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Community and Q&A

Working with XPS and ISO boards: Best glues, tapes, and circular saw blades

matt2021 | Posted in Energy Efficiency and Durability on

The title almost says it all:

– On an unvented roof, if I were to have a layer of polyiso or XPS (2″), topped by a layer of XPS (2″), which tape(s) are recommended for the XPS seams?  (This will not be T&G XPS, unfortunately; so, I’d like to seal the gap between the boards.)

– For the same roof, if I were to apply some XPS in the bays between the rafters, shall I use a little bit of GreatStuff Construction Adhesive, then tape?  I could also use some fasteners here and there; yet, right now I am wondering about the adhesive: something good and that might make the installation a bit easier, especially when working on a high ceiling.

– What are people’s experiences with circular saws, especially to cut 2″ XPS?  (Handheld circular saws, used with some type track, which I need to buy, not a table saw, which I don’t have).  I know I want a blade with small teeth, but I wonder how small, or perhaps what kind of blade.


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  1. Expert Member
    BILL WICHERS | | #1

    Most tapes don't stick particularly well to XPS. I would try tapes made for sealing crawlspace liners for best results, since those tapes are made to stick to polyethylene, and will probably stick fairly well to polystyrene (which XPS is made of) too. Some flashing tapes will also work, but I'd be sure to do a stick test first to be sure, and wait a day or two before trying to remove the tape. I've found many tapes will initially stick, but don't bond very well and will loosen over a day or two and eventually just fall off or stay stuck by "static cling" and not the adhesive, giving a false appearance of a passable tape job even though there is no actual seal.

    XPS makes a big mess of static sticky foamy dust when cut with a saw. I advise against using any kind of saw here. Use a knife, score the panels, then place the score line over a table edge or board and snap the panel at the score line. Hot wire cutters also work. I typically just score and snap XPS though, which works well, but you need to make sure you have a very sharp blade in your knife so that the knife doesn't "skip" through the materials as you cut. Angle of the knife also helps to get a clean cut. EPS doesn't score and snap as well as XPS, so a hot wire cutter or a hot knife is the best way to go. For polyiso, I usually score and snap, then carefully flip over the folded panel, open it a bit, then slice the remaining facer with a knife. Avoid saw blades like the plague (which is what the dust they make seems to be!) with rigid foam!

    All that fun stuff said, why are you considering XPS here? If you are planning on insulating on the exterior of the roof, which is what it sounds like, go with all polyiso. There is even "roofing" polyiso specifically made for this purpose, and it's usually cheaper than the foil faced polyiso! I would put all of your R value in polyiso on the exterior if you can, otherwise try for at least 50% of your overall R value on the exterior. Make up the rest of the R value with batts on the interior.

    I would not try to cut'n'cobble XPS in an unvented assembly. You could probably get it to work if you had enough exterior R value, but you CANNOT cut'n'cobble a roof assembly that has no exterior insulation. If I were to use a rigid foam material here, I would use low density EPS sealed in place with canned foam. The reason for EPS is for a little more drying ability to the interior. Ideally though, you really want to try to put as much of your R value on the exterior as possible and not rely on interior side insulation to try to keep moisture away from the roof sheathing.


  2. matt2021 | | #2

    Thanks Bill! For the XPS tapes and cutting, I am making a note of everything you said. I wonder whether those tapes would perform even worse because on a roof, subject to high temperatures in the summer, and temperature swings... Still, it would seem important to seal those seams between the boards before applying the plywood sheathing, underlayment, and shingles.

    I would go for XPS only because I have it. I, too, like polyiso better. Maybe I should change that plan.

    As for applying rigid foam between the rafters, I'll do that only if I have sufficient external insulation. Otherwise, the plan is closed-cell spray foam between the rafters.

    If it turns out that I am not using XPS for the roof or the ceiling, maybe filling the floor (I will have a 6"-7" cavity) with XPS would not be a bad idea. With a little patience, maybe I can do a neat job, and have a nicely sealed floor.

    1. Expert Member
      BILL WICHERS | | #4

      If you already have the XPS, I'd put it all on the exterior of the roof, which is where it will do the most good for you. I would only use spray foam under the roof if you have no way to make a vented assembly, and can't put rigid foam on the exterior. Putting rigid foam on the exterior of the roof is far superior to using spray foam on the underside of the roof.

      I wouldn't bother insulating the attic floor, especially with rigid foam in a cut'n'cobble style installation. Insulate the roof, or make a vented attic and insulate the floor. Don't try to do both.


      1. matt2021 | | #6

        “ I would only use spray foam under the roof if you have no way to make a vented assembly, and can't put rigid foam on the exterior.”

        Yes, absolutely, that’s the plan. The roof is and will remain unvented.

        And I, too, greatly prefer having the exterior insulation, and then more insulation between the rafters. Ideally, though it will be an extra expense, I’d like to have Rockwool between the rafters, as well as in the walls.

        I was referring to the room’s floor, which will be framed, with joists sitting on a concrete slab. Maybe that will be a good place for XPS boards. What do you think?

        I will have leftover XPS. Yet, I don’t know if using it in the walls would be a great idea (instead of the Rockwool that is). I should mention that I will be using Zip R6 as exterior sheathing. So, hopefully, if I do a good job, the wall cavities will be well air sealed. I’ll have to decide whether to add more foam in the walls, in the form of XPS, or Rockwool. Indeed, if XPS were to be a good idea for the walls cavities, I could get some additional panels (they are 2.5” thick) from a gentleman who has them. They are new and clean, and quite cheap. Any thoughts on this?

        I do worry that cutting and installing those XPS boards in the walls or floor might be a job I will hate — one involving a lot of cutting and mistakes and who knows what else. Rockwool batts would be so much easier. The roof job, if I take that route, instead, will be done by a roofer.

  3. seabornman | | #3

    I used a combination of tools when applying XPS to my walls. Score and snap with a sharp knife was first. Then a short coarse toothed toolbox saw for inside corners. Then if I had repetitive cuts, I'd use a bench top table saw or even a full-size table saw.

    1. matt2021 | | #7

      Thanks! To be honest, I’m not even acquainted with some of the tools you, but I will check them out!

  4. Patrick_OSullivan | | #5

    Circular saw with a normal blade flipped backwards creates way less dust. They also make blades meant for foam that again, make way less dust.

    3m 8067 adheres just fine to XPS.

    1. matt2021 | | #8

      Thank you! Noted on the tape. I’ll check out the blades for foam. I didn’t know they existed.

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