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Used 3″ Rigid Polyiso Insulation

canadianexpy | Posted in General Questions on


Had a quick question regarding used polyisocyanurate . I have an opportunity to purchase 3″ X 4′ x 8′ sheets that are reclaimed and 4 years old, they are paper faced on both sides.

The plan is to have 2×4 wall with 2 layers of the 3″ polyiso on the outside (6″ total) . All above ground.

Is there any reason I cannot use them for my exterior wall insulation?

Any issues with used Polyiso? or paper faced?


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is there any reason I cannot use them for my exterior wall insulation?"

    A. No.

    Q. "Any issues with used polyiso?"

    A. Some builders have complained about inconsistent thickness. This can usually be addressed by sorting or shimming, or by rejecting 10% of the panels that are unusually thick or thin. Quality varies -- some recycled polyiso is in perfect condition and of consistent thickness.

    Q. "Any issues with paper-faced polyiso?"

    A. No. Of course, with any installation of exterior rigid foam, you need a plan to limit air leaks. Most installers seal the seams between panels of rigid foam with canned spray foam, caulk, or high-quality tape. You also need to include a water-resistive barrier (WRB). For more information, see How to Install Rigid Foam Sheathing.

  2. canadianexpy | | #2

    Thanks Martin!!

    Any tapes recommended for paper face?
    The idea of spray foam between every seam, I think would be difficult and messy?!
    Caulk maybe, if I could find something that stick to the Polyiso?

    Tape seems the easiest way.

    My windows are going to be outies so, I thinking WRB being Tyvek over the foam?
    The wall sheathing will also be taped for the air barrier.


  3. GBA Editor
    Martin Holladay | | #3

    I think the best tape for paper-faced polyiso is probably Siga Wigluv, but Siga Wigluv isn't cheap.

    The other option is 3M All-Weather Flashing Tape. The 3M product costs less that Siga Wigluv, and it might work fine on your type of foam -- but I'm not sure. If I were you, I would buy one roll of 3M and test it before I bought a case of tape.

  4. canadianexpy | | #4

    Sounds good a will try the 3m tape but will test it first,

    Siga is $$$, priced it quickly up here in Toronto, Ontario
    Siga Wigluv 60 -60mmx40m roll. $80 a roll
    Siga Wigluv 100 $91
    Siga Wigluv 150 150mmx25 roll. $136

    Wow ....the tape could be the most expensive part of the build!

  5. canadianexpy | | #5

    Does anyone know what might happen to the paper facing if it rains before the siding is installed?
    How much time do I have before I need to install the house wrap?
    Just trying to figure out sequence/timing of the layers.


  6. Jabberwocky79 | | #6

    I have recent first hand experience with used 3" poly-iso with paper backing. We used it in our roofing system.

    The paper facing has fiberglass in it - wear gloves and long sleeves when handling it. Every part of your skin that makes contact will be irritated with fiberglass splinters.

    Don't bother with tape unless you are willing to spend the big bucks. We tried to use the generic equivalent to Huber Zip-tape and it failed spectacularly. The most insulting part about it is that it seemed to adhere perfectly well during application, but two days later it was peeling back and allowing air infiltration. So if you plan on using tape, make sure you allow adequate time for testing it - not hours, but days.

    We ended up sealing it with spray foam around all edges, which ultimately was cheaper than tape. I would liked to have used both, but couldn't.

    We never ran into a situation where the paper facing got wet AFTER installation, but we did get moisture in-between the pieces that were stacked prior to installation. They grew all kinds of fascinatingly patterned fungi. We brushed it all off and continued to utilize them. After the install, we sprayed a mold killing agent on the affected areas, and then painted over them with a mold killing paint. (Probably overkill, and yes, I'm dubious about the effectiveness of such products, but it kept the wife happy). A far better solution would have been to protect them from moisture to begin with. Unfortunately we just didn't have any climate-controlled storage available.

    I hope this insight helps you in your own project!

  7. user-2310254 | | #7

    Would the OP have better results applying a liquid sealant such as Prosoco R-Guard? It doesn't sound like this type of project would require a lot of material. It might be a better option for creating a reliable seal.

  8. canadianexpy | | #8

    Thanks for the comments John,I'm not sure the paper facing has fiberglass in it I haven't actually seen it in person yet. I was under the impression there was paper, foil and fiberglass faced Polyiso. I guess I will see when I pick-up the product. Was your Polyiso consistent in thickness? I'm concerned about this for my wall application and have heard it can be out 1/4 " or more.
    Steve I will look into this as an option, it looks like it might work, again I will need to see what the facing is made of. and pricing of the product up here in Ontario.
    Thanks for the input.

  9. Jabberwocky79 | | #9

    Dave, you may be right about the fiberglass vs. paper. The foam I purchased was advertised as paper backed, but since it had fiberglass in it, I assumed that's what paper-backed is. Hopefully you don't have that hassle. Really it wasn't a big deal, just a minor irritation that made the process a little less pleasant.

    I can't say for certain whether the thickness was consistent as I didn't measure each piece. Visually, when sitting in a neat stack, they appeared uniform, although I doubt a 1/4" variation would be that visible anyhow. Keep in mind that, as a previously used product, there will be some warping, along with some broken off pieces, some cracking, etc.. that you will likely have to account for.

    I was installing these in-between rafters along with a second layer of 2.5" thick foam. After both layers were installed, they were definitely not perfectly smooth across the bottom surface, and hung down past the rafter thickness in varying degrees. I feel the variations were due more to inconsistencies in the dimensional lumber (warping, bending, etc.) than in the foam itself. That said, it's certainly plausible that the foam could have also had inconsistencies..

    I have several boards left over, which I will be using on a section of my exterior wall, so I may be able to perceive variations better in that scenario. It will be a little while before I get to that point however.

  10. canadianexpy | | #10

    Thanks John for your info, I talked the guy with the product and he did measure some of the sheets and there was 3" and 3 1/4" in thickness. I assume I will be able to shim out the 1/4". I think it will still be worth it in the end in costing savings over new sheets.

  11. Jabberwocky79 | | #11

    I wish you the best of success!

    We are quite happy with our roof insulation system now, but during the install, it was pretty grueling and cumbersome, making us second-guess our choice. Of course we were installing these panels overhead in an existing structure, so your project will probably go smoother. I know we saved a pretty penny over doing the same R-equivalent in spray foam, but in retrospect I feel like we could have installed vented baffles, a membrane across the bottom of the rafters, and paid someone to dense-pack cellulose the whole roof for the same amount we spent in rigid foam, spray foam, tape, and most importantly, time.

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