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Community and Q&A

Venting Beneath Roof Deck

mjw7 | Posted in General Questions on

I have a detached garage in NJ zone 4a and I am going to install Tesla power wall lithium ion batteries inside as a home power backup. They prefer to operate in the range 32°F to 86°F (0°C to 30°C) .


My plan is to insulate with closed cell foam: 2″ on walls and 3″ under the roof deck and install a 6000 BTU Mitsubishi Minisplit.
I have limited space and every foot counts so I want the ground floor and attic of the garage to both be a conditioned space. There are soffits but currently no ridge vent (just a 4″ gap between the OSB at the ridge with the felt and shingle cap on top)

The spray foam contractor says that I don’t need any ventilation space between the spray foam and the underside of the roof and suggests spraying directly onto the underside of the roof OSB. What is your opinion? I want to avoid ice dams and roof rot.

Dr. Joseph Lstibuerk (building science expert) seems to think that the underside of roofs need to be ventilated:

A Crash Course in Roof Venting

Note that I have solar panels installed on top of the roof and there is a ventilation gap below the solar panels. Would this obviate the need for venting the underside of the roof from soffits to ridge?

If not, would installing a product like Smart Baffles we effective even though my garage does not see to have a ridge vent? I suppose one benefit of having baffles separating the foam from the roof would be that come time to replace the roof, the roof underside won’t be stuck to the spray foam and thus break when the roof deck is removed (so I won’t need to re-insulate).

Should I also install a ridge vent? The expense will of course add up.
I’ve also attached some photos in case it helps.

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  1. mjw7 | | #1

    Here's photos of solar panels and soffits

  2. bloedelbuilders | | #2

    Best to ventilate. If you are insulating with spray foam, that is much simpler to do because there is no pressure from the insulation crushing toward the cavity (as there would be in a dense-pack cathedral ceiling application). Cardboard chutes stapled from rafter to rafter maintaining 1-1.5" air gap under the roof deck would be sufficient, then the foam installer would mist the cardboard to make a rigid layer, then complete the install. You should install a ridge vent and verify that the there is a clear ventilation space from the soffit to your chutes. Likely, the ridge was left closed because the space was not insulated, allowing incidental heating/cooling, but that will no longer be the case. Also, installing the foam directly to the roof deck voids many shingle manufacturer's warranties, if that is a consideration. Good luck with your project!

  3. Expert Member
    Joshua Salinger | | #3


    There is no need to vent below the roof sheathing if you use enough closed cell spray foam to stop condensation. According to code in your area, R-20 closed cell is what is required. This would be roughly 3.5" of foam. If this was a living space, you would have to bring the rest of the insulation up to min R-38 for a 'hot roof' assembly such as this, but since it is just to keep a battery within a certain temp range, this wouldn't be needed. There is nothing wrong with spraying ccsf directly against the OSB sheathing. One could staple a layer of felt or a membrane to the bottom side of the OSB if one wanted to avoid the future tear off issue, but a vent chamber is unnecessary. If you did vent it you would have to create vents at the soffit and the ridge which would be a bunch of work and costs.

    It seems to me the biggest thermal loser here would be that large garage door. Those are notorious for being energy losers...


  4. Jon_R | | #4

    > 0°C to 30°C

    If you condition to this range, the usual issues (like ice dams) are mostly gone. I'd not condition the entire space (and maybe not at all), but otherwise I'd keep it simple and not vent.

  5. mjw7 | | #5

    Thanks for the advice, I have decided not to vent.

    1. user-5946022 | | #6

      Ok, reasonable enough. However you already have vented soffits. Will the spray foam be applied to block that soffit venting from the inside - which would be sort of unusual. May want to think about replacing the vented soffit with solid soffit so you are not introducing cold air directly to the underside of the roof at the lowest point.

  6. Deleted | | #7


  7. ranson | | #8

    Out of curiosity, have you checked the lowest temperature that you can set on the mini split thermostat?

  8. Renovations102 | | #9

    My struture looked just like yours, although w/ a bonus room above the garage. You have the opportunity to really do things correct when starting w/ the canvas presented to you.

    I talked to Joseph Lstibuerk about my garage & bonus room above. He recommended an unvented system which I did. I blocked all the soffits w/ 1.5" formular. However, instead of doing the foam in a vertical fashion to block them, I installed it horizontal, locked it in with some canned foam, & then they sprayed over that. This took their foam all the way out to the edge of the roof essentially instead of leaving the typical 1 to 2 foot gap of dead air space. I did this to do my best to prevent ice dams.

    Roof of the bonus room = 5.5" of closed cell w/the rafters sprayed to combat thermal bridging.

    Bonus room gable walls = 3" of closed cell

    Garage ceiling = 2" of closed cell

    Garage walls = R-23 Mineral wool w/ Membrain

    Garage common wall = 4" of closed cell

    P.S As none of the walls, etc. were covered or insulated, I did extensive air sealing w/ canned foam, Zip tape, & NP1 sealant throughout the entire structure before all the insulation went in. Everything is now tight & very comfortable before I have even done the mini-split...

    Also, to try something new, I did the windows as outlined in the fine homebuilding article, where the openings around them are filled almost full of canned foan, then a foam backer rod is installed, then I used NP1 sealant over the backer rod, then taped...

  9. mjw7 | | #10

    @Renovation102 - I also reached out to Joe and he was kind enough to reply and give me the same advice he gave you. He said that because I am not in what is considered to be a high snow load area, I don't need to vent.

    Do you have any photos of what you did? I'm having difficulty understanding what you did exactly and why.

    Why would you need to block the soffits with foamular? What benefit does this give above just spraying foam in and sealing off the soffits with foam?

    >> "instead of doing the foam in a vertical fashion to block them, I installed it horizontal"
    Do you mean you installed the "foamula" horizontally, or are you referring to the closed cell "foam"?

    >> "w/the rafters sprayed to combat thermal bridging"
    Presumably you can't install sheet rock on top of foam covering rafters because the spray foam will be of uneven thickness along the edge of each rafter?
    Doesn't the closed cell foam look a bit ugly all over the rafters when it's not covered by sheet rock?

    >> "I did extensive air sealing w/ canned foam, Zip tape, & NP1 sealant throughout the entire structure"
    Why would you need to seal roof and walls before spraying closed cell foam on top? Won't the multiple inches of closed cell foam already have sealed this completely?

    1. Renovations102 | | #11

      Hello, I'll reply tomorrow.

      1. mjw7 | | #12

        The closed cell foam insulation is going to be sprayed next week so this weekend would be the last opportunity for pre-installing anything. Just reaching out again to see if you could clarify your suggestions above.

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