I'm in Alaska and we are will be building on a shallow frost protected monoslab, it's on good, quick draining gravel, what's the best insulation for beneath the slab?  XPS like blue board?  Or EPS?  Insulfoam has a factory here and they have an R-tech for underslab and, if I'm reading their info right, a 40 psi and 60 psi EPS for under slab.  They claim that eps will take up water faster, but it also gives up water faster.  They pointed out a test where eps under a slab for 15 years was 4% water and the XPS was 18.9.  

But i don't know, blue board has sure been the old tried and true method.  (i think)

So what say ye?

Either will do you fine, but I believe XPS is a better choice as it absorbs less water and is denser.

Density counts too. Most XPS sold is in the 2lbs/ft^3 range, most EPS sold is around 1lbs/ft^3, but is also available in higher densities. (IIRC most insulated concrete forms use 2lb EPS or higher.) The higher density means a much higher percentage of closed cells, and much lower interstitial space between the beads, which adds up to lower water absorption as well as higher compressive strength.

On good draining soil and building and perimeter drain EPS should be fine. Under a slab you probably won't want to use anything less than 2lb stuff, which is probably about where the 60psi stuff weighs in at. The 40psi stuff is probably 1.5lb EPS- substantially more rugged than the stuff that goes into roofing, but unless you know the floor loads will always be light (in say, a gymnasium or hockey rink) going with the denser stuff is a safer bet. If you'll be parking loaded trucks, machinery, water tanks, etc on the slab, you'll definitely be wanting for the denser goods. If the slab loads are truly heavy duty you should be looking at 3lb XPS (I don't think anybody makes 3lb EPS.)

Insulfoam claims to make a 3 pound EPS, I wonder if it really is because it says "nominal".  They claim 60 psi compressive strength with it.

"...InsulFoam XV is a high-performance insulation consisting of a superior closed-cell, lightweight and resilient expanded polystyrene
(EPS). InsulFoam XV meets or exceeds the requirements
of ASTM C578, Type XV, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. InsulFoam XV has a nominal density of 3.0 lb/ft3. In addition, InsulFoam XV offers a long-term stable R-Value and has excellent dimensional
stability, compressive strength and water resistance properties...."

Thanks for the replies

The 2 lb. is all you need unless you are building a skyscraper.

Talk to the company that makes the foam.  I have been told by foam manufacturers that 1.5 pounds per cubic foot EPS is more than strong enough for a residential slab.  The structural engineer working with us for a concrete roof on top of 9" of EPS also says that 1.5#/cu.ft wil support the weight.  Do be aware that the lower densities will deform up to 10% but no more if the weight is within the range.

I'd defer to someone with arctic zone construction experience with slab on grade-  what DO the local Insulfoam folks say?

I'm thinkin' that in AK there will be seasonal & uneven frost-heave pressures under the slabs that are far greater than what most of us in the lower 48 will ever experience.  (This is the land of permafrost subsoil, remember...)  In my neighborhood simply insulating the slab edges to couple it to earth temp works, but I doubt that's the case in AK.  If in doubt stick with the higher density stuff, since fixing it later would be a disaster, and the difference in upfront cost per square foot isn't likely to be huge.

Cood be rong, offen am...

Densities for foam insulations are all "nominal", since tight precision is neither easy nor necessary.  If a 10% difference in density makes it or breaks it, it's WAY under-designed in the first place.

2# density XPS and EPS are about the same compression rating-- about 20psi. That's 2880 pounds per square foot!
1# EPS is rated at about 10psi. That is still 1440 pounds per square foot. As long as there is rebar in the slab, even 1# is usable for most residential
applications. I would use XPS since it is commonly used in this application and there is no question about its use if there was ever a lawsuit!
Life in the 21st century.

Oh, about water migration into the foam, XPS is a lot less likely to pick up water. Both XPS and EPS will pick up water. EPS will pick it up quicker and discharge it quicker, but XPS will lose a lot less insulation value if it becomes super-saturated. Most foams can pick up to 10x their original weight in water. They can be so heavy that they will break under the load in handling.
Use a vapor barrier under the foam and set up the site with good drainage and the moisture should never be an issue.
Site prep trumps everything else.

XPS or EPS? I would say that there are defiantly some advantages to both, and that both will work fine. EPS is made in different densities (type 1-5) and will work for any application (type 1-2 for residential construction). One thing to consider about XPS is that it is subject to thermal drift (Off gassing) where EPS is inert. I would personally use the EPS because it is cheeper and will work fine for the residential application, even in Alaska.

You won't have a water problem with either one if you put down a vapor barrier under the foam. I like the 2lbs geofoam eps for underslab insulation. How thick should it be in Alaska? What does code dictate for underslab insulation? The 2lbs density geofoam has the same R-value as xps about 5/in.

Tom Pittsley
ecobuilder@aol.com
www.eebt.org

I ended up using XPS under the interior part of the slab, I think it was something lijke 25 psi and I used 60 psi EPS under thickened edge part of the slab.  I used the xps around the vertical edge of the slab and a mixture of it for "wing" insulation.  (Stuff I had left over)  heh

 

And the ICF walls are up....just waiting for the monolithic dome roof now.

 

 

Oh ya, and I used a vapor barrier underneath it.

GreginAlaska,

Be sure to post a picture of the monolithic dome.