Helpful? 0

XPS versus EPS Underslab Foam

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.

Asked by Matthew Michaud
Posted Fri, 05/02/2014 - 11:34


20 Answers

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Helpful? 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.

Answered by Kevin Dickson, MSME
Posted Fri, 05/02/2014 - 13:44

Helpful? 1

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.


Answered by Stu Turner
Posted Fri, 05/02/2014 - 14:34

Helpful? 0

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.

Answered by Jin Kazama
Posted Sat, 05/03/2014 - 10:40

Helpful? 0

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?

Answered by Malcolm Taylor
Posted Sat, 05/03/2014 - 16:11

Helpful? 0

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.

Answered by aj builder, Upstate NY Zone 6a
Posted Sat, 05/03/2014 - 17:56

Helpful? 0

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

every material has its place though ...

Answered by Jin Kazama
Posted Sat, 05/03/2014 - 23:47

Helpful? -1

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

Answered by aj builder, Upstate NY Zone 6a
Posted Sun, 05/04/2014 - 09:33

Helpful? 0

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

Answered by Jin Kazama
Posted Mon, 05/05/2014 - 11:07

Helpful? -1

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

N961EZ.jpg space ship one.jpg
Answered by aj builder, Upstate NY Zone 6a
Posted Mon, 05/05/2014 - 11:39
Edited Tue, 05/06/2014 - 06:38.

Helpful? 0

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.

Answered by Jin Kazama
Posted Tue, 05/06/2014 - 09:23

Helpful? 2

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.

Answered by Dana Dorsett
Posted Tue, 05/06/2014 - 15:16

Helpful? 0

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

Answered by aj builder, Upstate NY Zone 6a
Posted Tue, 05/06/2014 - 17:00

Helpful? 0

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 .

Answered by Jin Kazama
Posted Wed, 05/07/2014 - 00:36

Helpful? 1

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.

Answered by Dana Dorsett
Posted Wed, 05/07/2014 - 12:58
Edited Wed, 05/07/2014 - 13:00.

Helpful? 0

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

Answered by Sean Wiens
Posted Sun, 08/31/2014 - 20:26

Helpful? 0


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?

Answered by Peter L
Posted Tue, 09/02/2014 - 00:56

Helpful? 0

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.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 09/02/2014 - 07:09
Edited Tue, 09/02/2014 - 07:10.

Helpful? 0

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.

Answered by Dana Dorsett
Posted Tue, 09/02/2014 - 11:10

Helpful? 0

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.

Answered by stephen sheehy
Posted Tue, 09/02/2014 - 18:55

Helpful? 0

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. .

Answered by Dana Dorsett
Posted Wed, 09/03/2014 - 09:56

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