How does XPS loose R-value over time?

it outgasses, which causes degradation of its R-value over time. This is true of most closed-cell foams, which is why an "aged R-value" is always what should be referred to.

they still retain some benefit over open cell or EPS, but it's not as great as the initial numbers indicate.

Which type of EPS would ya'll say is the best value for sub-slab application, including where live loads of vehicle exist?

Dana1, In residential construction you are correct but I do a fair amount of pole buildings with no footings that are subject to rodent damage to the sub slab insulation. I will look at using eps in residential applications were I have been using xps. How does it hold up at grade on a foundation wall and what do you recommend for a protective covering...Eric

Various stucco finishes work for protecting the above-grade portion of EPS insulated foundation walls, which is what I did to mine- 24-30" above grade, in my case. (Just like they do with EPS insulated concrete form foundations or CMU/wood-frame insulation retrofit systems such as R-ETRO, etc.) I'm sure Brad can give you the complete down-low on what's out there.

I can't really say how well it holds up over time- I only have a 15 year history on the EPS-insulated foundation of an addition at my house. (So far, so good... )

Brad: Unless your vehicle traffic is a Cat D9 or a 10 yard dump truck or something, type-II (1.5lb) goods will almost surely handle it, and type IX definitely will. If your vehicle really IS a heavy-cruiser you may need to bump up to the Type XIV. If you want to do the math on your actual weight, tire distribution, & slab thickness factors, knock yourself out:

http://www.insulfoam.com/images/stories/docs/6065_Below_SlabInstalGde.pdf

I happened to see some EPS and XPS today and the EPS was more expensive per R value. Probably about the same price if the XPS degrades a little.

I've never seen that. EPS was almost literally half the price per similar thickness for our wall EPS compared to XPS, cost per R probably about 2/3rds.

most places don't stock it, so maybe there is some economic issue there beyond material cost. we were ordering full lifts of the stuff.

Halfway through this thread someone threw out the weight of a 4" concrete slab as being about 4 psi. The nominal density of concrete is around 145 lb/cu.ft, or close to 1 psi. Only 4" of slab would load the foam under it by about one third of 1 psi. A 40 psf live load adds another 0.28 psi, for a total of 0.6 psi. That is well within the 25 psi compressive strength of XPS or 15 psi of lighter EPS.

Ya, that was me too. I divided by 144 instead of 1728. No wonder I got a D in algebra.

I emailed a manufacturer, I work with on ICFs, to see what they have/recommended for EPS under concrete.

I recently bought some 1" thick 4'x16' sheets of Type IX termite resistant EPS foam (Perform Guard) to put under my basement slab (deep soil temperatures here are ~60F). Cost was ~$0.35/sq ft. This compares to Dow Ag Board (1" thick 4'x21.5' sheets of XPS foam) at ~$0.55/sq ft locally. Both have a compressive strength rating of 25 psi.

My understanding is that termites tunnel through foam to avoid light - so they have no reason to be tunneling in foam below grade - they can just go around the foam.

I saw the XPS at $.45 per sqft-in and EPS at $.40.

Termites can tunnel up from the ground below the underslab foam.  Then they have to get through that foam to get to any edges or cracks in the concrete slab.

This is where I got my termite resistant EPS foam:  http://www.noarkrcontrol.com/EPS/termite-resistant.asp

Big Box stores in our area have the 1" thick 4'x8' sheets of blue XPS foam board at ~$19 per sheet.  That's over $0.59/sq ft.

If you need a large quantity your better off going to a real lumber yard or contractor supply and have them quote the project.

EPS is better for pools then?

I have yet to see a "wet" radiant floor. If you have running water EPS will not due. If you have a vapor barrier and the floor is operating, it can not get wet.

All this off-gassing is a lot - maybe not so much - hot air.

XPS is used nearly exclusively here in the midwest, except by those taken in by the "blanket" nonsense.

The EPS sales brochure you reference has nothing to do with a residential radiant floor application. With proper drainage and vapor barrier I see no advantage to EPS.

"With proper drainage and vapor barrier I see no advantage to EPS."

But if there is no disadvatage to EPS, it is cheaper.

Morgan, it's much cheaper and practically the same R-value when the XPS is done offgassing. those are the advantages.

I checked another place and there, NRT.Rob's price figures were right on. 1/2 price for the same thickness.

I seems a bit fragile for tube and bar installation, our friends plastic reinforced "Crete Heat" notwithstanding.

Posted By BadgerBoilerMN on 22 Aug 2010 05:46 PM
EPS is better for pools then?


All this off-gassing is a lot - maybe not so much - hot air.

XPS is used nearly exclusively here in the midwest, except by those taken in by the "blanket" nonsense.

I hate to re-open an old thread but I felt compelled to add a quick point or two for anyone who stumbles across this thread.

First off I applaud Dana1 for his excellent comments on EPS foam charatcteristics and recommend everyone interested in this topic give them a read. He clarified a lot of misconceptions.

EPS foam is indeed closed cell structure which will not absorb water. It does however have interstitial spaces available to absorb water, but they dont do so in frost wall or sub-slab applications.

I recall a study (pretty sure it was by the CMHC) where the authors performed destructive testing on 20 year old foundation assemblies which had been insulated with various types of foam including EPS type II and XTPS type IV foam. They specifically examined in-situ r-value performance as well as resistance to moisture penetration of the foam and examined the installation to determine if moisture was able to penetrate behind the foam which was only held in place by the pressure of the backfilled soil in heavy clay soils with poor drainage.

The results showed that no water penetrated behind either sheathing. They also showed that there was no significant absorption of water by the eps foam and no related degradation in r-values. In sub-slab applications the capillary break from your crushed stone base keeps slabs dry and therefore eliminates any real chance of absorption of water as well.

My firm has 20+ years experience in design and consulting on energy efficient homebuilding. We have designed 100s of heating systems with radiant floor slabs on the ground, and have used both eps and xtps with excellent results. However, more and more our builders are opting to boost sub-slab insulation levels for improved performance (dont forget thermal break at slab edge).  The result is that as soon as you transition into greater r-values EPS wins hands down due to it's superior cost/benefit ratio.

Interest in both the Passiv Haus standard and LEED for Homes is also driving more interest in greater levels of sub-slab insulation and the choice of blowing agents used in foamed plastics. EPS uses pentane which has a greener footprint (lower global warming potential) than xtps foam which use various HCFC based blowing agents.

Since the reality is that xtps off-gasses some refrigerant both during production, and over several years, off-gassing is a valid concern in addition to the long term reduction in r-values. Until such a time that the foamed plastics industry lowers the GWP of all refrigerant based blowing agents to the same levels as pentane or water blown foams off-gassing provides another metric by which you should be making an educated choice.

We recommend EPS.