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Is there any reason to think XPS at the big box stores would be of a lesser quality than what is sold at lumber/supply houses?

NewLaneProperties | Posted in General Questions on

I’m getting ready to demo and redo my basement and plan on air sealing the concrete wall, sills and band joist. I generally try not to purchase stuff from big box stores as I find their quality to be questionable. But if XPS is the same regardless of where it’s sold than cost becomes the primary variable. That’s where the big box stores do win out. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    I have no reason to believe that the rigid foam sold at big box stores is any different from the rigid foam sold elsewhere.

    Buyers of rigid foam will find the best bargains from sellers of reclaimed (recycled) rigid foam, which is often available at 1/3 the cost of new rigid foam.

    -- Martin Holladay

  2. brendanalbano | | #2

    An added bonus of reclaimed XPS is you sidestep the issues with XPS using a blowing agent with a very high global warming potential compared to other types of foam.

  3. Expert Member
    Dana Dorsett | | #3

    One way to find local foam reclaimers is to search your local craigslist for rigid insulation, eg:

    Over decades the performance of XPS falls to that of EPS of similar density, as the HFC blowing agents outgas. EPS doesn't have this issue- and uses a much more benign blowing agent (pentane). Over the lifecycle of the house don't assume more than R4.2/inch performance out of XPS.

    In applications that don't involve contact with soil one can usually use polyiso instead, which is also blown with pentane rather than HFCs, and will in most applications outperform EPS and XPS inch-for inch. As the above ad listings demonstrate there are sevearl square miles of roofing polyiso being reclaimed and sold every year, and the absolute greenest foam insulation out there is the stuff that doesn't have to be manufactured. Using reclaimed foam uses no new feedstocks or blowing agents, and only piles on to the benefit side of the cost-benefit balance sheet by extending it's lifecycle for another 50 years or more.

    In a basement wall application, as long as the bottom of polyiso isn't in contact with a potentially damp slab, it's a good solution, whether fiber faced or foil. If you're insulating over the top of the slab, used reclaimed EPS/XPS, and extend it under the polyiso and seal wall & floor foam with can-foam. If the basement slab has a history of dampeness, install a plastic vapor barrier over the floor-foam that extends a foot up the foundation wall behind the polyiso, and rest the wall foam on the vapor barrier.

  4. NewLaneProperties | | #4

    I was sold on XPS but now looking at polyiso instead. I found some on CL too. Here's my application. In Virginia where it's hot and humid more than cold and dry. Basement slab does not appear to be showing moisture issues. House was built in 04 so it may have poly under it but I doubt foam board. I do not plan on insulating the slab. I also have a crawl space. Can polyiso be left uncovered or should I stick with Thermax? Did I leave anything out?

  5. Expert Member
    Dana Dorsett | | #5

    Like XPS, most polyiso will need to be covered with half-inch wallboard or similar to meet fire codes.

    Dow Thermax has a fire rating on the facers that is good enough to be left uncovered in some locations, not in others. Adding a layer of 1/2-1" Thermax over a layer of 1.5-3" of reclaimed foam can be one way of dealing with it, provided your local inspectors will sign-off on Thermax. Using reclaimed foam for the majority of the R-value is still a lot cheaper than 2-3" of virgin-stock Thermax.

    In Virginia you need R10 to meet IRC minimum performance levels, which would take 2" of polyiso, but when using cheap reclaimed foam going 1"-1.5" deeper would usually still be financially rational on a long term basis.

  6. NewLaneProperties | | #6

    Thanks Dana, this is good info. I plan on adding extra insulation to the stud cavities after i install the foam board, so I'll meet code just fine. I am concerned about the foil on the polyiso, won't that block humid air in the wall cavity?

  7. Expert Member
    Dana Dorsett | | #7

    Don't sweat foil facers the basement never dries into the sub-soil through the foundation wall. The moisture drive is always from the damp soil toward the basement, not from the basement toward the dirt (except on extreme humidity summer days, if you were silly enough to leave the basement windows open when it's ridiculously hot & humid outdoors.)

    The stud cavities need to dry toward the conditioned basement air, so unfaced batts would be best.

    The interior wallboard of the studwall is sufficient for meeting fire-code on the foam, as long as there are fire-stops and/or sufficienty dense fluff in the studwall.

    From a performance point of view, even a half-inch of polyiso and with an R11 studwall would meet code min performance, and would have sufficient wintertime dew point control to keep the studwalls dry without interior vapor retarders (other than standard latex paint on wallboard) but 1.5-2" reclaimed polyiso trapped to the foundation with a 2x4/R13 wall would be 2x code, with HUGE dew point margin.

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