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Need help with recycled polyiso insulation in attic

ZLojs8DfxS | Posted in Energy Efficiency and Durability on

I found a great deal on recycled polyiso boards that were once a roof on a commercial building with EPDM rubber over it. Each board is 4 foot by 4 foot and 4 inches thick.

I am going to use them in a 1830 post-and-beam house. The boards are going to be going in the attic. There is currently no insulation or vapor barrier in there. The house is in upstate NY and sees some seriously cold winters (at least in the past).

I got the foam and my plan was to cut each board to fit between the ceiling joists and great stuff around the edges. I have enough foam to do 8 inches which will be two layers and I will overlap the seams. This should create a good tight envelope for the ceiling. The ceiling joists are 4x6s, 24 in. on center.

My question that I need help with is with the vapor barrier/retarder. The foam I have is foil-faced but the foil is deteriorated and not easily repaired. Does it make sense to cut strips of 10-mil poly 24 in. wide and lay them in the joists bays against the sheet rock ceiling and staple them to the side of the joists then lay the foam in and seal it in place? Should I not do a vapor barrier at all and just rely on the deteriorated and perforated foil facing?

My latest idea was to lay in Reflectix then the foam for added thermal barrier which would also act as a vapor barrier.

The space above the foam will be a good sized open unfinished attic ventilated at the peaks.

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  1. GBA Editor
    Martin Holladay | | #1

    Relax. There is no requirement in the building code for a vapor barrier -- only a vapor retarder. The paint on your ceiling should be adequate.

    Moreover, if the foil on the old polyisocyanurate is 80% intact, that means the vapor permeance of the foil is 80% as good as it used to be.

    Finally, 4 inches of rigid foam is also a vapor retarder.

    Remember, air sealing matters much more than vapor permeance.

  2. ZLojs8DfxS | | #2

    I am just worried about the foam getting wet because the dew point will be achieved somewhere in the foam and with out a vapor barrier it will condense and create water droplets. Which will ruin the insulation capabilities of the foam.

  3. GBA Editor
    Martin Holladay | | #3

    I understand your worry. But your worry is needless. It has no basis in physics.

  4. ZLojs8DfxS | | #4

    That sounds great to me. Less stuff to buy and less work to be done! Thank you for your help.

  5. jklingel | | #5

    "...recycled polyiso boards that were once a roof on a commercial building..." I asked a person at our university cooperative extension about getting pulled foam, and he cautioned me. It made sense. Why was this foam pulled? If it has been wet and gone through freeze/thaw cycles, apparently the freezing breaks the foam's integrity and allows water to more easily wick in. After MANY cycles, the foam can be pretty useless as far as shedding any water, reducing its R value to negligible. I've never studied the phenomenon any further, so I don't know, but caution may be advised.

  6. ZLojs8DfxS | | #6

    The foam seems to be in good shape. The actual foam is still in good condition. It wont be used to shed water anymore. Just using it for its insulation capability. It will be inside an attic. Do you still think that this foam will be ok to use without a barrier? As long as it is dry it should have a good R value still, correct? Thank you for all your help I really appreciate it.

  7. wjrobinson | | #7

    Larry, your foam is not being installed under rubber roofing trapping water in it. Not to worry about Johns post. Your problem is air bypassing the foam now or in the future since you are using lots of pieces. Air is going to do its best to go around all the pieces. Good luck.

    Now if you top coated it with spray foam or lots of cellulose you might really do a good job of stopping air leaks.

  8. NJFoam | | #8

    We have worked with the same recycled polyiso boards you've mentioned. We have used them in two configurations:
    1. create a chase/baffles for an insulated roof assembly where continuous roof venting from soffit to ridge is necessary. Closed cell spray foam underneath.
    2. Layed in ceiling joist cavities (your application), with closed cell spray foam to seal the edges and top coat for a monolithic assembly. (A customer did this to save money, we applied the SPF)
    Your idea should work, although it requires a lot of labor. There will be savings due to the low cost of the foam boards though. I might suggest talking to your local spray foam contractor about coming in for the sealing, and a flash coat on top, once the boards are in place. You might come up with a dollar figure and perhaps the contractor can do it on a flexible schedule to fill in a slow week, with you doing the prep to keep costs low. A Table saw is great for ripping down the boards, and don't attempt a friction fit, leave room for canned foam sealant or CC SPF. You might give some thought to insulating the roof instead, using the polyiso as a baffle and sealing with CC SPF. You can get the appropriate R-values and there will be no concerns with ice damming due to the continuous venting. If the rafter framing is a small dimension, consider encapsulating it with foam.

  9. NJFoam | | #9

    Oh, and skip the radiant barrier. You would need an airspace for it to work properly anyway.
    If you haven't done so, check out the podcast about this , here's the link:

  10. awhiteman | | #10

    I have done the first of the two methods that steve mentioned. I ripped 5/4 board to one inch strips and butted up to underside of roof sheathiing and nailed to rafters. Than cut my four inch polyiso board to fit in cavity with a 3/8 gap on either side and pushed up against the one inch strips tight. I used small pieces of the 5/4 board to hold the polyiso board in place until I foamed the gaps. I bought a touch and seal 600 board ft fire retardant foam kit. I used this to fill in the gaps.. I have 3 inch polyiso board which I was going to try to fit in the cavity on top of the four inch as a tight fit, no foam. I thought the air seal would have already been created so there was no need to foam the second layer. Any feedback would be appreciated as this is out of the box thinking.

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