Polyiso Exterior Insulation for Vaulted Octagon-Shaped Roof
Hello, thank you all for all you’ve taught me so far! I’m designing a freestanding kitchen/living building in Zone 2A, 50 miles east of Austin.
1) Dimensions: 28’ W octagon, cathedral ceiling (656 sq ft, 8279 cu ft).
2) Walls: ~R-9.4 *overall* (R-20 cavities, U-0.27 windows, 2×6 studs).
3) Windows: Lots of westish-facing glass (SHGC-0.21), which I’ll try to counter with white solar shades, some existing tree cover, and 2′ 4″ overhangs (via “real” rafter tails… aka thermal bridges).
4) Floor: ~R-1.88 (hardwood, plywood, 2×10 joists). ~3’ H fully open crawlspace (think ‘house on stilts’). No floor insulation — I’ve seen too many insects nesting in rigid foam in crawls.
5) Roof/ceiling: R-???. Insulation will solely consist of exterior polyiso – rafters will be exposed on the interior, as the ceiling. 86 SRI white metal roof, zero penetrations… and mostly unshaded, sun-drenched.
The Question: How much polyiso should I put on the roof? Or maybe: When it’s 120 on the roof, how hot is the ceiling, given R-X vs. R-Y? Let’s assume polyiso performs ~R-5.6 in Zone 2A.
1) Recommendations for Zone 2A: R-60 (Wilson/GBA), 30-60 (Energy Star), 40-50 (BSC), 38 (Austin). But I’m reticent to just adopt these because…
2) My concern is thermal comfort, not payback. I’ll hit net-zero with even meager polyiso. I’d rather keep heat out of the building in the first place… I don’t like to run/maintain AC and fans all the time. This is forever-ours building, and we may face 100 degree Octobers some day. With my big vaulted ceiling and windows, I feel this is even more important.
3) I’m not sure how to apply [Q = U x A x delta T] to heat gain. Some posts that indicate it can be done:
-Dana Dorsett: “Once you’re at R50 taking it to R100 would make less than a 1F difference in peak & average ceiling temp, and isn’t worth doing for cooling comfort or cooling energy use.”
-Allison Bailes: “[Although the energy savings of adding insulation here may not be substantial,] if we look at the percent reduction in heat transfer… it’s significant. The [assemblies with more insulation] reduce the heat flow through walls by about a quarter.”
4) Costs: I’m the builder and can snag recycled polyiso fairly cheap, so ignore costs, pretty much.
5) Gut feeling: Between 7″ and 10″. Assuming very good air-sealing all around, including on my silly glassy walls, by yours truly.
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