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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 - 12:34

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14 Answers

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1.
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 - 14:44

2.
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.

See:
http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-globa...

http://www.greenbuildingadvisor.com/green-basics/rigid-foam-insulation

Answered by Stu Turner
Posted Fri, 05/02/2014 - 15:34

3.
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 - 11:40

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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 - 17:11

5.
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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 - 18:56

6.
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AJ : could you please elaborate on EPS being crappy as OSB???

every material has its place though ...

Answered by Jin Kazama
Posted Sun, 05/04/2014 - 00:47

7.
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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 - 10:33

8.
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AJ : are you referring to the water issue or it's mechanical/installation qualities ?

Answered by Jin Kazama
Posted Mon, 05/05/2014 - 12:07

9.
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 - 12:39
Edited Tue, 05/06/2014 - 07:38.

10.
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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 - 10:23

11.
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 - 16:16

12.
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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 - 18:00

13.
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 - 01:36

14.
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 - 13:58
Edited Wed, 05/07/2014 - 14:00.

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