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How to best insulate and vapor barrier Room in Attic Truss?

haf45964 | Posted in Energy Efficiency and Durability on

I live in zone 7 Alaska. We have built at 24′ x 42″ detached garage/shop with Room in Attic Truss. The roof pitch is 10/12. The trusses are 24″ OC. The top chord of the truss is a 2×6, but we’ve furred it out where the top chord defines the interior ceiling, so essentially we have 7″ of room to insulate in the this part of the roof. The “wall” of the attic truss is 2×4. The roof consists of OSB, underlayment , 4″ of Rmax, OSB, then shingles. we have also wrapped the walls of building with 2″ of Rmax. We intend to have an unvented roof, use R38 batts between the trusses in the “attic” areas of truss and to install vapor barrier at the walls and ceiling of the room in attic truss. Obviously our goal here is to insulate as best we can due to the cold winters. After much research and talking with local building inspectors we still are uncertain about some of the finer details of our insulation/VB plan. The complexities all arising around the room in attic truss, and the fact that we have Rmax above the roof decking, so the webbing of the truss is already somewhat conditioned space.
Our big questions, which are two sides of the same idea . . .
1. Should the walls(2 x 4) of the Room in attic truss be insulated?
2. Should the webbing section of the truss be thermally isolated from the interior, or should we try and equalize it with the interior.

One additional complication, we have plumbing in the webbing section(hot and cold pex), and we plan on having baseboard heat in the space as well. The baseboard supply lines will run through the webbing, since we aren’t drilling holes in the trusses.

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  1. GBA Editor
    Martin Holladay | | #1

    Most building codes, including Section R806.4 of the 2009 IRC, specify the minimum R-value for rigid foam installed above roof sheathing if you are building an unvented roof assembly. In your case (climate zone 7), you need a minimum of R-30, so 4 inches of polyisocyanurate (R-24) isn't enough. You need 5 inches. (For more information on this topic, see How to Build an Insulated Cathedral Ceiling.)

    Once you have installed 5 inches of polyiso above your roof sheathing, I think it makes sense to insulate all of your rafter bays the same way, whether or not you have an "attic room." Remember, it can be tricky to secure R-38 batts to the underside of your roof sheathing if you have trusses rather than rafters -- so think through your installation details carefully before proceeding.

    I don't advise using polyethylene in this type of roof assembly. If your building inspector wants to see a vapor retarder that comes in a roll, use a "smart" retarder like MemBrain.

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