Hybrid roof insulation ratios + u-factor alternative
Hello all,
It’s time for a new roof, and I am interested in bringing the space behind the attic knee walls into the conditioned space in my mid-1800s New England farmhouse, Zone 6. There’s no amount of air sealing that I can do to stop cold air pouring down through the attic floor, and all the joints in the plate and posts/studs.
My plan is to basically follow this detail here https://www.greenbuildingadvisor.com/cad/detail/asphalt-roof-unvented-cathedral-exterior-3-rigid-insulation.
I am aiming for 3″ of exterior foam to the roof to avoid interfering (too much) with the windows on the shed dormers. I’m not being held to code, but the whole assembly should satisfy the u-factor alternative:
The walls of the lower story have been air sealed, new wrb, new windows and 3″ of exterior mineral wool, and a slightly compressed 3.5″ interior mineral wool. The dormers will get the same treatment.
I’ve read “The Ratio Rule for Hybrid Roof Insulation”, but how does that apply when my interior insulation doesn’t meet the code minimum for my area?
Is this assembly going to introduce new complications?
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Replies
Thermal code requirements are not important when it comes to the ratio, it's simply the vapor-impermeable R-value divided by the total R-value. In your case, R-18 polyiso plus R-20 cellulose is R-38. 18/38=0.47 or 47%. That's a good ratio in climate zones 5 or lower, but not enough polyiso for climate zones 6 or higher.
Because the issue here is long-term moisture accumulation, I use a conservative value for the polyiso layer--over time it loses its blowing agents and in cold weather it loses some of its R-value, so I'd use R-5.5/in as a good average. That changes your proposed ratio to 44%.
What is your climate zone?
Thanks Michael,
I am in climate zone 6 - southern Maine.
Then my options are to switch to another insulation type, like more mineral wool, or actually reduce the interior insulation to around R-15? That still gets me over the u-factor line
Yes, in CZ6, unvented roofs need at least 50% of the R-value to be in the impermeable layer. Your impermeable layer is R-16.5 (conservatively) so your cavity insulation should be no more than R-16.5.
Ruairi,
What code are you looking at for the U-factor alternative? This may not matter if 'code doesn't apply' as you say, but the U-factor alternative must consider the weighted average u value including thermal bridges (parallel path calculation)—not just simply adding up the nominal R values and then converting to U factors. That is no different than just using nominal R values so you would need to meet prescriptions.
In the 2018 IECC I see U-factor of 0.26 for ceilings.
2015 IECC, which is what Maine uses, but it's still 0.26 for ceilings. I'm not really being held to code, but also want to avoid any complications down the line.
I did the u values each for the insulation and the framing, and weighted them based on how much of the roof is insulated area vs framing.
Sounds like it doesn't much matter, but based on your above posted numbers, I don't see how you came up with better than U 0.26
You are using this method?:
https://www.greenbuildingadvisor.com/article/the-fundamentals-of-series-and-parallel-heat-flow
Here's the calculation of the framing, and yes I am using that method. The framing comprises 5.26% of the sq footage (2x5.25 actual, about 36" o.c.)
Layer R-Value
Asphalt shingles 0.21
Exterior air barrier 0.17
1/2-inch plywood 0.62
3 inches Polyiso 18
1" Board 1.41
5.25" lumber 6.74
Interior air film 0.68
1/2-inch drywall 0.45
Looks right! I guess my assumption based on adding your r values and seeing that they alone-- without framing factors-- barely made it was wrong. You have a very tiny framing factor going for you.
That said, it doesn't look to me like going with R-15 cavity passes this calculation. But being safe on the moisture side is of course the more important issue here.