Insulation location in unvented attic
LawrenceMartin
| Posted in Energy Efficiency and Durability on
Hello,
I have a client who is building his own home in Teton WY, Climate Zone 7. They were planning to install a vented attic with R-57 cellulose insulation on the floor of the attic. The roof will be standing seam metal with a galvalume finish. They have the water barrier already installed on the roof deck.
He has been told by the County that since the house is in a wildland urban interface fire area, eave/soffit vented attics are not allowed. He can put gable end vents in. But, I am not convinced they are effective. I just imagine snow and embers getting in there anyway, all while not providing adequate air flow since there is now low to high air flow. Would gable end venting be an acceptable solution? Maybe I am overreacting to this option.
If we went the route of the unvented attic, all the solutions that I have seen always keeps the insulation up against the sheathing (or on top). We suggested a spray foam solution but his budget did not allow for an R-57 spray foam. We don’t know of a good way to get cellulose insulation secured to the underside of the sheathing. I thought about spraying a minimum amount of foam on the sheathing to keep the dew line outside the sheathing and they putting cellulose on the floor for an added blanket. But, since I’ve never seen that, I presume there is a good reason.
I read a 1999 Article from Joe Lstiburek “Unventing Attics In Cold Climates” in Home Energy Magazine implying that in an unvented attic in a cold dry climate the insulation may not be needed against the sheathing as long as the dew point can be kept below the ambient temperature. Did I read that right, or was it wishful interpretation? If this is correct, one idea was to place the standing seam on sleepers to keep the sheathing cold on both sides. Install the insulation on the floor of the attic as planned, and install no soffit or gable end venting.
Any thoughts would be greatly appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Do a good job with air sealing/insulation/vapor retarder and I expect that any code compliant attic venting will work well. But I haven't seen any data indicating that gable vents at 150:1 are more or less effective than soffit-ridge vents at 300:1. If worried about it, you can oversize (like 100:1).
In California the vents need to be approved for the wildland fire zone. They have passed testing for embers. here is one brand --- https://www.brandguardvents.com/
We are allowed to vent the soffits with approved vents and construction details. San Diego County has lots of approved details on the county website.
Gable only venting should be fine, see website for an approved ridge vent.
Hi Lawrence.
Have you read these articles? I think you'll find the answers you are looking for between the two of them.
Building to Survive in Wildfire Country
How to Build an Insulated Cathedral Ceiling
Tim R: Thanks for the tip. We'll look into that. Unfortunately, Teton County does not seem to have any exceptions for this requirement. But, they may consider it if San Diego is successful with it.
Jon R: Thanks for the feedback. We may indeed go this path.
Brian Pontolilo: Thanks for the links. I didn't really look at the cathedral ceiling option since our ceiling was a flat ceiling. The ideas we had about keeping the insulation against the ceiling work with rafters but not so well with trusses.
Hey Lawrence.
Just so you know, that article is more about insulating a roofline properly than creating an actual cathedral ceiling. In other words, it's the same info you'd need to bring an attic into the conditioned space of the house.
Good advice! It pays to read past the headline.