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Cut-and-cobble polyiso, then spray foam?

jasonh37 | Posted in Energy Efficiency and Durability on

Hello everyone,

I am in the process of building a new home. The home has low pitched/flat roof trusses which start at approx. 30″ and taper to approx. 24″, the trusses have a 2×6″ top chord. I need to insulate from the bottom due to the fact that I already had my p.v.c. roof installed. The roof has approx 2,400 sq. ft. of area to be insulated. I know that the proper way would be to use closed cell spray foam sprayed to about 6″, but that will be way over my budget. I am considering doing 2 layers of 3″ polyiso, with 1″ of foam it green, closed cell spray foam, possibly 2″ if needed. The 2 layers of 3″ poly would bring the material a half inch past the 2×6″ top chord and then the 1″ spray foam will fill the remaining 1/2″ gaps between the sheathing covering the 2x’s. I would like to get some opinions on if this is a disaster in the making or will it work. Btw I am located right outside of Pittsburgh, PA.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, a note to GBA readers: It's important to finalize your insulation plan before you begin construction. Jason wants to install polyisocyanurate insulation, and the best place to install it is above the roof sheathing. He installed the roofing before he finalized his insulation plan, and now it's too late to do it the right way.

    I don't advocate the use of the cut-and-cobble approach with unvented roof assemblies, because there have been reports of failures (sheathing rot) when that method is attempted. For more information on cut-and-cobble (and reported failures), see Cut-and-Cobble Insulation.

    At this point, the best way to proceed is to install closed-cell spray polyurethane foam on the underside of your roof sheathing.

    In your climate zone (climate zone 5), the spray foam layer needs to have an R-value of at least R-20 to meet minimum code requirements (and keep your sheathing safe) before you can switch to an air-permeable insulation like fiberglass or mineral wool for the rest of your insulation sandwich. (You should aim for R-49 for the R-value total of spray foam + air-permeable insulation.)

    For more information to guide you, see Insulating Low-Slope Residential Roofs.

  2. jasonh37 | | #2

    Thank you for your answer on this matter Martin. The main reason that I didn't insulate above was because 6" of ridged foam plus the 4" tapered ridged foam for crickets would have interfered with a door and several windows, poor excuse I know, I could go into more detail as to why but typing on iPad.

    Is the main area of concern that the polish would not be adhered to the sheathing? Also, I could cut the poly tight for a friction fit and forgo the expanding foam. Like I said, after the poly is installed I will go over top of the entire surface with spray foam filling all gaps and in my opinion creating an airtight seal. Can you please elaborate why this specific plan would be disastrous? I have read your opinion that you do not like this method and have negative opinions about it. I had intended to do adequate spray foam from the beginning, but the budget is getting tight.

  3. GBA Editor
    Martin Holladay | | #3

    It's your house. You get to decide how to proceed. (Of course, your local building inspector may have a say -- and not all building inspectors approve of the cut-and-cobble method.)

    Framing members expand and shrink with changes in humidity. Spray foam installations are never perfect; voids and missed areas are always possible. Experience shows that cut-and-cobble jobs aren't always airtight; failures have been reported and documented.

    As I said, it's your house, and the decision is yours to make.

  4. jasonh37 | | #4

    Hello Martin,
    So I am going to bite the bullet and just pony up for the spray foam insulation. I do have one question though. We are going to install closed cell and then open cell on top of it. I am getting a lot of different information from different spray foam guys about thickness'. Some are saying all I need is 1.5 of closed and 7" of open. I was thinking 3" of closed cell, with 7" of open. What are your thoughts. Thank you in advance for your time.

  5. GBA Editor
    Martin Holladay | | #5

    Pittsburgh, PA is in Climate Zone 5. According to the 2012 International Residential Code, ceiling or roof insulation in your climate zone must have a minimum R-value of R-49.

    The contractors who are suggesting 1.5 inch of closed-cell spray foam and 7 inches of open-cell spray foam are suggesting that you install only R-29 of insulation. That is a code violation -- it's only 59% of the minimum required thickness of insulation.

    Your suggestion -- to install 3 inches of closed-cell and 7 inches of open-cell -- would result in a total R-value of R-45. That's still below the minimum code requirements, but it's much closer to your goal.

    It's fairly common for spray foam contractors to mislead homeowners about minimum R-value requirements in the code. For more information on this topic, see It’s OK to Skimp On Insulation, Icynene Says.

  6. Dana1 | | #6

    In zone 5 it takes 40% of the total-R on the exterior side of the stackup to have adequate dew point control. With 1.5" of ccSPF you have about R9-R10, and with 7" of ocSPF it adds another R25-R26, which is NOT a sufficient ratio. R10 is NOT 40% of R35, not even close! The consequence would be moisture accumulation in the ocSPF over the winter, which undercuts it's performance, and has potential-liquid-drip issues under some circumstances, and high humidity cycling in the attic spaces during the drying season.

    With 3" of closed cell you'd be at about R19-ish on the outside of R25-ish with an R-value in the mid-40s. R19 is then ~42-43% of the total R, so it'll make it from a dew point control perspective, but not with a lot of margin.

    With 2x6 top chords (5.5") and 10" of foam you'll have about R16 of ocSPF over the chords, which is a significant thermal break. To meet code on a U-factor basis you only need to duck under U0.026 (R38 "whole assembly-R" ), so it probably makes it, despite being only R44-R45 center cavity. With the ~ R6 ish R-value of the chords themselves and the R16 thermal break, the framing fraction has an R-value of about R22, and the roof deck & PVC roofing add up to an R-value of close to R1.

    To install ccSPF safely it has to be sprayed in lifts of no more than 2" , and with oc SPF it's no more than 6" (5" with some vendors), and since your depths are close but above those limits many foam installers are going to be tempted to cheat the system a bit, which can be very real fire hazard as it cured. It's probably better to discuss it ahead of time, and go with 4" of ccSPF in two lifts (R25-ish) and 5-6" of oc SPF (R18-22-ish), which will give you more dew point margin, and it can be installed safely in three lifts instead of four. It's more expensive, but it's a much better stackup,

  7. GBA Editor
    Martin Holladay | | #7

    Dana and I disagree on this point; you can make your own decision.

    In my view, the ratio he discusses is important when there is a sandwich installation of air-permeable insulation (fiberglass, cellulose, or rock wool) on the interior, and closed-cell spray foam or rigid foam on the exterior.

    There is no need to follow the "ratio rules" if you have a sandwich with closed-cell spray foam and open-cell spray foam. The reason is simple: the open-cell spray foam is an air barrier, so much less moisture reaches the interface than with an air-permeable insulation. The type of condensation Dana worries about does not occur when open-cell spray foam is installed on the interior side of a layer of closed-cell spray foam.

    As I said, Dana and I disagree on this point. You can choose whose advice to follow.

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