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Community and Q&A

XPS versus EPS Underslab Foam

Myrtleboone | Posted in Energy Efficiency and Durability on

Hi. I am back and forth on whether I should use XPS (blue board) or EPS for the insulation underneath my slab. I understand the qualities of each regarding water absorption and drying out, but would it (XPS particularly) affect the long term performance to the point where the slab is compromised? I also understand the pollution associated with XPS manufacturing. I have the option of purchasing 6″ (house in very cold climate) of EPS (R-24) or 4″ of XPS (R-20). The EPS is less expensive per inch, however, I need to pay for shipping which jacks up the price. It may be difficult to compare apples to oranges in this case, but when looking at R-value alone (and without heat load calcs performed), do you think I can recoup the $1000 more that I spend for the EPS for 4 extra “R’s”, in energy savings, or go with the R-20 XPS and save upfront? Any thoughts out there? Thanks in advance. The Q&A section of GBA has been very helpful.

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  1. kevin_in_denver | | #1

    The EPS you get at Home Depot does absorb water. I had some sheets laying outside for a year and their weight quadrupled. I have no idea how much that affects the R value. The XPS laying next to it was still perfect.

    If your site is graded properly, it will be pretty dry under the slab, but soil tends to wick water up.

  2. Stu Turner | | #2

    Both work fine as sub slab insulation, and the water absorption is not a factor, especially if installed correctly over a bed of gravel. One thing to consider is the blowing agent used to create the foam. EPS generally uses Pentane, which has a very low Global Warming Potential and results in a stable r value over time. XPS uses HCFCs as the blowing agent, which has a high Global Warming Potential. XPS also loses some of its R value over time, as the gas slowly escapes. One thing to keep in mind considering your climate, is the R values of both EPS and XPS increases the colder the outside temp is.


  3. jinmtvt | | #3

    Have you done a site analysis to determine the height of the water plane
    during melting/spring time ?

    If the foam is under your slab, and you've installed perimeter drains around the footing,
    which are at the same level as the bottom of the footings and you have a gravel layer before
    your foam + a poly under it ..why would it ever contact water at all ?

    If you want to be sure it never sees water, you could also
    add an interior footing french drain at the heigh of the gravel layer perhaps ..

    Wet foam degrades in performance quite drastically,
    but it should not be a problem here unless your footing height vs water level is wrong.

  4. Expert Member

    Kevin, The water absorption of EPS is something that worries me too. I have extensive experience e with hot tub lids that, as you say, over time easily quadruple their weight, and even when placed in a warm, dry atmosphere refuse to give up the water they contain for months. Does anyone have any insight into what the water content of EPS under slabs is?

  5. wjrobinson | | #5

    30 years of use... XPS is ten times better than EPS. EPS is almost as crappy as OSB.

    You get what yaa pay for.

    Hopefully XPS is made with good blowing agents soon. Great product.

  6. jinmtvt | | #6

    AJ : could you please elaborate on EPS being crappy as OSB???

    every material has its place though ...

  7. wjrobinson | | #7

    No Jin I can not. You go buy both use them and you will know.

  8. jinmtvt | | #8

    AJ : are you referring to the water issue or it's mechanical/installation qualities ?

  9. wjrobinson | | #9

    Jin. Be a paid GBAer for access to their wealth and of building details and more. Subscribe to Fine Homebuilding and JLC. Build and learn from doing. The others here have mentioned the downsides of EPS. My nice surfboard is made with XPS. My cheap boogie board is made with EPS. There are designs posted here at GBA using EPS but the projects are done by design and highly dedicated specialist low energy builders. If I build with EPS again I will follow their lead. EPS needs to stay dry.

    Burt Rutan has built dozens of amazing planes using XPS...from a Long EZE to planes that will take you to space

  10. jinmtvt | | #10

    AJ: as much as i like your input, what does building plane ( using mechanical properties such as tensil strength etc.. ) have to do with subslab and building materials ??

