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Pros and cons of closed cell foam on roof deck, non-vented attic

MSSUSR9501 | Posted in General Questions on

Looking at re-insulating the house after significant remodeling.
I am North of Grand Rapids MI, north edge zone 5/ south edge zone 6 transition area.  Roof is 7:12 pitch, truss construction.

I am interested in using closed cell foam directly on the roof deck.  This would provide vapor barrier and result in the attic being conditioned space.
I was quoted 6″ minimum /7″ average closed cell foam on roof deck.  Walls will be high density foam to R20

What are the pros and cons of this approach?
What do I need to watch out for?
I presume I may need to reevaluate the heating equipment (heat pump).
What risks am I incurring and what do I need to discuss with the insulating and HVAC contractors?

Objectives for this project are to make the home significantly more efficient and modernize to the degree I can afford.  Planning to retire in the home, want to get the work done while I am working.

Thanks for any comments, this site has been an invaluable learning tool for me.

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi MSSUSR9501.

    Some of the pros are high R per inch, good initial air sealing, and assuming you get a qualified installer, a less complicated assembly to detail than some of the options. The cons are that closed cell spray foam may not be cost-effective when compared to other options (of course that depends on actual costs in your area), depending on the install, it may not address thermal bridging, many people report that the air seal is not long lasting, when installations go wrong, it is not easy to remedy, repairs are labor intensive, an exterior insulated assembly done well is more durable (maybe that's not a fair con), and closed-cell spray foam is less climate-friendly than many alternatives. I assume you have seen this article, but if not, you may find it helpful: How to Build an Insulated Cathedral Ceiling.

  2. MSSUSR9501 | | #2

    Hi Brian,
    Thanks I had seen that article.
    Cost is not my primary concern although it is a factor.
    Mostly I want long term reliability. I don't want to do something that results in roof problems 5 years down the road.
    I have read that directly insulating the roof deck is the most effective approach but have concerns about humidity in attic, how the roof deck will dry with asphalt shingles above if it gets damp etc.
    One thought was to space foam or plastic chutes below the roof deck and apply the foam to the chutes, leaving an air gap above the foam and below the roof OSB.

    Since the roof is truss built, a 6 inch thickness would cover the upper chords of each truss completely. The bridging would be where the angle chords meet the top chord. The exterior walls of the house will be a bigger issue in that regard but not planning to tear off siding to install foam and reside

    Nice thing about insulating the roof deck is I can still get into the attic to evaluate the condition of the foam and if it pulls away or shrinks excessively I can have the contractor come back.

    The standard locally seems to be blown cellulose, had a tough time getting anyone to quote foam on the roof and could not find anyone who would do blown mineral wool.

    Cellulose just does not appeal to me at all for a variety of reasons.

    Thanks again for your comments.
    Bruce

  3. Jon_R | | #3

    Consider that shingles eventually fail and it may be hard to replace sheathing that is "glued down".

    1. JMrtns | | #4

      I've always thought about this. If you have to replace sheathing on a SPF roof, I'm sure ripping out that sheathing is going to bring some foam attached to it. Is this what you are getting at Jon?

      1. Jon_R | | #5

        Or maybe all of it. And the process might leave you without the good air sealing that SPF usually provides. Would be nice to hear from someone who has replaced sheathing on a few SPF roofs and monitored the results afterwards.

        Maybe a spray on release agent or membrane is a good idea. Or spray from above.

        1. Expert Member
          BILL WICHERS | | #6

          If you use a membrane, you're likely to have voids where you could have moisture accumulate in a roof assembly. I have used housewrap stapled to the inside of WALL sheathing as a way to keep spray foam from sticking though (to allow for possible future replacement of T1-11 siding that was also the structural sheathing), and it works but you need to be careful to staple it down to the sheathing to keep it flat. It's also important to keep the housewrap off the studs so that you avoid voids in corners, and to ensure the spray foam cures to a sort of "block" between the studs that will stay in place when the exterior siding is removed.

          I'm not aware of any spray-on release agents that could be used -- spray foam is notoriously sticky stuff, and I've even seen it adhere to polyethylene that is normally pretty resistant to things sticking to it.

          Bill

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