and installed directly below the roof structural sheathing, air must be supplied at a flow rate of at least 50 CFM (23.6 L/s) per 1,000 square feet (93 m²) of ceiling. The air must be supplied from ductwork that provides supply air to the occupiable space when the conditioning system is operating. Alternatively, air can be supplied by a fan when the conditioning system is operating.
https://codes.iccsafe.org/s/IRC2021P2/part-iii-building-planning-and-construction/IRC2021P2-Pt03-Ch08-SecR806.5
On the other hand, several sources suggest that cathedral ceilings with a diffusion port are viable:
https://www.jlconline.com/how-to/insulation/avoiding-wet-roofs-part-ii_o
https://buildingscience.com/documents/insights/bsi-088-venting-vapor
https://www.greenbuildingadvisor.com/article/filling-rafter-bays-with-fluffy-insulation
https://buildingadvisor.com/best-approach-to-hot-climate-cathedral-ceiling/
I can provide more references if needed.
No contradiction there.
You're looking at research and not code
Code hasnt caught up. That it quite normal.
That is-- if you built that, it would probably be fine, but it's not code compliant.
I haven't done an AMMR with San Jose, but it's a possible pathway there.
But on the prudential question-- I don't see why this is worth the fuss just so you can eliminate venting up there. Why not just go with unvented without the vapor diffusion ports? Eliminate the air gap, and do the materials you're considering-- look up Martin's article on cut&cobble.
Do you still have moisture concerns there? If so, do you have a particular moisture source that you're concerned about? Indoor hot tub? Tropical fish tank? (examples I see in the Bay Area which would give me pause)