    I understand that there is a substantial difference between both product as far as surface/cutting,
    but most building application do not require a perfect surface, nor does it require the material to resist water pressure filling it up .

    I've used EPS locally more than XPS because 1- greener in production 2- made locally 3- cheaper per R than XPS . Different location have different situation.
    But in all the ways i've used EPS for now, i fail to see how it was inferior to XPS.

    so please, explain YOUR pov about the downsides of EPS vs XPS in building situation,
    as you seem to think it has a large advantage , which i am curious to learn about.

  11. Expert Member
    Dana Dorsett | | #11

    The water absorption of EPS is not at all a problem, if you use the appropriate density for use under slabs. The cheap stuff often at box stores is 1.0lb density per cubic foot "Type-I" density, which doesn't even have sufficient compressive strength appropriate for use under slabs. If it's labeled you can tell- Type-I goods run about R3.9/inch, R7.8 @ 2".

    NEVER use Type-I EPS under slabs- it's compression rating is LESS than 10 psi!

    But Type-II (~1.5lbs nominal density) has no absorption issues- it not take on water. Type-II EPS is used for everything from crab-pot buoys to dock floats to surfboard cores- it's good stuff.. It is also rated 15psi, which is plenty for a residential slab. It's labeled R value is between R4.15- R4.2/inch- a 2" thick sheet would be R8.3- R8.4. Type-II EPS is manufactured in high quantity, and is more likely what you'd get when buying through distributors without specifying it.

  12. wjrobinson | | #12

    Jin, Dana just posted the info worth following. Box store EPS is what is poor.

  13. jinmtvt | | #13

    Yeah well AJ if you are comparin type 1 to XPS i understand why you vouch for the later.

    I have purcahsed/installed type 1, type 2 , some custom density and ICF ( quad lock ) EPS.
    The EPS used for Quad-Lock bloc was the best in terms of feeling,
    and it was much stronger than regular hardware store XPS .
    I also believe it is completely impossible to "wet" as i've left some parts outside for 2-3 years now without ever noticing any weight change.

    Anyhow, thanks for the input Dana it confirms as i thought that each material or version of as its right place in a building. So high density EPS if manufactured locally would be the best choice for subslab insulation if you consider price and impact .

  14. Expert Member
    Dana Dorsett | | #14

    Type-II EPS isn't generally considered "high density"- it's more like "standard density" for most applications.

    Type-IX EPS is 2.0 lbs/cubic foot, and Type XIV EPS is 2.5lbs. These high density versions are usually specified for their higher compressive strength. Type XIV goods are usually only specified when there is a higher static load, such as under a concrete footing for a foundation wall- typically rated at 40psi.

    Quad-Lock (and most ICFs) are made of Type-II EPS.

  15. user-871583 | | #15

    I wanted to get to the bottom of this debate as well and decided to perform my own long term tests. End result was that hands down XPS out performed EPS in a application where the insulation would regularly get wet.

    See for results

  16. jackofalltrades777 | | #16


    What about some SIP companies that use Type 1 EPS instead of Type 2 EPS? If Type1 EPS is not suitable for slab insulation how could it be suitable for roof or wall structural insulation in a SIP?

  17. GBA Editor
    Martin Holladay | | #17

    A couple of questions:

    1. What is "30# EPS"? Did you perform any testing on Type IX EPS?

    2. You wrote, "So why is EPS used in many 'green' projects? This stems from the EPS industries claims that it represents a lower Global Warming Potential vs XPS due to its use of Pentane as a blowing agent compared to the traditional HCFC agent used by the XPS industry. But XPS manufacturers like Owens Corning have already replaced their blowing agent with a Zero Ozone Depleting formula."

    In this paragraph, you are confusing two environmental issues -- global warming potential due to blowing agent release and ozone layer depletion due to blowing agent release. These are two separate issues. Just because a blowing agent doesn't damage the ozone layer, doesn't mean that it has a low global warming potential.

