Unvented attic conversion with closed cell – how to get the R-value or U-factor high enough?
Hi! What a helpful website this is! Hope you all can help me.
I’m planning an attic conversion for my 1892 home, zone 5 (northern illinois). It’s a cathedral ceiling, with one central ridge, basically. Unvented. 2×6 rafters, 16 on center. About 1100 sq feet of floor space, currently there’s insulation in the floor but none on the underside of the roof. About 300 sq was turned into a room, and plastered, about 100 years ago.
I’m getting a variety of proposals from folks. “Do 5″ of spray foam and drywall.” “Do 4″ and drywall.” “Do 3″ and don’t pull a permit” (not an option – morally and for sake of the long term, a permit is important).
I had a roofer look at venting it, so I could furr out the rafters and do batting or loose cellulose, but putting in the required soffit vents is cost-prohibitive. I looked at flash and batt, but the contractors are basically saying “no thanks” because of all the angles. I could have them flash and batt myself, but . . . ugh. Plus I’d have to furr out all the rafters.
I’m told our inspectors aren’t that curious, both in terms of looking beyond the manufacturer info, or digging below the surface.
I think U-factor is going to be easier than R-value.
If I did 3″ of spray foam, and then 1″ or 2″ of rigid foam board to reduce thermal bridging, would I hit the U-factor I need? (Oh, also, there are 3 layers of shingles — previous owner didn’t remove, just added. ) Other ideas?
I’m confident based on what I read here that 3 inches of foam would be sufficient for the air seal and climate control (I’ll have a dedicated gas furnace and AC unit for this space). I’m just trying to hit code with as little add’l cost as possible.
Thanks!!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
>"If I did 3″ of spray foam, and then 1″ or 2″ of rigid foam board to reduce thermal bridging, would I hit the U-factor I need?"
No. Not even close.
A continuous layer of rigid foam board would also create a moisture trap putting the rafters at risk.
To hit code on a U-factor basis it needs to be under U0.026, which is R38.5 "whole assembly". With a combined 5" of R6-R7 foam foam it wouldn't hit anywhere near R38.5 even at center cavity, no matter how many layers of shingles there are.
Full-dimension 2x rafters 16" o.c. delivers a fairly high framing fraction too, undercutting the U-factor.
To hit code on a U-factor start with with 3" of 2lb roofing polyiso above the roof deck (R17) and a 5/8" nailer deck + shingles (~R1) and on the under side some Bonfiglioli strips ( see: https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf )on the rafter edges sufficient to increase the depth to 7.25" to accommodate R30 rock wool batts. At center cavity it would only be R47, but with ~R20 of thermal break over the framing fraction it would still meet code on a U-factor basis.
>"I’m confident based on what I read here that 3 inches of foam would be sufficient for the air seal and climate control (I’ll have a dedicated electric furnace and AC unit for this space). "
An electric furnace is grossly inefficient. A through-the wall PTHP or small mini-split could deliver both heating and air conditioning using only a fraction of the amount of heating energy. A 1/2 ton or 3/4 ton PTHP can be DIYed for about a grand, and would use less than half the electricity of an electric furnace.
If you're installing 3" of closed cell foam on the under side of the roof deck the framing fraction would be less than R4. It would be "worth it" to fill remaining 3" of cavity with blown cellulose or fiberglass, doubling the R-value of the framing fraction, even if cheap batts or low density blown cellulose/fiberglass.
Thanks . . . I mis"typed" -- it's a gas furnace, not electric.... I'll edit the post to correct.
I don't want to pull up the whole roof, which is what it sounds like your suggestion would require . . . thanks for the suggestion on the foam furring strips, though -- If I did closed cell plus that plus loose cellulose maybe we'll get close enough? I can't install wool batts in an unvented attic, can I?
When you say "hit code", do you mean hit the U-value required by the latest International Residential Code for new construction? Or the U-value required by your local codes for new construction? Or the U-value required by your local codes for a renovation?
In some jurisdictions and in some types of renovations, when you expose a cavity, sometimes you are only required to fill the cavity to the best of your ability (they don't require you to furr out the wall to hit the code reqs for new construction for example). This may or may not be the case in your jurisdiction. And it may or may not be your goal.
I only bring this up because "code min." is not so simple of a number as it might seem and there are often many paths to compliance.
It might be worth figuring out your u-value goal from a energy efficiency/cost effectiveness standpoint, then try and figure out if there is a path to code compliance from there.
>"...it's a gas furnace, not electric..."
Any gas furnace (other than a Dettson: https://www.greenbuildingadvisor.com/article/finally-a-right-sized-furnace ) is likely to be grossly oversized for the heating load of a tight insulated 1100' attic. Take the time to run load calculations before picking a heating solution.
>"I don't want to pull up the whole roof, which is what it sounds like your suggestion would require . . . "
That wasn't really a recommendation, just an example of what it ACTUALLY takes to get there on a U-factor basis.
If you're going less than code the greenest approach would be 2" of HFO blown closed cell foam (R14), which in zone 5 would be sufficient dew point control for up to ~R20 or so of fiber insulation. So with 2" of foam there would be 4" of remaining cavity depth. Adding 1.5" Bonfiglioli strips consisting of 3/4" polyiso + 1x furring would be enough room for R20 cellulose R20-R21 fiberglass or R23 rock wool, and provides an R5 thermal break.
The 2" closed cell foam is sufficient moisture protection for the roof deck, but if fiberglass or rock wool is used an interior side vapor retarder would be a good idea, since they can't safely buffer moisture the way cellulose can. The vapor retarder could be "vapor barrier latex" primer on the ceiling gypsum, or a 2-mil nylon "smart" vapor retarder such as Certainteed MemBrain, or even heavier duty Intello Plus.
> I can't install wool batts in an unvented attic, can I?
You can, as long as 40% or more of the total R is closed cell foam a the roof deck (either above or below.
In the paragraphs above I'm violating that by a bit. The worst-case R23 rock wool + R14 closed cell foam would be R14/(R23+R14)= 38%, which is close enough that a smart vapor retarder or vapor barrier latex can deal with it. (If you're in the warmer half of zone 5 you could probably safely skip the vapor retarder, but it's cheap insurance.)
Thank you! That all makes sense, as best as I can tell . . . now to see if any of the contractors understand all of that . . . I fear we're all little behind the times on this.
My HVAC company recommended against the mini-split ductless unit because of maintenance problems, but I see a lot of folks on here use them . . .