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Community and Q&A

Cut and Cobble

user-466364 | Posted in General Questions on


This is a follow up to my original post seen here:

Getting ready to begin this ceiling insulation project using the “cut-and-cobble” method. I have a few more questions.
1. would it be better to use XPS product or something like R-Tech?
2. I will be using can spray foam for the gaps, but how do we attach to underside of the roof sheathing?
3. The article you forwarded mentioned the pancake method, but there were no details on what that is. Can I assume that means stacking two or more layers together to get the desired thickness (in my case 3.5 inches)? If so, how do you keep them sandwiched together? Should the foam be flush with the bottom of the rafters?
4. should we use a plastic vapor barrier once we’ve installed the foam and before installing drywall.

Thank you.

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  1. charlie_sullivan | | #1

    Sorry you never got your questions answered before. R-tech is EPS foam with a protective layer on the surface. It looks like it's about the same price as XPS. I'd recommend it over XPS because it has much lower environmental impact, but if you an find a local dealer selling EPS it will probably be significantly cheaper. And Martin's article says that maybe EPS is better than the others because it would allow some drying to the interior. That advantage goes away with the facing on the R-tech.

    Attaching to the roof--not addressed in the article, which is mostly about walls. It's light enough that the spray foam on the sides will hold it in fine later--you just need a way to hold it in place until then. One option would be a tight enough fit that it stays in place initially with no help. Another method would be to prop it up with some temporary supports, whether that's wood scraps temporarily screwed to the rafters or a step ladder positioned just right or some other improvised method. Some kind of adhesive could also be used, if it's sufficiently sticky to work before it's fully dry, and if it doesn't damage the foam.

    Yes, your understanding of the stack of pancakes is correct. holding them together has the same options as holding them up, although you could glue them together in advance and hold them together while the glue sets just by putting weight on top.

    Having the foam flush with the rafters is good, but only because that means you've used all the space available for insulation.

    I would not recommend a plastic vapor barrier, as XPS or R-Tech will already block vapor, and EPS would block it sufficiently that you wouldn't need it. And it would inhibit any drying to the inside that you might get.

    Given that it's only temporary, and that it's a small area, I think I'd got with a simple option, I wouldn't worry about any of this too much, except I'd carefully avoid XPS to avoid its global warming impact.

  2. user-466364 | | #2

    so perhaps EPS without the protective layer (R-Tech) would be the best bet because it's cheaper?

    Thanks for the help!

  3. charlie_sullivan | | #3

    The big orange box store caries EPS pre-cut for cut and cobble so all you need to do is cobble! (and stack, since they aren't very thick)

    That is if you have 2x4s on 16" centers.

  4. user-466364 | | #4

    Great! Thank you.

  5. KeithH | | #5

    As someone who has cut and cobbled, why? Unless there is a reason, check out roxul comfort batt. You can order it through lowes or Home Depot pro desk in various sizes. In my area, they have 2x4x nominal 16 on the shelf at lowes. If your joists are reasonably true to 16" o.c., it installs fast, has a high r value, is vapor open, is green guard rated, is fire resistant, doesn't mold or disintegrate like fb and tends to quiet a room.

    If you insist on cut and cobbling, I'd go with EPS. Cheaper, more environmentally sensitive, and almost as much R value.

    Regarding attaching foam board to the underside of the joists, are you going to attach drywall over the foam? Exposed foam is very likely a code violation due to flammability. Putting foam board over wall studs etc can make hitting wood with drywall screws (or longer screws) challenging especially at corners or fenestrations. The effort may not be worth the benefit, especially if the situation is temporary.

    Vapor barrier: if you use foam board over the joists you dont want or need a vapor barrier as foil faced foam is pretty vapor impermeable and the other foams and facings are not very vapor permeable. If you use Roxul, you probably want a vb in a humid environment. Siga Majpell or Proclima Intello Plus would both work well hut are ~$0.40/sf USD.

  6. Dana1 | | #6

    Cut'n'cobble tight to the roof deck gets riskier the colder the climate is. If you can vent the roof deck and only install 2.5" between the rafters it's very safe. When it's time to re-roof and add more insulation above the roof deck you'll have to seal up the vents. As long as those vents can be sealed tight you can either fill the space with blown fiber or not.

    If the plan is to eventually go unvented with insulation above the roof deck, do NOT put a vapor retarder on the interior or you'll have created a moisture trap. Thes also means you don't want to use XPS, or any EPS with facers. At 2.5" XPS is less than a half-perm- a bit on the tight side to allow rapid drying. But Type-I (1lbs density) or Type-II (1.5lbs density) EPS would be still over 1 perm. It would be fine to use a smart vapor retarder (Intello Plus, Certanteed MemBrain) between the foam and the gypsum board.

    With EPS you would also be OK with a continuous 1" of foam between the interior side of the rafter edges and the gypsum, which would dramatically improve the thermal performance due to the R4-ish thermal break over the R3-ish 2.5" worth of rafter. At 3.5" Type-I EPS would still be above 1 perm, and Type-II would be just a bit under 1 perm.

    To maintain the vent space, cut strips of 1" EPS, install a strip right at the rafters on both sides, and another mid-way between the rafters using foam-board construction adhesive (found in box stores), which uses solvents that won't attack the foam. For the first cut'n'cobble layer you can tack it to the strips with dab of foam board adhesive, but for subsequent layers toenail a few nails in at the edges to hold it in place for when you can-foam seal it in place. Cut the foam about 1/2" narrower than the space between the rafters so that you can insert the can-foam straw all the way in for laying the bead of foam, letting it expand toward you, for a full fill and perfect seal.

    The seal of the can foam will work better if the sides the rafters are clean- scrub them with a wet scouring sponge. When it's time to apply the can foam take a pump spray mister/sprayer of water, and lightly wet the surfaces before foaming it. Water is part of the chemical reaction, and you'll get higher expansion and better bonding if the surfaces are damp.

    If you wanted do a full 3" of EPS (in layers, or one 3" chunk) with a 1" vent gap, that would leave the surface of the foam 1/2" proud of the rafter edges. To thermally break the rafter edges tack in edge strips of 1/3" rigid polyisocyanurate (foil face or fiber faced- doesn't matter), which delivers about R3 over the ~R3 rafter, cutting the heat loss through the rafter by about half.

  7. user-466364 | | #7

    As usual it gets complicated quickly. I cannot do anything to the roof deck, we simply want to insulate the space for a year or so until we can afford to replace the roof, which will mean removing the entire roof in this section of the house and all this work in the process. So cheap and easy is the name of the game... I simply want my friend to not worry about condensation and water damage since this is in the bathroom. Wet, soggy drywall falling off the ceiling will be an issue and would like to avoid that.

    I am also limited by time. I only have two-weeks to work on her house, and will need to install exterior siding, install drywall in this section of the house (2 closets and a bathroom) and other miscellaneous tasks.

    I would like to do the cut and cobble with EPS, install a vapor barrier, overlay with mildew resistant drywall and a quality paint for bathroom purposes. Would this scenario be out of the question for the time being... say up to 2-years?

  8. Dana1 | | #8

    The roof deck won't fall apart in 2 years. Filling it with cellulose and installing an interior side smart vapor retarder would be cheaper, easier and still fairly protective of the unvented roof deck. The cellulose shares the moisture burden with the roof deck, the smart vapor retarder lowers the total burden and allows rapid drying.

    If blowing cellulose is too complicated, carefully /obsessively install rock wool batts to fit perfectly. The smart vapor redarder become more critical here than with a cellulose fill, however.

    Save the cut'n'cobble foam money to spend on foam above the roof deck.

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