I've looked through the posts for this information and need a little more than I see. I am installing an auto shop (for home use) in an existing pole building. I will install a radiant floor system. I have access to some very inexpensinve white polystyrene sheets, 2" thick. Can I use this under the slab or do I need the high density (blue) stuff?

I will be insulating with 4" around the building perimeter, to qa dept of 4 feet. (Wisconsin climate)

It depends on the nature of the EPS sheets you are getting inexpensively. Both EPS and XPS can go under slab, but you need to make sure the compressive strength is adequate for your use. I wouldn't use any old EPS, particularly under a "garage" floor because I like my parking spaces to have a little thicker slab and because of the added vehicle and machinery weights. If it had to be EPS, I'd use an engineered foam, like Insulfoam or Geofoam with known compressive ratings.

Building codes might not be an issue for that space, but EPS has a slightly lower R-value per inch such that 2" gives you R-8 or R-9, instead of the recommended R-10.

There is also the water issue If your site is well-drained, it probably doesn't matter, but if it is wet, you should know that EPS takes on a bit more water.

Considering that location is Wisconsin and the slab will be heated, the sub-slab foam ought to be thicker than 2", at least 4".

What Dick said.

The thickness of the slab affects the compressive strength required. Light trucks & cars don't present a huge load, but if you're skimping on concrete and going with a code-min thickness for garage slabs there's some potential for cracking the concrete even with blue-board. With a 6" slab you could park a tank on it and be OK. Thicker slabs are stiffer, and distribute the weight from the 4 points of contact with tires over a broader area. ( Post-tensioned slabs are also inherently stiffer.)

Contrary to ICFHybrid's statement, EPS (even low density Type-I EPS) takes on less not more water than XPS in wet/very-wet environments, an aspect that hasn't escaped the attention of EPS marketeers: http://www.epsmolders.org/PDF_FILES...ade103.pdf  EPS has a small amount of interstitial space between the beads, but it has a stronger closed-cell structure and fewer open or broken cells.

Dana1 quotes one study to support his statement that EPS absorbs more water than XPS. Who paid for that study? The competition to EPS (Extruded Polystyrene Foam Association) claims that the study was paid for by the EPS Molders Association. The Extruded Polystyrene Foam Association points out that building codes generally show a preference for XPS compared to EPS at http://www.xpsa.com/tech/index.html. Most of the standard ASTM tests show an advantage in compressive strength, flexural strength, thermal resistance, and water absorption for XPS compared to EPS, at least according to the results selected by an XPS manufacturer (DOW) and presented at: http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0266/0901b80380266d70.pdf?filepath=styrofoam/pdfs/noreg/179-06147.pdf&fromPage=GetDoc

I would tend to accept standard ASTM test protocol over one field test sponsored by an industry trade association with a vested interest in the results. I would also agree with ICFHybrid's inference that EPS can be variable in structural integrity. I have found some of it to be literally flaky when cut, producing an array of little polystyrene balls.

I have tested neither EPS nor XPS for water absorption, and do not have a commercial interest in either. However, I did select XPS to put under my conditioned crawl space in a high water table area, guessing that it might perform better than EPS, so I have that personal bias.

It would appear as if the XPS has the testing standards stacked in it's favor, but having spent a lot of time in Marine environments, I'm not too surprised by the notion that EPS will outperform XPS in the long run. If you are expecting a wet environment "under there", you had best go with something engineered to your particular needs, like GeoFoam. And you might be looking at 3" to start. The best plan might be to doe everything possible to make sure your insulation and slab isn't going to be exposed to a high water table in the first place.

Whoops. I said "Dana1 quotes one study to support his statement that EPS absorbs more water than XPS."
I should have said "Dana1 quotes one study to support his statement that EPS absorbs LESS water than XPS."

Just what would you insulate it with? What about the floor elevation change? What about utilities connections? What about whatever is connected to the slab?

Just what would you insulate it with?

In an existing pole building, I might go for the GeoFoam (EPS) first, because it is less likely that attention was paid to the surrounding water table. But, I would use a foam with a known compressive strength to avoid unpleasant surprises later. If I was going to work on vehicles in the shop, I would also make sure the slab was suitable by upgrading both the thickness and the reinforcing with either bar or mesh additives. In situations where water is not a problem, I still like the XPS a bit more.

What about the floor elevation change?

Extra physical effort is often required in situations where forethought has not been applied. In this situation, you would need to remove a few inches of dirt or gravel from the interior to make room for the extra slab thickness. Alternatively, you could have your floor stand a few inches higher, sacrificing some overhead.

What about utilities connections?

I like mine underground. They come up through the slab.

What about whatever is connected to the slab?

Like what, for example? Machinery mounts?

Lee- I clearly stated at the outset that the study was a piece of marketing hype by EPS holders to imply that in some regards it might be cherry picking results, but it was an in-situ test of 15 years duration of foundation insulation place along the drip-edge of a flat roofed commercial building with less-than perfect drainage, and comparing 1.6lb XPS to 1lb EPS rather than the 2.3lbs-XPS/1.0lbs-EPS straw man in the cited Dow lit.

By contrast the ASTM D2842 test used by the Dow marketing hype is an immersion test performed over the course of only 96 hours, where it is held under 2" of water for that period, which isn't representative of in-situ installations or durations. While the interstitial spaces between the beads in EPS will indeed be filled over the course of 4 days when fully immersed it's cellular structures are unaffected, and the water leaves those spaces about as rapidly as it enters. The skin of the XPS and the lack of interstitial spaces makes XPS look comparatively good in 96 hour soak, but unstated is the fact that the water absorption into the XPS is within the fine cellular structure, and you'd probably have to bake it to remove that water(!) They're also comparing 1lb EPS to 2.3lb XPS, not the more commonly use 1.5-1.6lbs density goods, which is another fat thumb on the scale. Was there a reason for that? Was Type-II EPS too expensive for the test? Was 1.5lbs XPS too cheap? :-) At more than 2x the density in a short-duration test, surely they were joking. Saturation levels over longer times, and the ability of the two materials to dry after immersions of weeks/months will differ dramatically. Water leaves the interstitial spaces of EPS fairly freely as the tide goes out, but getting the water out of cells within the XPS not so much, but third party testing of these factors isn't easy to find. A 96 hour immersion soak doesn't tell us much of anything about it's longer term performance where the tide may rise & fall or how the material fares in a high humidity possibly intermittently wetted to saturation, but not submerged situation such as most slabs & foundations. Cherry picking or not the EPS-molders' test is more real.

Don't confuse the bead structure of EPS with it's cellular structure. Yes, lowest density EPS is pretty crumbly stuff and easyto damage while handing, but there are much lower fraction of broken cells internally and it will not become waterlogged the way XPS will over long immersions. EPS has a long & successful history of long-term immersion use in floats, whereas XPS isn't (or can't be) used in those same applications. Even at 5% water by volume the R-value of EPS is largely unchanged. In a high-water table situation methinks EPS is still the right way to go.