I read many comments here posted in the past that EPS is as good or better for underslab ins.
I found this:

http://www.homedepot.com/p/CelloFoam-1-1-2-in-x-4-ft-x-8-ft-R5-9-Polystyrene-Foam-Insulation-Board-125755/202090239?N=1z0w1xr

It is cheaper than XPS but there is no compressive rating (psi) that I can see. Here it shows 10-14psi

http://cellofoam.com/BuildingMaterials.html

I am thinking this is not enough for a garage floor where I am getting recommendations for 25-40psi. What about the basement floor where no cars will be parked?

I suppose I should just do the 40psi everywhere...

The compressive strength, density, and R-value/inch @ 75 F of EPS depends on the ASTM 578 type rating. Type I is 10 psi, 0.9 pcf, and 3.85; Type II is 15 psi, 1.35 pcf, and 4.17; Type VIII is 13 psi, 1.15 pcf, and 3.82; and Type IX is 25 psi, 1.8 pcf, and 4.35.

The compressive strength, density, and R-value/inch @ 75 F of XPS depends on the ASTM 578 type rating. Type IV is 25 psi, 1.6 pcf, and 5.0; Type V is 100 psi, 3.0 pcf, and 5.0; Type VII is 40 psi, 1.8 pcf, and 5.0; and Type X is 15 psi, 1.3 pcf, and 5.0.

Please keep in mind that the slab application needs to be properly engineered and the slab application is limited based on the soil compressive strength. In other words, if you have low compressive strength soil, using EPS or XPS with a higher compressive strength than the soil is not going to allow you to achieve the higher compressive strength required by the application. Don't believe everything you read on the Internet. XPS is typically preferable when there are moisture/water issues that must be considered and addressed.

using EPS or XPS with a higher compressive strength than the soil is not going to allow you to achieve the higher compressive strength required by the application.

Actually, with thick enough foam, you can build on soils with zero compression strength.

EPS does as well or better than XPS underground.

Posted By sailawayrb on 28 Sep 2013 12:39 PM
The compressive strength, density, and R-value/inch @ 75 F of EPS depends on the ASTM 578 type rating. Type I is 10 psi, 0.9 pcf, and 3.85; Type II is 15 psi, 1.35 pcf, and 4.17; Type VIII is 13 psi, 1.15 pcf, and 3.82; and Type IX is 25 psi, 1.8 pcf, and 4.35.

The compressive strength, density, and R-value/inch @ 75 F of XPS depends on the ASTM 578 type rating. Type IV is 25 psi, 1.6 pcf, and 5.0; Type V is 100 psi, 3.0 pcf, and 5.0; Type VII is 40 psi, 1.8 pcf, and 5.0; and Type X is 15 psi, 1.3 pcf, and 5.0.

Please keep in mind that the slab application needs to be properly engineered and the slab application is limited based on the soil compressive strength. In other words, if you have low compressive strength soil, using EPS or XPS with a higher compressive strength than the soil is not going to allow you to achieve the higher compressive strength required by the application. Don't believe everything you read on the Internet. XPS is typically preferable when there are moisture/water issues that must be considered and addressed.

There is also Type XIV @ 40psi and Type XV @ 60psi (ASTM C578) EPS.

I will have to disagree with you on the soil and compressive strength comment you made. I've spoken to engineers and have seen applications myself where poor soil strength loads were overcome by putting in EPS with high psi loads. My friend who is a civil engineer has used EPS in road applications and it was applied as part of the road base substrate due to poor soils.

Engineers sign off on builds with poor soil loading conditions by using EPS to compensate for the poor soils. Good soil has around 3,000 psf, which is about 20 psi. EPS rated at 40psi would double that load bearing potential.