  18. Expert Member
    Dana Dorsett | | #18

    PeterL: Under a slab you can count on the humidity environment to be far higher than in above grade applications such as walls or roofs. If you have moisture levels in the above grade walls or roof high enough to affect the thermal performance of a SIP, the OSB facers will have long since rotted away.

    Martin: That's 30 kg/meter^3, not 30#. The conversion is 1lb/ft3 == 16kg/m3, so 30/16 would be about 1.9lbs per cubic foot, within the manufacturing minimum density of Type IX EPS.

    Mind you, burying it in undrained uncharacterized dirt isn't a very good comparison to the saturation levels you'd see in a code-legal slab that would have drained gravel under it (and would not have an overburden of wet soil.), but even the not-very characteristic test the hit in R value is only 16% (at saturation!).

    As the HFC134a bleeds out of the XPS over a few decades XPS takes a comparable hit in R-value that is PERMANENT, and unrelated to moisture saturation level.

    EPS is usually more than 15% cheaper than XPS at any labeled R value as well. If you're really concerned about moisture levels just derate the EPS by 15% from it's labeled R for performance characterization and call it a day. In a slab where you're looking for R10 performance that would be adding only a half-inch to the foam thickness.

    It's unclear how quickly the R-value of XPS falls when blown with low GWP HFO-based blowing agents, since it is (SFAIK) not commercially available in the US. In Europe most XPS is blown with CO2 (with only 1x CO2 GWP, strangely enough :-) ), but it has an R/inch identical to that of EPS. The low-ozone factor HFCs commonly used for XPS in the US pack far too great a GWP punch to be considered a "green: solution for sub-slab insulation, when low GWP solutions of fairly comparable performance & cost are available.

  19. STEPHEN SHEEHY | | #19

    Just a reminder that recycled xps is widely available and cheaper than new eps or new xps and whatever
    Co2 impact there is has already happened.

  20. Expert Member
    Dana Dorsett | | #20

    That's true- reclaimed foam is the greenest foam there is!

    For design robustness, derate any reclaimed XPS to it's fully-depleted ~R4.2/inch, even if it looks pretty "new". (XPS that has seen 2-3 decades of service will not be performing at R5/inch.)

    Reclaimed EPS & polyiso are widely available too. Take some care to only use EPS under slabs if it's at least 1.3lbs/cubic foot or higher. Don't ever use polyiso of any density under slabs or on the exterior of foundation walls, but it's fine for foundation interiors, above grade sheathing, roofs, etc. .

  21. UCS88 | | #21

    Hi I'm new here and just stumbled upon this string. Could EPS or XPS be used for ADA ramps?

  22. charlie_sullivan | | #22

    EPS can be and is used for ramps. I found an example on the Universal Construction Foam web site. I'd include a link but the spam filter blocked it.

    XPS would be possible, but worse from a global warming impact perspective, would cost much more, and would be harder to get in custom shapes. EPS can be directly molded into a wedge, whereas XPS comes out of the machine as a slab and would need to be cut to make a wedge.

    In that example it had concrete poured on top. You'd need something on top as a floor surface--for interior use it could be a different material like two overlapped layers of OSB sub-floor.

  23. GBA Editor
    Martin Holladay | | #23

    Here is the link:
    Handicapped access ramp.

  24. charlie_sullivan | | #24

    AJ, thanks for the list! One that I'm curious about is you don't like hardwood floors, and you don't like carpets. So what floors do you like? Pine? Presumably tile? Linoleum? Vinyl?

  25. wjrobinson | | #25

    Charlie, long post on mini screen, I screwed it up.. just edited it. I love wood.

