Logistics of exterior roof insulation
Dear GBA readers,
I'm starting the process of finding a roofer who can add up to 6 inches of insulation on top of the roof. I have an unvented cathedral ceiling in the attic, with rafters filled with mineral wool. I would like to get a clearer picture of what is involved before describing my needs to prospective contractors.
I'm in climate zone 5 and I've read Mr. Holladay's post about performance of different types of foam in cold weather. I'm leaning toward EPS at this point. Or maybe 2" polyiso and 3" EPS on top. But I've also heard a bit about dense mineral wool board for exterior walls. Is there a similar product for roofs? (If so, would you recommend investigating this further?)
Also, I don't really have a clear picture of exactly how this is done. From what I've read, the foam is laid in overlapping layers, taped, and nailed to the rafters with long nails. Then new sheathing is laid on top -presumably on furring strips so that the top roof is vented like an umbrella over the foam. Is it really structurally sound for this entire assembly to be held in place with nails passing through foam?
I haven't found anything online that really spells out this process- if anyone has such a link it would be most appreciated. The attached photo from one of Holladay's articles gives a very good idea- I certainly plan to show it to contractors. It doesn't seem to have any space between the foam and the top layer of sheathing though. Maybe that's not necessary?
Posted Thu, 04/10/2014 - 10:12
Edited Thu, 04/10/2014 - 10:13
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