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Community and Q&A

Recycled Polyiso Foam Board Insulation

GibsonGuy | Posted in Green Products and Materials on

I have read on this site that used polyiso is a viable solution if one is considering foam board insulation. It appears to be in decent shape and the 2″ boards were removed from a built-up roof. However, the polyiso is 40 years old.   Should I stay away from polyiso that is that old? Thanks in advance for your input.

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  1. Expert Member
    BILL WICHERS | | #1

    If it seems ok physically, it’s probably fine. What you want to look for is obvious holes and voids, and signs of it having been saturated with water. If the sheets are in good shape and haven’t soaked up any water the material should be fine to use. Maybe try haggling a better price due to the age though :-)

    A plus with older material is that it was made before the newer blowing agents came into use, so older polyiso doesn’t suffer from the “R value drops in cold temperatures” issue that some of the newer polyiso does. If you’re worried about offgassing, older material has aged already, so any offgassing has already happened and won’t be an issue for you.

    I have 1/2” exterior polyiso on most of my home that is near 40 years old (1978), and it’s fine. The edges have discolored and darkened, and the foil facers are thicker than is used on new material, but the insulation still performs just fine.

    Do be careful of any published R values on old polyiso though — those values were published before the manufactures were forced to use more honest measuring methodologies. Use the same published R values per unit thickness for modern polyiso instead. An example is my old 1/2” polyiso being rated “R5”. Suuure. I use R2.7 instead, which is what modern 1/2” polyiso is rated.


  2. GibsonGuy | | #2

    Thank you for your prompt thorough reply, you offered some excellent info. As a follow up, could I install the polyiso above the roof deck, but below metal roofing without topping the polyiso without another layer of sheathing? I plan to have energy heal scissor trusses so I will have reasonable depth of blown in insulation. I do plan on having an active air space under the standing seam roof using a grid made from 1 x or 2 x material.

    1. Expert Member
      Michael Maines | | #3

      John, you need a certain ratio of foam to cellulose or else you'll get moisture accumulation (aka condensation) inside the roof assembly. The ratio varies with climate zone; what is yours?

  3. GibsonGuy | | #4

    Thanks Michael, I'm in Climate Zone 7 so I'm sure that won't be an option for me. I had not considered using foam board on the roof prior to the availability of the polyiso being referenced. I do seem to remember that I would need 3" on the walls which is something I plan on using at least that thickness. I like the idea of the foil faced foam being a good candidate for taping the seams. Possibly it would be best installed on the interior? Or no foam at all in the roof system.

    BTW, I listened to you on one of the BS & Beer Show and discovered that you are a "Brother of the Tangle Tongue". A term a former professor who stuttered referred to us. I too am a stutterer and a retired Speech/Language Pathologist.

    1. Expert Member
      Michael Maines | | #6

      John, table R806.5 shows how much exterior insulation you need to minimize the risk of condensation: It's not stated but the background information assumes a total of R-49. Climate zone 7 requires R-30 on the exterior, or a little over 60% of the total roof R-value. Proportions for roofs are more robust than for walls. The proportion is what matters when it comes to protecting the assembly from moisture accumulation.

      Providing an air space under the metal roofing can help prevent ice dams and reduce summer heat gain. Be sure to include a robust WRB below the roofing, though, as cool, clear nights following warm days will make the roofing into a still, condensing water on the underside.

      Haha, yeah I'm a lifelong stutterer. One reason I had started our local discussion group was to practice speaking in public. Everyone deals with their own challenges and I feel fortunate that aside from having to work harder than others to speak fluently, I'm otherwise very lucky. And there are silver linings to stuttering, though given the option I'd choose to be fluent. I'm sure being a stutterer (I know, I'm supposed to say PWS) made you a better pathologist. None of the few speech therapists I've had stuttered.

      1. GibsonGuy | | #7

        Thanks again for the info. I assume by robust WRB, I might want to consider a self adhering membrane. I was seriously considering a fairly basic synthetic membrane and using TESCON NAIDECK at the penetrations. Some folks run it the entire length of the vertical portion of the grid, but I don't believe it would be too difficult to determine where the penetrations are.

        I won't continue with the PWS topic, but if you ever have any questions, please feel free to reach out. I feel we are certainly wired differently and with that are gifts.

  4. Expert Member
    BILL WICHERS | | #5

    Foil facing is the easiest to tape. I like to use foil tape from Nashua for this purpose since it gives a clean look and sticks like crazy. Flashing tape will work too.

    Rigid foam is usually installed on the EXTERIOR of walls. It can be installed on the interior, but exterior is more common, and probably also easier to do in most cases. You need R10 on the exterior for a 2x4 wall, or R15 for a 2x6 wall. 2" polyiso is around R13, but it may be better to size it as R10 to have a bit of extra margin here. I'm assuming you have 2x6 walls if you're thinking you need 3" of rigid foam on the walls?


    1. GibsonGuy | | #8

      Bill, thanks for the tape tip. I do plan on using the Polyiso on the exterior and am considering covering it with EPS as I have read that Polyiso doesn't perform as well as the temperatures drops.

      1. Expert Member
        BILL WICHERS | | #11

        >" have read that Polyiso doesn't perform as well as the temperatures drops."

        This comes up here periodically. It's not as big of an issue as people think. The cause is that the newer blowing agents are thought to condense in the cells within the polyiso foam, and that results in the reduced R value at low temperatures.

        Really old polyiso -- like the 40 year old stuff you're looking at -- uses blowing agents that don't have that problem! That's something of a free bonus for the old polyiso.

        I would skip the EPS step.


  5. dfvellone | | #9

    I bought a large amount of reclaimed 4'x3'x3" sheets of polyiso which after deliver I discovered didn't all weigh the same. They were removed from a school roof which I suspect had been leaking for some time, or poorly stored after being reclaimed and the heavy sheets were waterlogged. Take note of any sheets that might seem heavier than they should.

  6. GibsonGuy | | #10

    Thanks, good to know.

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