  26. wjrobinson | | #26

    I thought I would list good building products that I say are good because they have been good to me over the decades. And the bad

    high density fiberglass batts, easily installed correctly
    Silicone for glass installs
    higher quality vinyl siding for a low maintenance exterior.
    Factory coated cement board siding has a very nice finish compared to field finishing
    PVC trim and PVC deck boards
    Hidden fasteners when decking
    copper plumbing, PEX plumbing, have had good luck with all three PEX fitting systems
    natural gas heat, mini splits, pellet stoves, new wood stoves, if you have the coin the Euro stoves
    In my area Andersen windows, Marvin and Pella... same for doors, I love the doors. Fiberglass Thermotru too.
    Open cell Icynene spray foam, but am trying not to use spray foam
    Never drywall in a cellar or shower
    Schluter products are amazing, so is Redguard.
    Electric mat heat under bath tiles
    Recessed lights are fine in first floor uninsulated ceilings
    DuraSupreme cabinets
    Custom cabinets
    custom stairs by myself
    Avoid carpet or install wool
    hardwood floors
    Mahogany should be your favorite wood. It is mine. Also all the other tropic hardwoods, Brazilian
    cherry, IPE, Teak and all it's cousins, Much of it is plantation grown now Great for floors and decks and stairs and trim and doors and everything.
    PV is neat, hydro power in a stream is awesome
    bicycles are a good way to be green IMO
    hang gliding is a great green activity, paragliding even better, hike up a mountain with it in your backpack and fly down, or up if you find the thermals.
    Solar sailplanes, how can you not like them

    Bad luck;
    low cost vinyl siding
    3 ply low cost plywood, bought by mistake, didn't notice the ply's
    cheap EPS
    cedar deck material, looks great for two years, rots out in 5 years in my area!
    latex deck stains, suck. they come off in spots over time and then you have to use chemical strippers to redo the deck. Insanity.
    standard 5/4 deck boards not as good as 2x6 pressure treated southern yellow pine if using PT pine
    MDF particle board is the worst idea ever brought into home building. If you want 1" material buy 3/4", wet it and the next day you'll have 1". Crap. No particle board goes in homes I work on.

    This is my list, yours I'm sure is.... yours. Hope at least one item is an eye opener for somebody somewhere on the planet or off...

  27. user-871583 | | #27

    Sorry for the delayed response. I find the GBA site crazy to use in the fact that you cannot subscribe to comments on threads you are interested in (like you can on LinkedIN) and so do not typically visit here often as it just takes too much work and I do not have the time to check to see if there is a comment of interest or not. I was brought back here by a comment on my blog. GBA - Please add a subscription service for the comments.

    Dana and others advised that the water absorption of Type 2 EPS (High Density) is not a problem at all. Nothing is further from the true and this is just a urban myth based on some biased research by a EPS company.

    I did my own independent testing including have the products HOT box tested at a local Technical Institute ( EPS soaked up 258% of its original mass compared to just 31% for XPS in the same conditions. What is true is that EPS drains quickly and will dry out relatively fast if removed from a water source for a period of time. But in a sub grade environment where the humidity alone will be 100% for much of the year, it is going to wet up.

    Martin - No I did not test Type IX EPS – I only tested materials that were reasonable for underslab applications. Re Global Warming vs Ozone Depletion – not mixing anything up. The Owens Corning product also has 70% less global warming potential over previous formulations.

    Dana – the test is actually quite legitimate as a lot of people in my region are not always diligent about getting a good granular layer below the slab and even if they are, do not necessarily drain that layer, meaning the water table still rises up close to the sub slab insulation. The test also emulates foundation insulation where back fill is often placed directly against the rigid board. You may not think 16% is high – but it could mean the difference between comfortable floors or cold floors (or even worse, floors that condense). I agree you could just derate and add more.

  28. charlie_sullivan | | #29

    " EPS soaked up 258% of its original mass compared to just 31% for XPS in the same conditions"

    That's a red herring. FIrst, stating it as % of it's original mass is misleading. Mass per unit volume would be more relevant. Otherwise, you are penalizing it for starting out as being a light weight material. The EPS at 258% moisture content is at about 2.6 lbs of water/cu foot. A piece of wood at 2.6 lbs. of water per cubic foot is at about 10% moisture content. That's considered nice and dry. Rather than throwing around a number like 258% that sounds scary but isn't relevant, consider why you care. Probably the only reason is that it has lower R value when it is wet. But as Dana pointed out, the effect of that is only ~15%, so all you need to do is get EPS 15% thicker.

