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Community and Q&A

Mix and match rigid foam types and sizes

jasonhoetger | Posted in General Questions on
I’m working on insulation details for a new construction basement (main home) and slab (ADU). I’m hoping to source reclaimed rigid foam for at least some portion of it. Since I have a few thousand square feet of slab and basement wall to insulate, I may end up with a hodge-podge of materials and even thicknesses. I have two questions about mixing material types and sizes:
1) From what I’ve read, “layering” different types of rigid foam, either on the wall or under the slab, should be fine–2″ of EPS on top of 1″ of XPS, for example. But what about using different materials at the same “layer”? Like a 3″ XPS panel next to a 3″ EPS panel under the slab, or 1.5″ EPS next to 1.5″ polyiso on the wall. Or even a 3″ EPS panel next to 2 layers of 1.5″ XPS. Assuming the R value is sufficient to meet code, are there any problems mixing materials and even thicknesses like this?
2) I’m trying to avoid framing and drywalling my basement so we have more design freedom if/when we start to use it as a livable space (though perhaps this is a silly goal). To avoid drywall, I could leave a layer of Thermax or similar no-thermal-barrier-required insulation exposed. Is there some minimum amount of Thermax/similar that is required to satisfy the thermal barrier requirement, or will any amount do? For example, would 2″ of EPS against the concrete and 1″ of exposed Thermax on top of that work, or does having any layer of non-thermal-barrier foam require drywall?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Jason.

    Because all of the insulation types you mention are suitable for the uses you have mentioned, I can't think of any issues that mixing and matching would cause, though I've never thought about this before or asked anyone about it (don't use polyiso in ground contact applications, of course). Make sure you use tapes and sealants you use compatible with all of the different materials (also shouldn't be a problem). If possible when layering, stagger the seams. I'll be curious if anyone else has done this...

    Maybe a GBA member will have a better answer for you, but I think the second question is one for your local building official. Thermax insulation is allowed to be installed without a thermal or ignition barrier over it because of it's favorable surface burning characteristics, but I'm not sure that qualifies it as a thermal or ignition barrier to protect other materials installed behind it. You can look for it's evaluation at, which also may help you answer this question.

    1. jasonhoetger | | #6

      Thanks, Brian. I searched extensively for reports of mix-and-match, but couldn't find any (other than the "layering" I mentioned). People must be much better at finding large quantities of reclaimed foam than I am. Thanks for the advice on the tape -- definitely will keep that in mind.

      1. Expert Member
        BILL WICHERS | | #7

        Look on craigs list for your area, and make sure to include surrounding areas in your search. There are foam reclaimers scattered around the country, so there is a good chance there will be one in your area.


  2. Expert Member
    BILL WICHERS | | #2

    You can use polyiso under the slab. Polyiso needs to be protected from pooling water, so sometimes that means keeping its edges from directly contacting the slab too.

    There is no problem mixing EPS and XPS under the slab as long as you are careful to match the load bearing ratings to your application. If you use 25 pound/sqft and 10 pound/sqft together, you’re limited to the lower 10 pound/sqft rating, for example, when the sheets are stacked on top of each other.

    I don’t see an issue with thermax covering lesser fire rated materials behind it and still being safe, the same way drywall works. Once you have an ignition barrier, you have that barrier for whatever is behind it. Using some thinner expensive thermax as a top layer and cheaper, non-fire rated polyiso behind it to build up additional R value should be OK.


  3. Expert Member
    Michael Maines | | #3

    While I agree with Bill's advice that it would be best to match compression ratings, which are determined by how much weight a square inch of foam can support at 10% deflection, the typical loads on slabs are so far below the ratings that I would be shocked if you actually got differential settling. I would not use polyiso below the slab as it can absorb water, which there could be in some situations, and the facings make it hard for the water to drain. It won't hurt anything but it won't be insulating if the cells are filled with thermally conductive water.

    Thermax and the building code specifically prohibit using Thermax to protect other materials. My guess is that it's more lawyer-driven than science-driven, but you won't meet code without adding a thermal barrier at the face.

    1. Expert Member
      BILL WICHERS | | #4

      >”Thermax and the building code specifically prohibit using Thermax to protect other materials. ”

      I think that’s only if you were trying to build a fire rated assembly, like a 1 hour wall where you’d normally have a layer of 5/8” type X on both sides. If the thermax is just layered over another layer of non-rated insulation I think you’re ok, since you’re only “protecting” the other layer of insulation and not anything else in the structure.

      Thermax achieves its fire rating with a thicker metallic facing on one side. Code allows a layer of sheet metal (I forget what gauge) as an ignition barrier to protect insulation in the same way it allows drywall or hardboard. This only applies when an IGNITION barrier is needed, not a THERMAL barrier. The metal facing in thermax should pass as an ignition barrier regardless of how much insulation is behind it.

      Since there is the possibility to interpret this both ways I suppose, it would be worth a call to your local building department before starting work to be sure if they’ll be ok with it or not.


      1. jasonhoetger | | #5

        Thanks, good advice -- I did reach out to my building department (King County, WA), and the inspector told me that it doesn't matter what the exposed thermal barrier is, whether gypsum or some other product, as long as he can see the product label that states it is a valid thermal barrier. He is fine with other foam behind whatever thermal barrier you choose.

        I got the impression that he doesn't see this too often, so like many things, it's probably going to vary by jurisdiction and even by inspector. If you plan to take this approach, you have to be prepared for a little skepticism and be ready to make a case for it.

      2. Expert Member
        Michael Maines | | #8

        I've been through this several times with building inspectors and with Dow technical support. Thermax has passed fire tests, up to 4" thick, but fire ratings are based on assemblies and they don't have testing for using Thermax over other insulation. If your building inspector is ok with it, that's great, but if there was a fire or damage don't expect Dow to take any responsibility.

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