Maine unvented low-slope roof efficiency
Last Post 02 Apr 2021 10:37 PM by GoGreenSelene. 2 Replies.
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jdblosUser is Offline
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23 Mar 2021 05:41 PM
Hi, We are building a home in coastal Maine, zone 6, with a low-slope roof (2/12). We'd prefer exposed beams so have been exploring rigid insulation. Currently we plan the following layup (top down): Solar Panels-- EDPM rubber roofing-- 3/4 ply-- rigid foam insulation-- 1/4 birch ply-- exposed beams. This will primarily be a 2-season house (summer & fall) but heated during the winter to 50. A decent percentage of the walls will contain a lot glass (triple pane as long as fits within budget). Our town has no codes. Question is whether insulating the roof above R-30 rigid foam board is worth the extra expense? My thought is that above 6" of foam board, additional framing would be necessary because otherwise it would be difficult to attach two layers of ply and 10-12" of foam with fasteners alone. Additional framing increases cost quite a bit and would probably also require deeper beams to carry the additional weight of the add'l framing on top. To go from r-30 to r-60 will probably double the cost of the roof structure. The house's heat load will probably be handled by the solar power generation. My research has led me to believe that in a properly sealed house, the cold-weather difference between r-30 and r-60 is not dramatic. Am I missing anything?
newbostonconstUser is Offline
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24 Mar 2021 10:32 AM
Not optimal but couldn't you put a layer of foam above the 3/4 plywood?

Betting with what you have said that triple pain windows will not make the cut....you seem very intelligent and I think you will calculate out that they aren't worth it.

Are you able maximize solar are gains through windows? This will greatly reduce your heating cost and need for insulation.

On windows, they pretty much can coat 4 sides of the glass when making them. The more number of coatings the more cost and the less heat that can travel through them in both directions(into and out of the house - it isn't picky and blocks both). Cardinal Glass has the majority of the patents and thus they supply everyone the glass. Every window company adds a tricky name to it but anyway it is all the same. I would put one layer on south facing windows, 2 layers on east and west and 4 layers on the northern windows.

Also because of your location I would use Canadian windows....US manufactures suck at cold window....We went to Canada for ours and are happy and they were cheap.

Good luck with your project.
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
GoGreenSeleneUser is Offline
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02 Apr 2021 10:37 PM
OMG I thought I was the only who thought that the US was not exactly the greatest at creating cold windows!
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