    It's good to know that wet EPS has its R-value degraded by 15%, but that's not a reason to avoid it.

  29. wjrobinson | | #30

    So we build floating docks with foam.... XPS foam.

    I built some ultralight aircraft floats back when... XPS foam.

    EPS... have run into wet EPS... and we did have some floats made with EPS.... poo poo... real poo poo.

    Use XPS under a slab folks unless you are going 8" passive level and have all it takes to do all with the proper EPS and proper crew, and proper plans like some examples bouncing around GBA.

    If you decide to have no offspring you get two giant offsets that will credit you for any and all less green choices you could ever make building a home... Go XPS, vinyl and load up on concrete... why not, you deserve it for your ultimate green choice of cutting out the not so green kid making factory religions promote.

    You'll be an order of magnitude greener than all your Prius hugging neighbors with 2.1 replacements riding their green bikes up and down your street.

  30. user-871583 | | #31

    All the info is posted to allow you to cut this what ever way you want. I set out to put an end to the inconsistencies between the published specs of these materials and the urban myths circulating amongst green building circles as to which absorb more moisture. The testing is pretty clear - in equal conditions, EPS absorbs a lot more moisture than XPS just as the specifications indicate.

    Yes you can compensate, but does saturated EPS have a reduced life span??? Will the reductions in resistance continue as it gets even wetter?? Do the costs for the extra EPS pencil out?

    Lets look at two examples utilizing pricing from the Home Depot site (an easy to access and consistant pricing) using the same density materials. Lets say we want a R10 slab.

    EPS Option:
    3.75/inch degarded to 3.15 after the 16% 'wet' drop
    Need 4" to meet requirement (utilizing standard thicknesses).
    2" x 48" x 8' = $16 and 1" x 48" x 8' = $7.78 so $23.78 for the full 3" (R9.45)

    XPS Option:
    R5/inch degraded to R4.85 for the 'wet' reduction per the same testing.
    Need 2" to meet requirements (utilizing standard thicknesses)
    2" x 24" x 8' = $33.30 (R9.7)

    So for $9.52/16 sq ft difference for this example. For my basement, this is an extra $1000.

    Not much premium to ensure a better performing product.

  31. charlie_sullivan | | #32

    Or, if you want to spend over $30, how about 4" of EPS. At $32, That's still slightly cheaper than the XPS, and the degraded performance is R-12.6. 30% higher actual in-service R-value for a slightly lower price. That's what I'd call a better performing product.

  32. user-871583 | | #33

    Better compare would be 4" EPS for $32 (R12.6) vs 2.5" of XPS for $41.61 (R12.13). Still a reasonable $1000 premium for my basement for a product that I know will not significantly reduce its performance down the road. It would also be interesting in also looking at the embodied energy of 4" EPS vs 2.5" of XPS in terms of manufacturing and also shipping.

  33. STEPHEN SHEEHY | | #34

    Using reclaimed insulation makes the numbers better. I paid $19 for 4" xps last fall. R20ish. Plus what amounted to about $4 per piece for shipping 130 pieces about 200 miles.

  34. Dana1 | | #35

    "Dana – the test is actually quite legitimate as a lot of people in my region are not always diligent about getting a good granular layer below the slab and even if they are, do not necessarily drain that layer, meaning the water table still rises up close to the sub slab insulation."


    Seriously, drainage gravel notwithstanding, that is a "quite legitimate" test only if folks are building with standing water above the top of the slab, or the foam being used as a float to keep their house from sinking into the swamp, or doesn't build a roof or onstall a vapor barrier so that the slab stay rain-soaked.

    Most people to build foundations in stable soil rather than swamp muck next to a pond, install a roof with overhangs &/or gutters, and at least grade the surface soil so that surface water drains away from rather than into the house. A "...quite legitmate ..." test would be to install it under 4" of concrete in a conditioned basement in soil conditions that a sane person would actually build a foundation in, not 4' down on the swampy edge of a pond, then buried in rained-on saturated soil.

