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Community and Q&A

Clarify cut-and-cobble vs. peanut brittle for an unvented cathedral ceiling

Phil Andrews | Posted in Green Building Techniques on

I’m looking to add insulation to a separate, enclosed porch addition that the previous owners of my house in MA (Zone 5) built sometime in the 90s.  I probably won’t be upgrading this to a permanently open part of the house and don’t plan on running any of my baseboard heating to it, I just want to make it a bit comfortable and expand my usable window of time before it gets too hot or too cold.  Which it does fairly quickly since there is 0 insulation currently in it and the air sealing is abysmal (I can see light at one corner where it abuts the house).

Obviously as a homeowner methods like cut-and-cobble are appealing to me as a way of cutting down costs a bit vs a full cavity fill of spray foam, but I’ve also read many accounts (mostly here) of cut-and-cobble installations failing due to air infiltration bringing moisture and causing rot.   However I’ve also seen Martin saying “The peanut brittle approach is definitely preferable to the standard cut-and-cobble approach” in other Q&A posts discussing unvented roof assemblies (that quote from: )

But the Cut-and-cobble article ( doesn’t make it clear to me what the difference between standard cut-and-cobble and peanut brittle is.

So my question is: What is the difference between the cut-and-cobble advice to cut the panels for a loose fit and fill in around them with spray foam to seal the sides and the “peanut brittle” variation described as “tack up the scraps in the stud bays like jigsaw puzzle pieces, all you have to do is seal the edges of the rigid foam and then install a skim coat of foam on top of everything.”?

Is the failure of CNC installations in ceilings due more to the slight gap (or possibly not so slight if the entire cavity isn’t filled) between the wide face of the rigid foam and the roof sheathing, or due to the side friction fit becoming loose and forming gaps between the foam and the rafters?

Or put differently, in regards to CNC installations in ceilings, what makes PB better than standard CNC?  Could I further minimize the risk by a light flash of spray foam and pressing the loose-fit rigid foam into it before it cures then filling the cavity around the sides so the rigid foam is entirely encased and all parts of the assembly are only in contact with spray foam?  Then would like 1/4″ drywall, taped and painted be an appropriate air barrier or would I need to do anything since this is entirely foam?

Thanks for the clarification!

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    There are many ways that a cut-and-cobble job can fail. The original air sealing at the perimeter of the rigid foam might be imperfect. More commonly, the rafters can change dimension due to changes in loading (for example, heavy snow loads), changes in temperature, or (especially) changes in humidity. When the rafters shrink in hot, dry weather, gaps can open up. Long rafters can also cup and twist as they age, pulling away from the rigid foam.

    A peanut-brittle job involves the addition of an inch or two of spray foam that completely covers the rigid foam and seals to the rafter edges. That's likely to result in a more effective (and longer lasting) air seal than the cut-and-cobble method -- but it's still somewhat risky, and I generally don't recommend it.

    1. Phil Andrews | | #2

      Martin, I've updated my profile with actual names. The options weren't there at sign up that I remember so I just entered a screen name.

      Thanks so much for the exceedingly quick answer!

      I think because of your mention of goosing concrete fills with large rocks I was imagining some weird soup of rigid foam chunks swimming in foam and curing into place and wondering how in reality to actually DO that.

      I've mostly talked myself out of the trouble for the ceiling install. I feel I may have been overcomplicating things especially for NOT a living space...and frankly I think I was doing bad math anyway. Just like Brian in my linked thread I only have 5.5" of rafter bay without sistering in deeper boards so a simple flash and batt with mineral wool would get a titch over R27. 4" of XPS with 1.5" spray over top adds...R2. Some weird combination of XPS on the cold side and polyiso inside that might just barely break R30 for the extra labor of cutting polyiso.

      Plus I also have to do the walls and if I buy all the same rockwool I can get a bulk deal.

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