What are my options for fastening R-40+ rigid insulation on exposed rafters?
I have 2×10 rafters which I would like to leave exposed. My idea is to put T&G 1x’s above the rafters.
On top of the T&G I plan on stacking rigid insulation (R-40 min.)
My question is regarding screwing OSB on top of the rigid foam all the way through to the rafters. Other than using 8″, 10″, 12″ long screws (depending on the type of rigid foam) is there a different way to fasten this together?
Is it possible to top the first 4″ of foam with 1/2″ ply and screw that through to the rafters then stack an additional 4″ of foam topped with OSB and then screw that into the ply?
I’ve been a lurker here for some time and I appreciate all that GBA provides.
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Long screws are the usual method used. For more information, see this article: How to Install Rigid Foam On Top of Roof Sheathing.
-- Martin Holladay
If it's continuous foam you really don't need more than ~R37 of foam to hit code-min performance on a U-factor basis, which demands a U-factor lower than U0.026, which R38.5 "whole-assembly", with credit for the thermal performance of the roof deck & shingles (~R1), and the interior & exterior air films (another ~R1).
With the additional ~R0.5 thermal performance credit for a 1/2" nailer deck over the foam, a very buildable 6" of polyisocyanurate (R36-ish), R2 of structural deck / air films / shingles gets you down to U0.026 or a shade under.
That's do-able with 8.5-9" screws.
Martin, Yes that article prompted me to as this question. The article asks: "How hard is it to screw through thick foam?"
Six inches of foam and 10" screws is the scenario I'm looking at.
USER=1004076, Thank-You. If it makes a difference here's what I'm working with:
• Zone 5
The stack is something like this:
• 1x6 T&G Knotty Pine on the rafters
• 1/2" OSB
• 6" Foil Faced Polyiso, staggered, taped seams
• Zip System 1/2" as an overlay and membrane
• 1x3 furring strips for venting
• Metal Roofing
I'm confused by the performance of Polyiso vs. EPS. We see temperatures much colder than 40°F. We can use EPS and stagger and tape the seams but how much is needed to meet code?
It's the AVERAGE temp through the foam, not the cold side temp that determines the derating & uprating factors. When it's 70F indoors and +10F outdoors and the foam is the only insulation layer, the average temp through the foam is 40F. When it's 40F outside the average temp through the foam is 55F.
EPS will run about R4.5/inch when it's mean temp is 40F and is usually (and tested using ASTM C 518 at that temp by the manufacturers. Most polyiso will perform above R5/inch at that temp, and many will be above R5.5/inch
https://www.greenbuildingadvisor.com/sites/default/files/Polyiso%20penalty%20-%20BSC%201.jpg
In most zone 5 locations the binned hourly mean outdoor temperature will be above 20F even in January, so your average January performance would be at an average temp through the foam of about 45F, where most of the polyiso samples in that graphic tested north of R5.5/inch. The rest of the heating season would be at an even higher mean temperature.
Codes only care about the labeled R and U-factors based on the labeled R, which is tested at a mean-foam-temp of 75F. While your seasonal average performance of 6" polyiso might really only be R33 rather than the labeled R36, you'll note that bumping it to 6.5" would deliver the performance even at the derated value. But from a total energy use point or peak heating load point of view the difference is really pretty tiny.
User, Thanks. Great Information. I can work it from there.