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Advice needed: Recycled XPS rigid foam

drewintoledo | Posted in Green Products and Materials on

I’ve located 4×8 sheets of 3″ recycled foam board. This is DOW blue board, claiming to be 3″ thick.

Should I nab this up for exterior insulation around the foundation? The price seems right and I love to recycle.

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure what your question is.

    Are you looking for advice on the ethical question -- that is, whether XPS (manufactured with a blowing agent that has a high global warming potential) is more ethical when purchased used than when purchased new?

    Or are you wondering whether exterior foundation insulation is a good idea?

    Or something else?

  2. drewintoledo | | #2

    Sorry - I realized I introduced ambiguity in my question but for whatever reason I could not edit the post.
    I'm wondering if this material is recommended for exterior insulation around the foundation? If not there, under the slab or otherwise? I don't want to miss this opportunity if I am able to save monies and also recycle.

  3. GBA Editor
    Martin Holladay | | #3

    XPS can be used vertically to insulate the exterior of a foundation wall. It can also be used horizontally under a concrete slab.

    For more information on this issue, see these three articles:

    How to Insulate a Basement Wall

    Choosing Rigid Foam

    Polyethylene Under Concrete Slabs

  4. drewintoledo | | #4

    Thanks Martin! I'm going to scoop it up!

  5. Expert Member
    Dana Dorsett | | #5

    Re-used foam is by far the greenest foam there is- go for it!

    When re-using XPS for dew point control or insulation at code minimums, assume no more than R4.2 per inch of thickness, since that's where it roughly settles once the blowing agent has dissipated. It takes decades performance to drop to that level, but they have been making XPS for decades too (with a range of blowing agents.)

  6. drewintoledo | | #6

    The product is actually DOW Deckmate PLUS. It's a specialty board for the deck of a roof. It sports a higher compression strength specifically for installation on roofs. Hey.. Maybe I'll use it there. Here's what I've found. Per Dana's response, I'll downgrade the actual R value so I'll consider these 3" boards R12.6. Thanks Dana!

    Thermal Resistance per inch, ASTM C518 @ 75°F mean temp.,
    •h•°F/Btu, R-value(1), min. 5.0
    Compressive Strength(2), ASTM D1621, psi, min. 25
    Water Absorption, ASTM C272, % by volume, max. 0.1
    Water Vapor Permeance(3), ASTM E96, perm, max. 1.1
    Maximum Operating Temperature, °F 165
    Coefficient of Linear Thermal Expansion, in/in•°F 3.5 x 10-5
    Flexural Strength, ASTM C203, psi, min. 50
    Dimensional Stability, ASTM D2126, % linear change, max. 2.0
    Flame Spread(4), ASTM E84 5
    Smoke Developed, ASTM E84 165

  7. drewintoledo | | #7

    UPDATE: I just used video conferencing to see a sample of the boards. He only has 4 at his place and the rest are in a barn down the road. The 4 I saw look like they have a fairly smooth side but the opposite side is very pitted as if gravel was smashed down into the surface - It would make sense to me to remove another R value or two because of this. I'm hoping the other sheets in the barn are similar quality. I offered $350 for 25 sheets bringing the cost to $14 each. Does this seem to be a good price given the quality I described?

  8. Expert Member
    Dana Dorsett | | #8

    A scarred up surface barely affects the thermal performance- derating to R4.2 is sufficient.

    $14/ea for 4' x 8' x 3" high density polystyrene is a decent price, even if it's a bit scarred. As long as most don't have huge corners broken off from dropping it 30' into a dumpster, or broken in half by backing the truck over it it'll be pretty easy to make useful.

    If using it on the exterior of the foundation put the flat side to the foundation, scarred side out. A roughed up side holds cemeticious EIFS finishes like Quikrete Foam Coat a bit better than super-smooth XPS, and you'll need to cover it with something like that to protect from UV degradation.

  9. drewintoledo | | #9


    Thanks for your assistance guys!

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