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Using 1″ XPS as the vent baffle

Tobias | Posted in Energy Efficiency and Durability on

I am thinking of using 1″ XPS as a vent baffle in a cathedral ceiling, similar to what is shown here:

I wouldn’t actual use that product, but the same idea. I am thinking of spacing the xps 1.5″ off of the underside of the roof sheathing, and sealing any gaps with a combination of sprayfoam and tape. The roof would have soffit and ridge vents. The rest of the bay (approx 7″) would be insulated with fiberglass bats. I am building in a cold climate, and the space is a conditioned room over an unconditioned garage. The roof is 2×10 stick framed.

Is this a sound plan? The client has decided to go with fiberglass insulation as a cost-saving measure, so I am trying to make the best of that with good air sealing. I am figuring that having the roof vent detailed in this way will limit the amount of air moving though the f/g batts, making them more effective. I am concerned, however, that the conditioned side of the xps will become a condensing surface within the roof assembly.

I appreciate any input on this,

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  1. Peter Yost | | #1

    Hi Tobias -

    I have not used the vent baffle product you cite, but it looks pretty slick (There are 4 or so insulation baffle products listed in the GBA Product Guide, but this is not one of them). So does your site-made approach. Getting an air seal on all six sides of a cavity insulated with a highly air-permeable cavity insulation such as fiberglass batt is really important. My main concern with the assembly you describe is the overall thermal resistance or R-value that you will achieve with fiberglass batt in just a 7-inch deep space. This sounds pretty slim for a cold climate in a cathedral ceiling.

  2. Tobias | | #2

    Peter, agreed that the situation is less than ideal. I figure the whole assembly will be about R 30 (mid-bay). For the use of the space (it will be a studio, not living space, so not heated all the time) and the budget, I think it will be okay. I'm more concerned that the xps will become a condensing surface within the roof.

  3. Riversong | | #3

    You're right in being concerned about creating a condensation plane on the foam baffles, though it would be far less problematic than installing batts tight to the roof sheathing with no vent cavity. Continuous soffit-to-ridge ventilation is "best practice" in all but the most humid climates. And ridge vents need to have an exterior wind baffle (like Air Vent and Lomanco) to be effective and prevent infiltration of moisture.

    The key on any cathedral ceiling (or attic) to prevent condensation problems is thorough air-sealing at the ceiling plane and at top plates of walls.

    With KD 2x10 rafters, a 1.5" vent channel and 1" XPS will leave 6.75" for insulation. Whether you were planning to install 5.5" batts or compress thicker batts into that cavity, you'll end up with an effective assembly R-value (whith 16" oc framing) of R-20 to R-24.

    I would suggest trying to talk the client into blown cellulose, as it would offer better R-value and air sealing and create a moisture buffer to compensate for minor moisture intrusions. Do not, however, intall a vapor barrier at the ceiling plane, as that will prevent drying to the interior. The XPS (perm=1.1) will allow modest drying to the exterior.

  4. Tobias | | #4

    Thanks for the info. Think I might of sold the H/O on the insulation upgrade to cellulose. Spend more on shell, less on HVAC (now and forever). No vapor barrier anywhere. I'll look into the ridge vents you spoke of.


  5. Expert Member
    Michael Maines | | #5

    When faced with a limited depth as you are, I generally prefer to fill the bay with dense-pack cellulose. A properly vented roof is desireable, but when it means the difference between R-values in the mid-20's and values in the mid-30's, I would vote for more insulation. As Robert says, cellulose can absorb a bit of moisture now and then which makes it perfect for an unventilated roof. The ZIP sheathing we use is an air barrier.

    Are you strapping the underside of the rafters, as we do in the northeast? The bays between straps can be filled with 3/4" rigid foam. Or strap with 2x2's and get reduced thermal bridging along with 1-1/2" of extra cellulose. Don't use vapor barriers on the inside, but vapor-retarding paint and other airtight drywall approaches are a critical to good performance with an unventilated roof.

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