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Community and Q&A

XPS vs. EPS Below Grade

Midwayman | Posted in General Questions on

So I know there lots of information regarding XPS vs EPS for below grade applications.  The consistency of this information is questionable, and now  I am trying to figure out what to do for my situation.

My concern right now is insulating the sides of the trench for the footing/stem wall which sits 30″ below grade

Based on a recommendation from a friend, I set out and collected as much used XPS foam as I could find.  I have almost all foamular 250 and 150.  Some of it is 150 with a thin plastic membrane for moisture control (old stuff?).  Additionally, some of this has been stored in less than optimal conditions and has its share of mouse damage (both chewed on, and a few sheets had some mouse pee smell).  None of this bothered me as it was all slated to be buried.

Fast forward to now, and after some research, found some compelling research on how after 15 years underground, the EPS retained far more of its R value and retained less moisture (here is an artical I saw a few times since then:

So now I have to wonder, should I be using EPS instead of XPS and save the XPS for the walls of the building?  (the roof will be getting a 2″ layer and the walls 1″).   The crummy part about this, is I want to avoid using XPS that has any kind of bad smells from storage/mice/etc in a place that might make the building smell.

So the question is:  What would you do?  Are there places below grade, say, the INSIDE of the trench footing, that would be ok for the XPS then use EPS on the outside?  Can smells be effectively washed off of the XPS?

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  1. benwolk | | #1

    That article is from an EPS manufacturer so take it with a grain of salt. Both are closed cell foam insulation and thus should absorb a negligible amount of moisture.

    You are doing the right thing by sourcing used XPS foam and re-using it instead of buying new. Go ahead and use the XPS in the below grade situations and use EPS for all the above grade locations where you might want to use new products or are able to source less questionable used boards. That way you limit your purchasing of new foam to EPS which has a much lower GWP and carbon footprint than XPS.

    A better option would be use a foam free insulation product for the above grade walls and roof. Something like mineral wool, cork, or wood-fiber (Gutex). All of these options would be better for the environment and be vapor open to allow your wall and roof assembly to dry to the exterior. Unless you are in a hot-humid climate where you want a good vapor barrier on the exterior to prevent reverse vapor drive in the summer.

    1. Midwayman | | #2

      I would hope it goes without saying that I would not be discarding any of this foam. I just want to make sure I use the right materials where they are most effective. If EPS is actually better for this scenario, I will buy used EPS and supplement with new. The XPS I have will get washed, patched, and used on the walls. Is the consensus still that XPS is just as effective (or more) than EPS in these environments?

  2. Expert Member
    Michael Maines | | #3

    If you use freely draining backfill, which you should, and the foundation drains lead to open air, which they should, there won't be enough water at the XPS to matter.

  3. onslow | | #4


    Michael just said the 25 words or less version, but here goes.

    FWIW I have found the recycled XPS I used to be very durable and water resistant, even the random pieces I have had outside for 5 years. One piece that was quite sun struck with surface degrading was practically new looking when I snapped it in half. I will post a pic later. To be clear it is 3" 25 psi grade removed from a commercial roof, so maybe the stuff you have is lighter duty. I covered my foundation with same as well as the walls of my garage. The balance of the walls and roofs are mostly EPS and some Polyiso simply because of the products I used.

    To prevent water aborption issues, I encapsulated the entire foundation from sill to over footing with Grace Bituthene sheeting. The more practical concerns that led me to encapsulate the foam on my foundation were the gaps between sheets and the hundreds of holes created while attaching the sheets to the foundation. All of the gaps and holes would freely allow even low pressure water to migrate to the foundation. How much effect this would have on net R value is a bit questionable, but I do enjoy a dry basement with good thermal profile.

    It is easy to find competeing claims about how much water gets into either XPS or EPS. XPS has a very fine texture that appears to be very tiny bubbles in a solid mass, where EPS is essentially separate bubbles molded into a mass. A potentially important difference lies in the interstitial spaces between bubbles. In XPS it is styrene itself, in EPS it can be air. For insulation reasons, static air is just fine and will behave much like the static gases inside the EPS bead or the foamed bubbles of XPS styrene. For resistance to water immersion an arguement can be made in favor of the XPS despite the outgassing properties.

    Low density EPS bead structure tends toward larger individual bead size and less tight packing. Imagine oranges in a crate. The interstitial spaces are going to be large until you squish the oranges more tightly. High density EPS will typically have smaller bead size and smush them more tightly. Not a good technique for orange shipments. For EPS beads, how you squish them is important.

    The various packing foams used in product shipping can be instructive. Some grades are so loosely bound that merely rubbing them will separate out the individual expanded beads, which then statically cling to everything. Foam packing around heavy items like copiers will typically be comparatively dense and even difficult to break. The beads are harder to dislodge as they have been smushed together more tightly. Disposable coffe cups get around the interstitial problem of low density beads by processing in a way that creates a surface skin to contain liquids. Breaking one will reveal the lower packing density. Holding a cup up to the light will also reveal the generally large bead structure.

    The pertinence to your plans is how much ground water either foam will be subject to. With good drainage either foam will not likely pick up or retain much water. It can be argued that EPS, if it does take on water, will also release the water more easily due to the spaces in the structure. If XPS takes on water it is sometimes argued that it will stay put due to its inherent structure. My belief is that it is harder to force water into the XPS structure. How hard is unknown to me beyond the rudimentary 5 yr test in my yard. The practical effect for foundation walls is likely moot unless you are floating your house in a lake.

    For sub-slab insulation it can be argued that XPS is less apt to allow water migration than EPS. However, if the foam is placed over a proper bed of washed rock to break capillary transfer and keep bulk water away, then the choice needs only to be made on other parameters. I feel XPS is more durable under foot and will survive the rigors of construction better than the EPS grades commonly found at big box stores. I do acknowledge the GWP question and for that reason I used recycled foam like yourself. At long last (at least in my state) low(er)GWP XPS foams are available. The new Corning version is most reasonable for cost.

    EPS vs XPS on walls takes things into the realm of permeability properties. As noted, your local conditions and siding choices will drive the choices you make.

    Mouse pee unfortunately is one of the most irritating things to deal with. Burying it is a good choice. A possible alternate would be to spray a couple of times with a puppy pee neutralizer of which there are many to choose from. Maybe a wash down with OxyClean would work too.

  4. Expert Member
    BILL WICHERS | | #5

    There is a pretty long-term study done by a group in Alaska, I think their road commission, that showed XPS outperforming EPS over time due to XPS retaining less moisture. Your study is in conflict with the Alaskan study. That complicates things :-) Now we need a third study so that we know which of the first two studies to believe.

    I typically recommend XPS underground, mainly due to the findings of the Alaskan study (which I don't have a link to handy, unfortunately). I agree with Michael though, as long as you use backfill that will let water drain off, either type of foam is likely to perform pretty well underground. If you don't have the foam -- either kind of foam -- sitting in liquid water much, there shouldn't be much issue with moisture retention to begin with. Keeping water away is beneficial in other ways too, so it's really important regardless of the type of foam you use here.


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