I would like to build a metal-clad roof.
rockinroger
| Posted in Green Building Techniques on
The roof will have 2×6 16″o.c., rbs, and R-13 batts stapled to the inside of the attic, leaving an air space below the rbs, so air can flow from soffit vents in each bay to a ridge vent at the top. Does this make sense? I plan to insulate the 2×6 ceiling with closed-cell foam. Thanks.
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Royal Bank of Scotland? What is rbs? Perhaps you can describe your proposed roof system less cryptically and more thoroughly.
rbs=Radiant Barrier Sheathing.
If I understand you correctly, you plan to intall fiberglass batts between the rafters, leaving an air space between the radiant sheathing and the fiberglass for a vent channel. And you plan to separately insulate the ceiling assembly with sprayed foam, leaving the attic as unconditioned space.
What, then, is the purpose of the fiberglass batts? Simply to create an air channel? Fiberglass loses most of its insulating ability when air moves through it and will add nothing to the radiant barrier. There are simpler and less costly ways of creating an air channel, but none is necessary if the attic is unconditioned. Warmed air will still rise up the rafter bays, provided there is enough pitch to create a minimum of 3' of height difference between intake and exhaust vents.
I'm having to make assumptions because you're still not offering a comprehensive description.
Roger,
If your house has an insulated flat ceiling and an unconditioned attic, there is no need to insulate the roof rafters, unless you have attic ductwork.
1. If there is no attic ductwork, your plan to insulate the flat ceiling with closed cell foam is adequate. You're done. There is no need to insulate the rafters.
2. If there is attic ductwork, you should plan to insulate the sloped rafters and bring the attic into the home's conditioned space -- in other words, you should cathedralize your attic. If you are planning to have a spray foam contractor on site, by all means use closed-cell spray foam to cathedralize your attic -- not fiberglass batts. If you are cathedralizing your attic, you don't need any insulation at your flat ceiling (attic floor).
Thank you both. You have answered my question.