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Community and Q&A

Spray foam layer over board foam in Northern Zone 5a?

gordy_b | Posted in Green Building Techniques on

Mr. Holladay once wrote of rafters, “Yes, you can combine rigid foam and spray foam if you want to save some money. I would be inclined to install the layers of rigid foam first [I, Gordon, assume this means the rigid foam would go against the roof decking] — cut undersized, and with canned foam sealing the perimeter of each rectangle — and cap everything with spray foam [I’m seeing this as under the rigid foam and further away from the roof] from a contractor’s truck.”

My interior attic edges will only be about 5.5″ from the floor below’s ceiling to the bottom of the air vent baffles below the roof; height increases to the interior as the roof angles up. Can I safely turn the idea above upside down to get a good R value? That is, cut rigid board to fit up against the band joist, between the ceiling joists, and extend inward until the roof height is such that cellulose can give a good R. The first board(s) down would rest on the sill plate and top of the floor below’s ceiling and more would be stacked until about 4″. Then closed cell spray foam would not just be used to seal all wood/rigid foam and rigid-foam-board-on-top-of-rigid-foam-board joints (this is suggested by many) BUT (this is the different part, which I haven’t found much precedent for) WOULD ALSO COVER the top of the stack of boards to the level of the air baffles to provide an extra kick of R without the cost of filling the whole perimeter with just closed cell spray foam. For instance, 4″ of polyiso at R6.5 and 1.5″ of closed cell spray at R6.8 would be R36 in 5.5″.

Two questions before we get to cost. 1) Will this work, or are there condensation issues, will spray foam eat away the board foam over time, etc.? And 2) Does anyone know someone who did this or know of any articles or pictures of this framing/ceiling-foam board-spray foam cap idea?



  1. GBA Editor
    Martin Holladay | | #1

    Q. "Will this work?"

    A. Yes, as long as you are conscientious with your air-sealing details, and as long as the total R-value at the perimeter of your roof meets minimum code requirements. If your R-value is less than the minimum code requirement, you could still get ice damming problems.

    Q. "Or are there condensation issues?"

    A. No, as long as there are no air leaks from below.

    Q. "Will spray foam eat away the board foam over time?"

    A. No.

    Q. "Does anyone know someone who did this or know of any articles or pictures of this framing/ceiling-foam board-spray foam cap idea?"

    A. I have used the cut-and-cobble method (polyiso plus canned spray foam), as have many other GBA readers.

  2. gordy_b | | #2


    Thanks for the advice. So the general idea sounds good, but specific materials questions... Part of the roof will only be accessible while some decking is off and we can crawl through. After insulating and re-roofing, this part will NEVER be able to checked for leaks, damage, etc. You wrote of polyiso that "it “absorbs water easily, so it can’t be used below grade” (Smart-Energy Homes, Fine Home Builders, Winter 2012, p. 114). Even though this is above grade, would you (or others) recommend a different rigid foam seeing if there's a leak/excess water vapor, the polyiso might soak it up and mold/rot nearby things instead of draining the water to and through the plaster ceiling (my leak detector!)? If people think polyiso's degree of soaking up water is safe enough for never checking on again, what about if it's kraft or aluminum faced - will this cause the dreaded multiple moisture barrier problem? If one or both of these suggestions might truly be negatives in an inaccessible space, which rigid foam would be safest to be encapsulated by closed cell spray in an inaccessible attic?

    Thanks to whomever answers. And, Martin, the few times I've poked around the site make it apparent how much work you do trying to let those of us who want to be energy efficient but don't know how get a clue as to how to successfully execute - so in addition to your specific answers, thank you (and other moderators/regular contributors) more broadly.


  3. GBA Editor
    Martin Holladay | | #3

    It sounds like you are planning to include a ventilation gap between the top of the insulation and the underside of the roof sheathing -- right?

    I think polyiso is a good choice. It can be foil-faced or kraft-faced -- your choice. Any facing is fine. Multiple layers of polyiso will not trap moisture.

    As long as you do a good job of air sealing, I wouldn't worry about moisture. Of course, a roof leak is always possible -- all roofs leak eventually -- but an unfixed roof leak will lead to problems no matter what kind of insulation you install.

  4. gordy_b | | #4

    Yes, there will be air baffles below the roof decking. So, sounds like a plan... From ceiling/sill plate up and extending about a foot in from the band joist - good air sealing (especially at the ceiling and sill plate level); 4" or so of horizontally stacked polyiso; 1.5" or so of closed cell spray foam that will seal the ceiling/iso, iso/ceiling joists, and iso/band joist joints AND provide the 1.5" of extra R on top of and to the attic side of the iso; then air baffle (near the perimeter - further in, some of the cellulose that will provide the insulation in the interior will lay on the last few inches of spray-foam on the attic side of the board-spray foam construct).

    I'll check back, but no reply necessary unless this summary sparks a criticism or improvement someone can think of. Thanks!

  5. RCFW | | #5

    Idea sounds great, I presently have an open cavity attic in a 1 1/2 story home. Knee walls in the second level rooms. I have been thinking of doing a simaler insulation project, however I wish to turn my attic space into a useable storage area. I was thinking of using closed cell foam board directly against the underside of the roof deck between the rafters. Two layers thick which would give me R31 as the foam board chosen is R15.5. Then two inch of spray foam over the foam board at an R-value of R6.7. This should give me a total of R44.4
    Any thoughts

  6. GBA Editor
    Martin Holladay | | #6

    As with any similar hybrid approach, good results depend on paying attention to airtightness. It's important to avoid any cracks or air channels that can allow warm, humid interior air to contact the cold roof sheathing.

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