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Community and Q&A

GPS vs Polyiso for basement insulation

Nathan_PA | Posted in Green Products and Materials on

Hello all,

I am beginning a basement finishing project (basement was partially finished in the past, but not done well, had flooding and mold issues).

I am having a sump pump with perimeter drain installed, and then plan to insulate exterior walls and floor with rigid foam.

I am debating between gps (proboard 2″) and Polyiso (rmax thermasheath 3) for the insulation.

I am currently leaning towards Polyiso for the walls and gps for the floor, with the thought that the Polyiso on the wall comes with an air barrier in the form of the facing, but the GPS has a higher compressive strength for the floor (25 psi vs 20).

Currently thinking I would then put down a dimpled plastic liner above the GPS on the floor, followed by osb board for weight distribution, follow by rubber tiles to provide a soft top surface for the kids.

My questions are:
1. Is using the Polyiso on the walls a good choice? I read one article suggesting that it’s r value falls off if it gets colder than 50 F or wet, which my basement shouldn’t usually get that cold, but….
2. Are my flooring plans overkill? Should I skip the dimpled plastic or osb? Do I need something as a thermal barrier over the GPS on the floor for fire protection? If do, what can I use?
3. For hanging drywall in front of the Polyiso, can I use 2×2 instead of 2×4 studs to save space?

I live in South eastern PA.

Any thoughts or suggestions appreciated.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Nathan.

    If you are installing a perimeter drain and sump pump because you have had water issues in the past, you need to do all of this insulating and finishing with a plan towards allowing water to get to that drain. You'll need something to allow water to drain down the walls, behind the insulation to get to the drain, like a dimple matt. And a dimple mat on the floor or a subfloor product like Dricore is a good idea in a potentially wet basement. Here a couple of articles that you migh find helpful: How to Insulate a Basement Wall and Using a Dimple Mat to Keep a Basement Dry.

    As far as your insulation choices, I think it is perfectly fine to use GPS on the floor and polyiso on the walls, though I will point out that GPS sealed at the perimeter and seams will create an air barrier too.

    I'm not sure if your building inspector will have an opinion on this, but 2x2 strapping for the drywall is fine (I've seen builders use 1x strapping in this application). It just doesn't give you a lot of room in the walls for more insulation, electrical, plumbing, etc.

    I'm not a fan of finished basements. I've seen too many expensive finished basements destroyed by a single flood. But I understand the need for additional living space and accept why people do it. Since you have had water issues in the past, and it is great that you installed a perimeter drain, I would recommend making sure everything outside is up to par as well--working gutters, downspouts that drain far away from the house, proper grading away from the house, etc.

  2. Expert Member
    BILL WICHERS | | #2

    Polyiso on the walls will work just fine, just keep the bottom edge a little above the slab to avoid any potential moisture wicking issues in case you ever have any water on the floor.

    Do NOT use polyiso on the floor due to the potential for problems if you ever have any water on the floor. GPS is fine here, although I’d save some money and just use EPS. GPS has slightly better R value per inch, but not enough to be worth the extra cost in my opinion.


    1. brendanalbano | | #3

      I didn't get to fully put this to the test, because the contractor ended up not using GPS for lead time issues (wasn't willing to use foam that wasn't in stock at a local lumberyard), but I got a quote from Insulfoam where their GPS product was cheaper on a $/R basis than their EPS product. Cost will probably not be the same in all markets or for all manufacturers, and perhaps Insulfoam's EPS is more expensive than average (which would make their GPS look better by comparison.) But food for thought! GPS might not be as big of a premium as you might assume.

      That said, if EPS is cheaper for the same performance, and you don't need the thinness of GPS, then I agree with Bill that you mind as well just use EPS and save a little money.

    2. gary__b | | #4

      Indeed, and as to the OP's concerns on compressive strength: EPS is going to be just fine. I just got done doing a floor with R-Tech EPS (and its meager 13 psi) covered with OSB, and there's no discernible deflection. With the OSB to distribute the load, there's nothing you'll put on your basement floor that's going to challenge it.

      1. Expert Member
        Dana Dorsett | | #5

        Fifty gallon water heaters get put on Type-I EPS pads all the time, even without a subfloor to distribute the weight.

        Maybe a couple of 200lb Czechs dancing the polka with a keg on each shoulder on the corner of your antique full-slate-slab pool table might show some deflection under that corner's leg, but it's really a non-problem that people seem to enjoy worrying about.

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