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Open Cell vs Rockwool vs Closed Cell

nynick | Posted in Energy Efficiency and Durability on

Coastal Connecticut, on the water. Zone 5 ish

Please don’t chastise us for the possible use of spray foam.

The plans for our NEW garage call for Flash and Batt in the ceiling. The insulating sub has quoted 2″ CC and the balance OC in the rafters instead of Rockwool batts.

They also quoted 6″ OC in the 2×6 walls instead of Rockwool for a $750 premium. A little research on my part showed this would require a Vapor Barrier in our climate. The insulating sub recommended not using a Vapor Barrier with OC. I told them no thanks to OC in the walls as a result.

Two questions:

1) is the roof 2″ CC+OC ok or do we need a Vapor Barrier also for this type of install? Seems to me this would work ok.

2) Should I just go with the Rockwool walls or spring for the CC? Can you use the same type of CC+OC in the walls?



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  1. Expert Member
    Michael Maines | | #1

    When mixing vapor-closed and vapor-open insulation the important thing to understand is the R-value ratio between the two. In CZ5 roofs, at least 40% of the total R-value should be in the closed-cell layer. How deep is the open cell layer? In any case you don't want a vapor barrier in the roof. You might benefit from a variable permeance membrane (aka vapor retarder) but with a safe ratio of open cell to closed cell it shouldn't be necessary.

    As for open cell foam vs mineral wool in walls: they have similar levels of embodied carbon emissions--neither are very good on that front--and mineral wool insulates better. Open cell foam does provide air-sealing in the stud bays; with mineral wool that requires a separate step. But open cell doesn't address the edges and transitions where a lot of air leakage occurs, so you should have a different air control strategy anyway.

    1. nynick | | #2

      Thank you Michael. I believe our rafters are 2x10's. There would be 2" CC against the roof sheathing and then another 6 or 8 of OC. This would meet code.

      I asked them to quote me the same setup for the walls: 2" CC against the sheathing and the rest OC. Haven't heard back yet.

      We've got Zip sheathing everywhere and so far air sealing has been pretty good.

      1. Expert Member
        BILL WICHERS | | #3

        "Meeting code" is not necassarily the same thing as meeting the R value ratios. For your question #1, you're light on CCspf for the ratios. You need at least 40% of the total R value in the form of closed cell here, as Michael mentioned. Be careful with spray foam contractors quoting overly optimistic R values too -- closed cell spray foam is more around R6 per inch, for example, than the R7+ numbers you sometimes hear.

        For 2, go with mineral wool in the walls. If you go with spray foam, go with open cell only. There isn't much reason to go with a mix of OC and CC spray foam in a wall. While spray foam is better for air sealing, you can air seal other ways, and spray foam does miss some areas (like between sistered studs). I think relying on spray foam for all the air sealing makes you more likely to miss leaks, since less attention will be paid to other areas since "the spray foam does the air sealing" becomes the primary thinking about the job. I usually use a smart vapor retarder detailed as an air barrier, then I detail the interior drywall airtight too for some redundancy. This isn't that difficult to do.


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