Possiblity of moisture within the layers of polyiso?
I need to meet code (R38- zone 5- on a cathedral ceiling) and would like to do this as cheaply as possible. The exterior of my roof assembly looks like: 3/4 ply, ice and water shield, asphalt shingles. The proposed interior would look like: 1 inch vent (ripped 1×1 w/ 1/4 ply), roxul (r 15), and 3 1/2 inches polyiso rigid insulation. The 3 inches of polyiso would be cut to fit in between rafters while the other 1/2 inch would go over the strapping to give me my thermal break as well as vapor barrier.
Although the polyiso will be layered one on top of the other, I’m still concerned about the inevitable gaps and the possibility of moisture breaching the first layer and becoming trapped in the subsequent layers.
As an aside, I didn’t like the idea of closed cell spray foam being applied directly to the underside of the roof as it already had one impermeable membrane on the exterior.
I would appreciate any feedback on this design.
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Assuming the polyiso pieces fit between the rafters are foam-sealed against the rafters, and mainly that the half inch laid over the rafters and covered with sheetrock together give you a good air barrier, then I wouldn't expect significant breaching of inside air with moisture into the roof cavity.
From the R values of the layers, it sounds like your rafters are 2x8. I assume you want to fit the polyiso between the rafters to push as much of the required R value up above the bottom edges of the rafters as possible, so as not to lose ceiling height. That's labor intensive. Also, adding just a half inch of insulation below the rafters adds little more than R3 to the total R of the rafter and sheathing, perhaps R9. That addition certainly helps, but not in a huge way. Why not fill the rafter bays with Roxul (~R25) and add a 2" layer of polyiso below the rafters? That would give you R38 total, with more significant reduction in thermal bridging, and the extra 1.5" drop in ceiling height might not be noticed away from the walls. Or you could raise the wall height to compensate.
I note here: https://www.greenbuildingadvisor.com/blogs/dept/musings/overview-2012-energy-code that the 2012 energy code calls for R49, not R38 for zone 5.
How can you be half finished and not have had your insulation plan to start with?
Fitting foam is the poorest idea of insulating. Continuous foam done right us better.
Derck,
I vote with Dick. Your plan is fussy. It will be hard to air-seal the little rectangles of foam that you want to insert between the rafters, because you will be pushing them against fluffy Roxul.
Skip the polyiso, except for the continuous layer under the rafters, and make it at least 2 inches thick.
Also -- I hate to see a continuous layer of Ice & Water Shield on top of your roof sheathing. It limits the ability of the sheathing to dry upward; there is some evidence that wind and solar effects can allow roof sheathing to dry upward (somewhat), even through asphalt shingles, as long as you have a vapor-permeable underlayment like asphalt felt.
Of course, if you you have a convoluted roof with many valleys in snow country, you may have no choice but to go with the Ice & Water Shield. But I prefer to see designers address ice dam problems with better roof design principles, not Ice & Water Shield.
AJ Builder- I assume you do this for a living, I don't. While you may have the contacts and resources to know all of your roofing and insulating options way ahead of time, many home owners (including me) are not afforded that luxury. I had planned on using continuous foam until I received the quote. Yes, it's the best option- one I can't afford. Thank you to Martin and Dick for their helpful feedback.