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Community and Q&A

Foam roof deck or seal ceiling?

xeHJ6QUaDY | Posted in Energy Efficiency and Durability on

I am looking to improve the insulation and reduce air infiltration in an attic which has no lower ventilation because there is no overhang on the roof on all four sides. The ceiling area of the attic is about 865 sq. ft and it is constructed of 2 x 6, 24 inches on center. Currently, there is blown insulation which has settled or been disturbed throughout. At some point, a previous owner opened up three 16” x 16” open registers, simply pass thru-vents from the second floor ceiling to the attic, apparently to provide lower ventilation to the attic. They provide unnecessary air infiltration to the attic and we would like to close them off. There are mushroom vents on the top of the roof, (4) and a power vent which does not work. The roof deck is also 2 x 6, approximately 1180 square feet. We would like to improve the insulation levels in the attic as well reduce the air infiltration so blocking the registers makes sense, but that would leave us with no lower ventilation. So my question is, which would be a better alternative, sealing and insulating the second floor ceiling joists (providing lower ventilation with a vented drip edge system) or spray foam the roof deck and conditioning the attic?

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  1. Jim Bannon | | #1

    Depends on your budget. Each approach would improve your current situation, but foaming the roof deck will cost substantially more. There is still some controversy on foaming roof decks, so I'd advise air sealing and adding cellulose to the 2nd floor ceiling.

  2. Riversong | | #2


    The grills through the ceiling are not lower attic vents. The purpose of attic ventilation is to allow the unconditioned area above the thermal envelope (ceiling, in this case) to remain cold in winter, to vent out summer heat gain, and to allow any moisture to dry out.

    The ceiling registers merely help heat the outdoors, so by all means close and seal them.

    You don't indicate where you are (what climate zone) or what kind of blown insulation is currently in the attic floor.

    The most cost-effective solution would be to remove all existing insulation (throw it out of it's old and mouse-eaten (if it's vermiculite, you'll have to have an asbestos mitigator remove it), air seal the entire attic floor with canned foam including the interface between attic floor and exterior wall plates, and blow cellulose insulation to whatever depth you care to purchase (12" minimum).

    The attic can survive unvented as long as its relatively open and can dry itself out, but the fact that you have only exhaust vents (however inadequate) means you'll have negative pressure that can draw air from the conditioned space below if it's not very well sealed.

  3. GBA Editor
    Martin Holladay | | #3

    I agree with previous posters. In this case, insulation on the attic floor is better than at the roof slope (assuming there is no HVAC equipment or ductwork up there).

    What's your climate?

    Air-sealing the ceiling plane is essential. That will go a long way towards keeping your attic dry. Then bring in a cellulose hose and pile it deep.

  4. terrell wong | | #4

    My favorite new air sealing of attics - Just did it at Rosedale House and at my cottage:.

    1 Make sure you don't have any potlights - especially in bathrooms in the ceiling under the attic.
    2 Spray 2lb foam min 2" - then its a vapour barrier and the damn easiest way to air seal such an awkward space. (Professional or do it yourself)
    3 fill the space with as much cellulose as you can afford.

    In your case you might need to pop in some roof vents low on the roof, if you only have ones high on the roof, to allow for ventilation. Be sure to direct the air flow away from the cellulose with bafflesbetween the rafters right below the new vents.

  5. xeHJ6QUaDY | | #5

    Thanks for the input. I live in the Chicago area. I'm not sure what the exisitng insualtion is, may be vermiculite? I like the idea of blowing in cellulose, but I'm worried about no lower ventilation.

  6. Riversong | | #6


    You say no roof overhangs on all four sides. Does that mean it's a hip roof with eaves on all sides and very little ridge?

    In any case, the next time you replace the roofing, I would recommend removing the "mushroom vents" and installing either vented drip edge ( or the Edge Vent (, and full ridge ( and hip vents (

    This is the only way to get adequate attic ventilation and keep it sufficiently balanced to avoid negative pressure problems.

  7. xeHJ6QUaDY | | #7

    Exactly. hip roof with very little ridge. No soffit. Maybe like 12 feet of ridge. Thanks for all your help.

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