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Attic insulation upgrade — 4/12 hip roof

phumer1 | Posted in General Questions on


I am upgrading the attic insulation in my house and have two questions for you all.  First let me give some background.

The Roof is a hip roof built with 2×6 24″ OC and the pitch of the roof is roughly 4/12.  The Roof Joists do not line up withe ceiling joists. I am considering converting the roof assembly from a vented to an unvented attic using spayfoam directly adhered to the underneath of the roof deck.  I live in Zone 4 roughly 20 south miles from the Zone 4/5 border.  Specifically I live in the south east corner of Colorado.

I have recently obtained quotes and in the process of deciding whether to go with closed cell or open cell spray foam.  The contractor that I am considering noted within his estimate that he can spray 8 inches of open cell for the same quoted price as 2 inches of closed cell.

Initially I was going to spray 2″ of closed cell foam and then fill the remaining 3.5″ with rockwool insulation.  Then at a later date apply 3.5″ reused polyiso over the top of the roof deck apply 1/2 OSB then metal roofing.  An online whole wall calculator I am using rates this assembly at R-49 not considering the metal roofing.  This assembly will need to be completed in steps and make take years to come to fruition.

Therefore I am now considering filling the roof joist bay with open cell roughly 5.5 inches then spaying the remaining 2.5 inches over the roof joists in order to compensate for thermal bridging.  The same online whole wall calculator I am using rates this assembly at R-45, which is close than the initial plan and quite a bit cheaper both labor wise and time wise.  Given I have been obsessing about this project for quite some time I wish to be done with it as soon as possible so that I can get back to my true hobby of woodworking.

So my first question is would I be setting myself up for a failed roof deck by pursuing the cheaper option of open cell foam over the closed cell foam + Rockwool + Polyiso + Reroofing?

My second question what would be the easiest way to block between the roof joists so that I can ensure that the unvented attic seals at the top plate, given that the roof joists don’t line up with the ceiling joists, and the low pitch of my roof?  Would it make sense to remove the soffit and block from the exterior in this situation?

Thank you for your help both in this specific question and many other problems that I have used your site in order to address.


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  1. Expert Member
    Dana Dorsett | | #1

    What's a "roof joist"? Is that what most people call "rafter"?

    At 5.5" + 2.5" = 8" most open cell foam is only good for R30. Even 0.7lb open cell won't be much over R32, and that's at center-cavity. How then are you hitting R45 whole-assembly- I assume that's including the ~R20 exterior polyiso? (If you're referring an online calculator it's worth adding a link or URL- some tools are way better than others.)

    In zone 4B Colorado there isn't a huge risk to the roof deck to go with 8" of open cell foam UNLESS there is going to be a high solar reflectivity "cool roof" installed, or if the house is actively humidified in winter, but it's not completely risk free. In summer there can be some daily swings of attic humidity to deal with as the sun bakes moisture out of the roof deck & foam, only to retake it overnight when the roof deck is cooler, and the north facing roof pitches might have excessive moisture level averages high enough over the long term that could lead to rot.

    To be done correctly it has to be installed in two passes, with a cooling/curing period between passes. With R30 at center cavity installing R20 on the exterior at some later date would reduce the risk to the roof deck even further.

    A mere inch of closed cell foam on the underside of the roof deck is enough to protect the roof deck, but there can still be moisture cycling in/out of the fiber or open cell foam installed to the interior of that.

    See Table 3, the column labeled 1" ccSPF + spray fiberglass, and the 4A Kansas City and 5B Boulder rows. See also the column labeled "Full depth ocSPF", which is effectively what you're installing.

    You'll note that with open cell foam and asphalt shingles the 4A Kansas City roof sees 28% moisture content for only a week, and above 16% for only 4 weeks. That's not TOO risky, but not zero risk. That's what to expect if there is no cool roofing shingles and the indoor RH is only ~30% RH or during the coldest weeks of winter. But if kept at 40%RH indoors in winter the north facing roof deck be above 28% m.c. for 4 weeks out of the year or longer, which is much riskier over the long term.