    From the linked-to test description:

    " EPS looses 15.7% of its thermal resistance when in a wet environment and saturated compared to only 3% for XPS."

    Any comment on the fact that XPS loses about 15% of it's R value in the first 50 years as the "... Zero Ozone Depleting formula..." (that still has a global warming potential greater than 1000x CO2), leaks out? With XPS the 15% hit in performance over time is guaranteed. With EPS it takes the hit only if you have fully saturated soil or standing water in your basement for months at a time, and when the tide recedes the performance is restored. But once the HFC134a gets out of the XPS it's gone forever.

    Did you test the comparative drying rates of EPS vs. EPS after the tide went out?

    Anybody who builds with so little foundation drainage and or in soil with such high moisture content directly below the slab that the EPS really DOES take that 15% hit in performance has MUCH bigger problems to worry about than the modest increase in energy use that performance hit represents.

  35. user-871583 | | #36

    Dana - not sure about conditions in your part of the world - but most of your comments were irrelevant to below grade slabs.

    I am not talking about storm water, so efforts at or above grade are irrelevant to this test or conversation. Storm water is not going to filter down through 10-12' of soil. I am talking about below grade ground water which I am pumping out of my construction pit every single day, and no I am not in a swamp and in fact am building on stable solid ground. In fact I am on the side of a mountain. That is part of the problem, there is quite a bit of ground water travelling from higher up the mountain along the division between glacial cobble and glacial till. On my site, this interface is about 10' down and it continually flows all day every day and has nothing to do with the current days weather. In fact, it was flowing in August when it had not been raining for many weeks. Ground water is pretty common in my region of the world and so is below grade basements. Then we have the fact that many homes do not have a thick granular layer below the slab (code only requires 4" under slab and none under footings) and the perimeter drain is often installed above most of this granular layer. Finish this off by another fact that many homes in teh region have to pump up that ground water because the aging services were never anticipating below grade basements and the result is that many homes will have standing water below the slab and even if almost none is present, the humidity will still be 100%.

    - XPS only looses 10% of its resistance in the first 20 years not 15%.
    - Owens Corning product is not using HCFC-142b. It has phased it out in advance of the Montreal Protocol deadline. You need to update your information.

    "Anybody who builds with so little foundation drainage and or in soil with such high moisture content directly below the slab that the EPS really DOES take that 15% hit in performance has MUCH bigger problems to worry about than the modest increase in energy use that performance hit represents. " You clearly live in Nirvana. In the Pacific North West, this could be every house on the block.

    Listen - the goal of this experiment was to refute the claim by the EPS industry that it is XPS that wets up and not EPS, even though the technical specifications of the products and ASTT testing show otherwise. My testing confirmed that the EPS wets up much faster and to much higher levels than the XPS of similar density. I am satisfied that this test was relevant and mirrored the conditions present on many build sites in the region.

    I will be setting up a much longer test prior to back fill of my foundation where samples will be buried in a vertical orientation in well draining areas as well as virgin soil areas. These samples will probably be instrumented, but then will be dug up a few years down the road to send back to BCIT to test again.

  36. charlie_sullivan | | #37

    HCFC-142b was phased out by the Montreal Protocol in 2010. Yes, Owens Corning phased it out in advance of the deadline, in 2009. That had a GWP of 2310. They now use a "proprietary blend" now that has "70% less GWP", so that would be a GWP of 693. That's more than 100X the GWP of the hydrocarbon blowing agent in EPS.

    Most North American XPS uses HFC-134a, with a GWP with a GWP of 1430. So Owens Corning is apparently doing better. They may be using a blend of HFC-134a and HFC-152a. HFC-152a has a GWP of 124. But the blend, at 693, is still making un unnecessary, unjustified contribution to global warming.

