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Yet another insulation question

user-7213032 | Posted in Energy Efficiency and Durability on

I’m building in New Hampshire and have three opinions plus my own about how to insulate my cathedral ceiling.
First insulation guy–all closed cell foam for a “hot” roof.
Second insulation guy–1-2″ closed cell foam and fill the bay with open cell, still a “hot” roof.
Third insulation guy–all open cell.  Says closed cell is a waste of money.
Carpenter, and my own inclination,  soffit vents and a ridge vent with Accuvent running from the soffits to the ridge and some sort of foam sprayed over the Accuvent.  
When the first insulator suggested a “hot” roof, which I had never heard of before, I called the shingle manufacturer (GAF) and was told that there would be a warranty issue if there were problems with the shingles and the roof was unvented.  I wouldn’t expect problems but the excavator who did my dirt work told me that he was talked into a “hot” roof and five years later had to reroof.  Incidentally, he was right about most everything else he suggested from who to get to drill the well to how to situate the septic tank, so I give what he said about his roof a lot of credence, even though he usually deals with the bottom of projects rather than the top.
So, given that I am going to vent the roof using Accuvent (unless someone can give me a compelling reason not to) what foam should I use over it:  closed cell, mixed or open cell?
Finally,  has anyone had experience with “Accuvent”.  The last time vent strip I used was made out of the same stuff they pack a dozen eggs in and I was not at all impressed by it. It probably would disintegrate if sprayed with foam.  I really don’t want to (and am not going to) create vent channels with furring strips and plywood or masonite or whatever, which might be the best way to go but is a little too labor intensive for my taste.   (Same reason I’m not going to use Roxul in the ceiling but will in the walls.)
Thanks very much.

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  1. Expert Member
    Akos | | #1

    If you can vent the roof, why are you even thinking about SPF? Go with batts, way cheaper. Except for air sealing there is really no performance benefit for SPF.

    If you really want to go with SPF, for a vented roof open cell is fine. The foam puts no load on the vent channels, pretty much anything works there. I've sprayed closed cell over the stuff sold at the box stores without issues.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi user-7213032 (would be great to get your real name).

    I don't believe that you mentioned the depth of your rafters. Unless they are pretty deep, with a vent channel taking up some of the space, you may not be able to hit the current International Residential Code minimum R-values for New Hampshire. Maybe you don't need to or care to. But no matter what you do, there's a lot of cost and labor involved and there are other, and better options than you are considering. Have you read this: How to Build an Insulated Cathedral Ceiling?

    1. user-7213032 | | #3

      Thanks very much for your replies. I did read the article and this is my take from it:
      "If possible, include a ventilation channel above the top of your insulation layer. The ventilation channel will provide cheap insurance against moisture build-up, and will lower the chance of ice damming."
      The rafters are 2x12 and I will be able to meet the minimum R values. The reason I am using foam is because it's too much hassle to install batts in the rafters.. I am using Roxul and Membrain in the walls and will foam the rim joists and ceiling (with some sort of foam--which type is one of my questions) and probably some other little weird holes and spaces with open cell foam.
      I've considered the options and don't want a "hot" roof, whether it's a good idea or not. I'm old enough to remember the fiasco caused by the then revolutionary idea of plastic over unfaced fiberglass batts in "all electric houses" which then sort of rotted from the inside out. I assume that in today's air tight houses moisture leading to rot isn't a problem but the builders that built the "all electric" houses were pretty sure of what they were doing then. I also remember the "revolutionary" polybutylene plumbing and aluminum house wiring and we all know how those worked out. Enough said about that. I was always taught that wood (including roofs) has to breathe and I'm hoping the Accuvent will accomplish that. I'd like to hear from someone who has used it with good or bad experience. Thanks again.

      1. Expert Member
        Dana Dorsett | | #4

        >"I’m building in New Hampshire..."
        >"The rafters are 2x12 and I will be able to meet the minimum R values."

        How are you planning to do that?

        The IRC 2018 code minimums for US climate zones 5 & 6 is R49. A 2x12 is 11.25" deep. With a 1" vent gap that leaves 10.25", which would require a foam with R4.8 per inch if done with just one type of foam. It won't get there with rock wool + MemBrain.

        That's do-able with Icynene's ~2lbs per cubic foot density water-blown MD-R-20o or MD-R-210, (assuming they still offer those products after getting into bed with LaPolla) but not any other products.

        From a total verditude point of view it's comparable to (slightly worse than) doing it with 7" of HFO blown 2lb foam, but would outperform the 7" foam solution slightly due to the longer path through the framing (10.25" of rafter is ~R12.5, whereas 7" of rafter is ~R8.5). A 2lbs water blown Icynene solution is WAY greener than an 8" of HFC blown 2lb foam solution.

        In zone 6 parts of NH (most of the state) going with half the total-R as HFO blown foam one could install 4" 2lb foam on the exterior (~R28) , with 6.25" of half-pound open cell ( ~R23) without any risk of wintertime moisture accumulation in the open cell foam at the boundary between layers.

        In zone 5 parts of NH (all the counties bordering MA plus Strafford County) one could get away with 3" of HFO blown 2lb foam (R21-R22) and 7.25" of half-pound goods (R27), which close enough to call it R49, or R30 rock wool batts, without a need for interior side vapor retarders (though MemBrain would be cheap insurance.)

        Even HFO blown foam (and 2lbs water blown Icynene) has a fairly hefty CO2e footprint- it's about twice the footprint per R of half pound open cell foam or rock wool:

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