    1. phumer1 | | #7


      I appreciate the linked article. That is exactly what I was looking for. Perhaps the main reason I was considering open cell spray foam is the blowing agents used with the closed cell and their global warming consequences of their use.

      Admittedly the whole roof calculator that I was using is wrong. I'm not sure why. When I reran the calculator I'm reaching the values you listed. To clarify the calculator that I was using is If this is inferior, I am open to suggestions of better options.

      I believe I will go with the 5.5" of open cell between rafters, followed by 2.5" of open cell covering the bottom of the rafters. I will then plan on adding polyiso onto the exterior side of the roof deck as time and funds become available. This shouldn't be more than a year or so. I will make sure to not use any humidifiers during this period.

      When completing my ducted minisplit installation I will consider adding a very small outlet into the branch in order to condition air as Martin suggested. I will seal up this duct after I have added the polyiso to the exterior.

      Finally do any of you have any suggestions of how to block between the rafters without removing the facia? I believe I read in a previous post that sliding 1/2 eps or polyiso over the top plate up to the roof deck and then sealing with canned spray foam is a valid solution, however I cannot locate this specific post. Anyways this is what I am planning on doing, unless of course there are obvious problems with this approach.

      Thank you all again for your support. I only wish I had known about this site before I began construction on an addition that was completed a few years ago, by a skilled drywaller turned capenter, whom had some antiquated opinions on house construction. In a nutshell he was of the opinion that houses should breath, while he attempted to construct an unvented roof assembly on the cathedral roof on my addition, a pitfall of potential problems I hope to address when I finally add polyiso to both the old house and the new one.

      Thank you again.


  2. walta100 | | #2

    Why do you want a conditioned attic?
    Are you unlucky enough have an attic full of HVAC equipment and ductwork?


    1. phumer1 | | #5


      I'm leaning towards a conditioned attic mainly because I have a hip roof that is difficult to vent properly. Second, there is a portion of my roof, a mud room addition that was build with next to no attic space necessary maintain the vented attic or install code minimum leveles of insulation. Third, during recent multizone minisplit upgrade that I have 85% completed requires me to consider placing ducted minsplit equipment that I have already purchased in the attic due to the construction constrains of my original plan of placing this equipment in the hallway. Long story short the hallway is too narrow to put it within the building envelope there, and the space above the doorways that I had planned running the ducts through are occupied by 2x12 headers that I don't want to cut into.


      1. walta100 | | #8

        If you are stuck with duck work in your attic then a conditioned attic is the least bad solution left.

        If you do it, understand you are taking a risk and bending the rules in construction or operating a humidifier could put you in a bad spot.


  3. GBA Editor
    Martin Holladay | | #3

    You wrote, "I am now considering filling the roof joist bay with open cell roughly 5.5 inches then spaying the remaining 2.5 inches over the roof joists in order to compensate for thermal bridging."

    I'm confused. Did you really mean "over the roof joists [rafters]"? I'm guessing that you meant to write "under the roof joists [rafters]."

    1. phumer1 | | #6


      Rafter, as Dana corrected me. I am planning on spraying the remaining 2.5" on the bottom of the rafter. Please excuse my relative ignorance concerning terminology. I don't work in this field, yet live in an area too remote to hire the level of expertise that I desire, so I do much of these things myself. I've successfully installed 4 out of 5 zones of a multizone mini split for example. Im a Jack of all trades, master of none.


  4. GBA Editor
    Martin Holladay | | #4

    For more information on the risks of installing open-cell spray foam on the underside of roof sheathing, see this article: "High Humidity in Unvented Conditioned Attics."

    If you end up installing open-cell spray foam, remember to include a supply register connected to a forced-air heating and cooling system in your attic, so that the attic is conditioned.

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