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Unvented roof

JCDIY | Posted in General Questions on

I’m building my first ICF home in climate zone 5a and am concentrating heavily on energy efficiency due to high electrical rates in our area. I’m wanting to utilize an unvented roof with 5 1/2″ of polyiso on the roof deck and air permeable insulation under the roof deck. My roof structure (tobacco barn style) utilizes fairly shallow trusses. I wanted to dense pack 10″ of  cellulose under the roof deck which is one of the few jobs I will contract out on the build. My insulation guy said netting webbed trusses is tricky and suggested loose fill in the truss bays. My concern is settling of the loose fill which would create an air gap under the roof deck (possible moisture trouble and local code dictates the insulation must be against the roof deck).  My goal is at least an R-60 roof so would I be better using the loose fill, netting the trusses for dense pack, or maybe installing unfaced glass batts against the roof deck? **update** For clarity the roof style is high pitched (12/12) in the center of the home and sheds to 4/12 either side of  the home. My polyiso  satisfies the code requirements for exterior rigid board (R20 min) in my climate zone and i was trying to follow a formula I found in an older GBA article which used dense pack under the decking.

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Replies

  1. ssnellings | | #1

    I'm not sure what 'tobacco barn style' means and googling didn't help.

    Installing loose fill in a sloped cavity is asking for (or perhaps demanding) trouble. Maybe it will work out fine with the 5.5" of polyiso on the exterior helping to mitigate the issue, but without a compelling reason to play with moisture issues.... I wouldn't.

    Dense pack or unfaced batts are better choices, as long as the math is right on the polyiso to avoid a condensing surface. Someone who knows the math better than I do will come along eventually to answer that question. An easy solution, since you've already started putting insulation outside the sheathing, is to just keep going until you hit the number you want.

    With the dense pack I've seen contractors net off each truss bay as a separate fill to make it easier. Perhaps doing this extra netting is the 'tricky' bit your contractor refers to, or perhaps he hasn't seen that before and it's worth mentioning. It's a lot of extra netting work.

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