    Sean, I appreciate your doing tests and sharing the results. They give me the confidence to use EPS below grade, knowing that worst-case moisture uptake is not a big deal, and that even in that case, I'm getting better final degraded R-value per dollar than I'd get with XPS, while also producing lower environmental impact.

  37. wjrobinson | | #38

    Choose a foam, insulate your slab
    pop a cold one upon completion
    smile broadly
    eat a great breakfast
    Do it again
    stop to watch the sun set


  38. user-5480158 | | #39

    Here is an interesting link on repeated saturation/freeze cycles and water absorption for XPS and EPS.

    "With frozen EPS specimen the absorption continued increasing even after 48 months of immersion. Presumably the reason for such behaviour is that the pore structure of EPS is not able to resist the tension caused by freezing water and therefore cracks are formed. Thus, more water gets absorbed inside the EPS through the cracks and it causes cracking deeper in the specimen which is why absorption increases after every freezing period."

    Owens Corning also has a document showing that after 1000 saturated freeze thaw cycles EPS retained 41% of R value and XPS retained 81% of R value. (page 4)

    Again, the effects would probably be much less dramatic with proper drainage and/or not living in a swamp that regularly freezes.

    I'm still on the fence on EPS or XPS for my basement. XPS definitely seems like the better product, but the GWP does concern me. I'm not sure of where to source reclaimed XPS in Winnipeg. EPS seems like a great choice ($ and environment) if you have confidence in your drainage.

    Thanks to everyone for providing such useful information.

  39. Dana1 | | #40

    If you have freeze/thaw happening in saturated soil under the slab you have much bigger problems than the degradation of the EPS, eh? ;-) (Say, a frost heaved, badly broken slab, for instance!)

    The only place this is even possible for an occupied house in Winnipeg is at the slab edges, with worst-case drainage conditions.

  40. user-968917 | | #41


    Salvage Supermarket in Winnipeg frequently has reclaimed foam insulation available. Clients of mine recently purchased 4" XPS from them at a very good price.

  41. t_t | | #42

    I hope it's ok to resurrect this thread...

    A test showing there is little performance difference between EPS and XPS

    And "XPS is far better" tests:

    I've been thinking how tests can greatly contradict on a simple thing like absorption. Maybe the fact that EPS dries out so quickly holds part of the answer. If a sample is tested at the end of 3 rainy weeks the results may be very different from testing after 3 sunny weeks.

    So, as an example, the test results from that 15 year long field test may have been very different if they had waited a few more days and dug up the samples after a few days of heavy rain.
    All that said, I don't care about the amount of moisture. All that really matters is the R-value.

  42. Mark_Nagel | | #43

    While researching insulation materials for under-slab use I ran across this from DuPont:

    Here's the summation (at the front of their article). It seems to run counter to what people have said in this thread.

    A new study from the Alaska University Transportation Center, University of Alaska Fairbanks (the Connor study [1]) recently reported on R-values measured on insulation removed from below grade applications in harsh climates, supplementing data from two previous similar studies [2, 3]. The three studies reported the in-service R-values per inch of extruded polystyrene (XPS) and expanded polystyrene (EPS) insulation installed under roads and airport runways with the time in service ranging from 1 to 31 years. The data from these three studies was combined for a more reliable determination of both XPS and EPS in-service performance in demanding below grade applications. The new data confirms previous results indicating higher R-value per inch for XPS compared to EPS. In fact, the new results indicate that the previous results underestimate the decrease in EPS thermal resistance over time. To deliver the same in-service R-value, EPS needs to be 1.5 to 2 times thicker than XPS.


    They are saying that XPS R-values decline (much) less than EPS and that in longer-term studies EPS performs worse than had been thought.

    Curious as to what people's thoughts are today (some 5 years since the last post to this thread).

    One cannot, in the future, change or fix any problems with an insulated slab-on-grade, unlike is possible with walls and roofs. If EPS performance is dropping off as this DuPont article suggests then it's not far off from pouring money into the ground. How can a home buyer identify any problem, and wouldn't it likely happen outside of any warranty period?

