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Spray foam insulation in low-slope shed dormer roof

michco | Posted in General Questions on

I would like to insulate a shed dormer roof for a second story bathroom.  (Located in Oregon, Zone 4C).  It is a low slope, unvented roof assembly, 2×6 construction.  The roofing material is 3-ply roof system (self-adhering SBS modified bitumen) on 5/8″ OSB.  The size of the roof is approximately 10’x7′.

Due to the limited cavity space created by the 2×6 rafters and blocking, my plan is to use spray foam insulation directly on to the underside of the roof sheathing.  The rest of the roof on the house is vented.  I plan to make this  section of the roof over the bathroom airtight and self contained with foam.

My question is:  I believe to achieve R21 it would be approximately 3 1/2″ of foam.   This leaves about 2 inches of space above the green board ceiling.  Should I leave this gap?  I have read it is inadvisable to put any type of a vapor barrier on the green board ceiling.  Is this correct?  Thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    Most building codes require insulated ceilings in Climate Zone 4 to have a minimum R-value of R-49. So you want more than 3.5 inches of insulation.

    If you are using closed-cell spray foam, you should be able to install 5 inches of spray foam in your 2x6 rafters -- meaning that you'll end up with an R-value of something between R-30 and R-32.5 . That is still shy of what you want, but better than R-21.

    If you end up with a small air space between your greenboard ceiling and the spray foam insulation, the air space will do no harm. You don't need to install a polyethylene vapor barrier -- closed-cell spray foam is already a vapor barrier.

  2. michco | | #2

    Thanks for your response. You have answered my concerns succinctly.

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