Unvented Attic/roof assembly
My question is regarding unvented attic assemblies. I am designing a house that has a 5/12 gable that is attempting to replicate a traditional craftsman style rake/eave condition. So soffits and other venting methods are not an option. There is also a concern about fire so producing a completely sealed attic rather than relying on fire resistant vents is part of the goal. The project is located in Los Angeles County California so climate zone 3b i believe, so low moisture/humidity but relatively hot. I am attempting to avoid the cost of spray foam in the rafter bays and the added labor/complexity of rigid foam over the roof deck. My primary concern is rotting the roof deck/roof framing or creating mold in the attic with trapped moisture.
According to R806.5.1.3 both air-impermeable and air-permeable insulation can be used and rigid insulation board can be used as the air impermeable layer applied directly to the underside of the roof deck. Since it’s climate zone 3 the rigid board would need to be r5 minimum which is one layer of 1” foam. I have already consulted a title 24 engineer and they prescribed r-19 at the roof deck and r-38 for the ceiling deck (which seems overkill to me).
So the full attic/roof assembly would be (from the top down):
Asphalt shingle (cool roof shingle required), synthetic underlayment, ⅝” zip roof sheathing, 1” r5 rigid board, r-19 batt insulation (likely Rockwool) between 2x rafters (so the result would be r-21), then attic space, r-38 batt insulation between ceiling joists and then drywall, no vapor barrier at the underside of attic.
The attic would be “semi-conditioned” ; it won’t have a direct hvac supply but with the roof insulated and the ceiling deck insulated it would be considered within the thermal envelope. The HVAC system will be a concealed duct mini split heat pump. So there isn’t an air handler or major ducting in the attic, just the small duct from the concealed ceiling cassette to the ceiling register. No other mechanicals are planned in the attic.
I have attached two details that demonstrate what I am explaining. (Note the details don’t show insulation at the ceiling deck but their will be)
Any guidance on this would be helpful, I definitely appreciate the work on this site and use it regularly thank you.
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Replies
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
As stated in my original post vented assemblies are not possible so the first few solutions do not work and the rest require spray foam which I am try to avoid.
Check out #3:
"Unvented assembly with continuous rigid foam above the roof sheathing and fiberglass batts, mineral wool batts, or cellulose under the roof sheathing."
As mentioned in the original post I am trying to avoid this due to a more complex and costly assembly. It would require essentially assembling the roof twice and therefore increases costs significantly.
PatSauce,
Since you are in climate zone 3 I would skip the rigid insulation and use entirely permeable ones (either cellulose or batts), coupled with a vapour-diffusion port at the ridge.
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
With either that assembly or the one you have suggested, I would install an air-barrier on the underside of the rafters. That will stop air-movement through the permeable insulation, and contain whatever insulation you use. House-wrap would work fine for that.
When I say "the assembly you are suggesting", I mean a combination of rigid and fluffy insulation in the rafters - and none on the ceiling. Installing insulation in both places is risky. The attic is then neither fully inside nor outside the thermal boundary, and is prone to developing unpredictable moisture issues.
Similarly, in the codes I'm familiar with, and in practice, there are no "semi-conditioned" attics. They need to be fully in or out of the conditioned space to function properly.
A vapor diffusion port would be more risky due to fire. As far as I know there aren't any effective vapor diffusion port assemblies that have an adequate fire rated assembly that is any different than a traditional vented assembly.
Title 24 engineer has prescribed ceiling insulation. I can't just nix the insulation here.
In addition when I refer to "semi conditioned" attic it means the attic is within the thermal envelope but doesn't have any direct air supply.
Am i just stuck with spray foam here? If need I can maybe do a flash and batt style strategy with a few inches of closed cell spray foam and then batt below but the cost may be prohibitive.
The other alternative is obviously a vented system but to preserve the traditional architectural look it would only be able to be a gable end vent with a fire rated screen. but then i run into the issue of not venting the rafter bay if i insulate them.
any further assistance would be helpful.
Pat Sauce,
Here is what BSC recommends - and includes vapor diffusion ports
https://buildingscience.com/documents/building-science-insights/bsi-129-wildfire
I think what you drew will work fine - but it depends on diligent air-sealing of the foam board for its success.
But again:
- Being "inside the thermal boundary" is not conditioning the space, which is necessary if it is to work.
- It is also not inside the thermal boundary if there is insulation on the ceiling.
See fig6 for a fire rated diffusion vent.
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
You say the Title 24 consultant has prescribed this.
The combination of Ceiling and roofline insulation prescribed in Title 24 only applies to vented attics. An unvented attic will not be able to demonstrate compliance prescriptively. And as others are describing on this Q&A, this approach may not be prudent in an unvented assembly regardless.
You can refer to this Section 150.1(c)1A:
Roof and Ceiling insulation shall be installed in a ventilated attic with an R-value equal to or greater than that shown in TABLE 150.1-A meeting options ii or iii below....
https://energycodeace.com/content/section-1501-performance-and-prescriptive-compliance-approa#ra-chunk--135439
If your attic is unvented, then it MUST be a part of the thermal enclosure and as such, your R19 assembly (using your numbers there) at the roofline fails Title 24, which requires R-22 minimum per section 150.0(a). The rafter depth here will matter with the effective performance of those batts, so you may find yourself increasing rafter depth and enclosing the rafters to create a rafter roof assembly one way or another.