    1. Expert Member
      MALCOLM TAYLOR | | #45


      All I know about the performance of the two c0mes from GBA. The only direct experience I've had is with the EPS foam in hot tub lids at a resort I did work for. After a couple of years they became saturated and very heavy, which I found disturbing.

  43. Expert Member
    BILL WICHERS | | #44

    I've seen some documentation showing EPS tending to take on water more readily than XPS when underground, and performing worse as an insulating material as a result. I did a quick read of the info at that link, and it doesn't seem to get into the reasons behind the reduction in R value, which is unfortunate.

    EPS is basically working with trapped air, so there are no blowing agents to diffuse out and reduce it's R value from the rated R4.2 or so per inch. XPS blowing agents diffuse out over time, and reduce it from it's rated R5 per inch down to pretty close to EPS's R4.2 per inch. For applications where immersion in water isn't an issue. XPS should gradually decline in R value performance until it's about the same as EPS, and EPS should stay pretty consistent with no degradation in R value perforamnce over time.

    Underground, if EPS is taking on some water, that would explain the drop in R value. XPS is less prone to taking on water. There is one big thing to keep in mind here though: we can design systems to limit the possibility of either insulation material being subjected to water -- just put drainage (clean stone and drain tile) in under the slab. This is usually done under roadways (I don't know about airport construction, but it's probably to an even higher standard than interstate roads), so it's possible something else other than water is at play here.

    It would be interesting to see more detail into their study. They could have easily tested to see if water retention was the cause of the drop in R value.


  44. Mark_Nagel | | #46

    Seems each insulation consortium has it's "studies" indicating their product as being superior.

    Yes, it's NOT a good idea to take the "floating raft" description to its fullest- setting it ON/IN water! :-) For me, I have to consider water intrusion (and, really, it's water that's the biggest threat to nearly ALL construction, so one shouldn't just push such concerns off to the side) as I clock over 100" of precip a year. With that much water it's like dog hair, no matter how you try to contain it it IS going to get where you don't want it to get in to! (having a mud room and a good porch are two requirements for a new home for me!)

    Kore industries uses XPS: DuPont and Dow Corning use XPS. These are top industry leaders here.

    The saying that "you get what you pay for" seems as though it might take on some added importance given the gravity of the situation (once it's installed that's it until everything gets torn down). If there's any situation that demands the best means of risk reduction I'd think it would be the foundation.

    One thing that I seemed to glean from some quick reading is that it seems like XPS might be more sensitive to UV than EPS. This would be a concern on above grade protrusions (support beam/walls).

    I'd agree that given proper site work that a floating raft slab wouldn't seem to have the concerns as say a regular floating slab or a perimeter wall foundation would (the later having footing area well down, below grade). However, there's still a bit of concern as pertains to grade fill/material that is against the vertical sides of the outer insulation. While degradation here is a problem I'd think that it would be one that could be dealt with (unlike with degradation UNDER the slab, past the outer edges). I suppose that if all things being equal and there is proper drainage (and, of course, we're talking floating raft slab construction) that the material that handles UV best might be the better selection (even though one would apply some sort of coating on the exposed portions there's always the possibility of that material degrading or being damaged and thus exposing the foam).

    Sadly, it appears that us lower-level consumers will be the ones to pay the costs for long-term studies.

  45. charlie_sullivan | | #47

    I suspect that many of us here on GBA have been hostile to XPS and supportive of EPS beyond what the data supports because we have been horrified by the global warming impact of North American XPS. With Owens Corning now producing low-global-warming-impact XPS, there is an option to use XPS with being as egregiously irresponsible to the climate. It's not as low impact as EPS, but the impact is reduced drastically. But you do have to special order the "NGX" foam.

    I doubt the UV issues are any different--it's the same polymer. But the carbon black in the new Dow foam and the graphite is Neopor and other brands of graphite EPS (gps) w0uld hold up better with UV. They probably still need protection from UV, but would do better with incidental exposure.

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