Converting a Vented Attic to an Unvented Attic
TBallister
| Posted in General Questions on
Hi!
I’m about to get a new roof, and I’m planning to add 2” of exterior rigid polyiso foam insulation on top of the existing roof sheathing. This is primarily to insulate a room which has a vaulted ceiling, but this roof also covers a room which has an uninsulated attic.
Can I close up the existing vents to effectively bring this attic into the conditioned space with an unvented hot roof design? I want to gain the benefits of the rigid polyiso on the roof, but every roofer I am talking to wants to vent this attic (which will eliminate the value of the polyiso over this room of course).
Currently this attic has only one small gable vent, and the attic floor is not air sealed from the living space. I am in zone 3C, which for new builds only requires R-5 exterior insulation for an unvented roof design. Given I’ll be doing R-13, I should be clear there, I’m just not sure if I’m overlooking something with respect to retrofitting this currently vented space to be unvented. Anyone have some insight on whether my proposal will cause moisture issues and will it pass inspection? Is there anything I would need to do beyond just sealing up this gable vent?
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Replies
TBallister,
It sounds like it would work to me, as long as you condition the newly un-vented space.
https://www.greenbuildingadvisor.com/article/a-new-look-at-conditioned-attics
My approach in CZ3 is to install 1.5" R10 polyiso to match the 2x4 perimeter nailer. If I plan to install shingles, we install a 2nd layer of sheathing. If I plan to install metal roof, we install a P&S WRB, then 1x4 battens and/or counterbattens for tile (if required). No vents but install supply ventillation per code, or better, install an independent dehumidifier.
You do need to install 8"-11" impermeable insulation against the bottom of the roof decking to meet R38 or R49 depending what code you use.
Armando,
Using 1 1/2" rigid makes a lot of sense from a practical point of view.
Did you mean 8" to 11" of permeable insulation?
Malcolm - I wrote 8"-11" and it shows the same in my laptop. Maybe 11" american reads 1" in canadian? 🤪
He's asking whether you meant impermeable or permeable.
A. Malcolm had written 8"-1", I made a joke and he edited it.
B. Here is that pesky code again... R806.5.1.2 tells us that if we install rigid foam on top of the roof decking, we must install permeable insulation against the underside of the decking.
Thank you both! Follow up question to you Armando. Is R806.5.1.2 applicable with an existing house? I was assuming I didn’t need to get all the way to R-30 (current code for my area) in this retrofit scenario given installing even the R-13 polyiso would be voluntary and an improvement over my current situation of having no insulation (I.e I don’t have to add insulation to my house to be able to get a new roof - right?).
And out of curiosity, is there an actual moisture management reason why the permeable insulation would be required or is it just to meet current building code for energy efficiency?
You need to check with your building officials. Some municipalities require homes to be brought up to code with certain jobs or dollar amount. Having said that, the money spent on energy efficiency has the best ROI on anything you do in your house.. See weatherization information and professional home energy assessments.
https://www.energy.gov/energysaver/weatherization
https://www.energy.gov/energysaver/home-energy-assessments
You should check for federal assistance with the Inflation Reduction Act of 2022 (IRA) that will allow homeowners claim tax credits on qualifying energy efficiency home improvements. You can also check on dsireusa.org for incentives in your area. https://www.dsireusa.org/
The rigid foam on top of the roof decking is to avoid possible condensation issues. The permeable insulation under the decking is to complete the total R-value required in CZ3 for energy efficiency.
Thanks, this makes sense! Do either of you know if I would also need to insulate the gable end walls? And if so, would they also need the exterior polyiso insulation?
Yes, you must insulate the gable walls. IMO, installing rigd foam on the outside is always the Cadillac of wall assemblies, it mends thermal bridging. Having said that, it depends if you're willing to insulate the exterior walls on the rest of the house, and how extensive of a job you are going for. As I said, energy retrofits are the best ROI and better if there are tax credits or energy credits coming your way.
Armando,
That was my typo not yours. Sorry.
I was more interested in the impermeable vs permeable for the below sheathing insulation.