An 8x4 sheet of Type XIV / 40psi EPS runs around $20 for a 2" thick sheet. Not super cheap but not that outrageous either, especially if you need 40psi bearing capabilities, that is the answer. Most homes can get away with 15 or 25 psi

It is true that you can spread a given load over a larger structural area to reduce the effective psi that the structure applies to the soil. This is why multi-story buildings have wider footings than single-story buildings. However, for a given area (e.g., a standard size garage) and a given load (e.g., the weight of a couple vehicles, shop equipment, and the slab), you will end up with a given psi (i.e., the total load divided by the total area). If this given garage slab psi exceeds the soil psi, just making the EPS/XPS thicker or placing EPS/XPS with a higher psi load rating than the soil does NOT solve this problem. There are only three solutions that will address this problem: 1) reduce the slab load so as to reduce the effective psi that the loaded garage slab applies to the soil, 2) make the garage area larger so as to reduce the effective psi that the loaded garage slab applies to the soil, or 3) increase the effective psi of the soil before building the garage (e.g., add piers/piles). The aforementioned is true because concrete structures (e.g., concrete reinforced slabs) will readily share the loads and evenly distribute them across the structural area in contact the soil. The addition of EPS/XPS does NOT improve the ability of a concrete reinforced slab structure to share these loads and evenly distribute them to the soil.

With regard to road construction, many of the typical materials used for road construction (e.g., asphalt) will NOT share loads and evenly distribute them to the soil very well. If the soil is poor where the road is being constructed, an additional solution to the aforementioned garage example solutions is to construct a road substrate that will provide the required additional structural support to enable the road substrate to share the loads and evenly distribute them across a larger effective road area in contact with the soil. So in this case, the addition of EPS/XPS may improve the overall ability of the road substrate to share these loads and evenly distribute them to the soil. However, like the aforementioned garage example, once the road load divided by the effective road area exceeds the soil psi, you can only solve this problem by using the aforementioned garage example solutions.

I have provided expert testimony against contractors/engineers who have followed this practice with the result being failed foundations and badly cracked slabs. You really don't want to be on the wrong side of this argument in court.

Posted By sailawayrb on 28 Sep 2013 11:19 PM

I have provided expert testimony against contractors/engineers who have followed this practice with the result being failed foundations and badly cracked slabs. You really don't want to be on the wrong side of this argument in court.

I guess there is disagreement on the topic. We will have to agree to disagree as it appears there are two school of thoughts on this topic.

I am not trying to denegrate your qualificaions, so please don't take this the wrong way. As you know, "expert testimony" is given by both defense and plaintiff, each side has their own expert witness. Yet both expert witnesses give completely opposing testimonies. That is the dichotomy of proving ones point.

EPS is used in SIPs and the strength of EPS is quite evident in producing panels that can take some insane loads.

EPS FOAM CASE STUDIES

Calling those case studies is at best generous, I would expect anything called a case study to be much more rigorous, those are examples intended as advertising copy. This is not intended in anyway to disparage the company or product, I'm sure their engineer could show the data required to support inclusion of their product in each of these examples. Now as to expert opinions: I fire the people who cannot show the rigor necessary to support those opinions, if a person can not, or will not demonstrate this rigor then what they have to say is meaningless noise. Enough said.

The issue of whether a foundation/slab is going to be fully supported by a given psi soil or settle into a given psi soil has nothing to do with the thickness/strength of the foundation/slab...i.e., once the foundation/slab is of sufficient strength to share the building design loads and evenly distribute them to the ground. Bottom line, once the loads divided by the effective structure distribution area exceed the soil psi, settling will occur even if the foundation/slab is infinitely strong...and uneven settling caused by non-uniform soil psi within the building zone will create sufficiently large reactive loads that can cause reinforced concrete foundation/slab failure.

So lets perform a simple experiment that I have found will convince even those folks who are not comfortable with physics/math. Fill a box with several inches of vermiculite which will represent our poor soil. Get some assorted weights (e.g., perhaps from your weight lifting setup) which will represent our building design load. Cut out one, 1 foot square sheet of paper. Cut out two, 1 foot square pieces of corrugated cardboard. The sheet of paper will represent the case where the initial structure has insufficient strength to share the building design loads and evenly distribute them to the ground (e.g., our aforementioned road construction example). The corrugated cardboard will represent the case where the structure does have sufficient strength to share the building design loads and evenly distribute them to ground (e.g., our aforementioned reinforced concrete garage slab example).

Place the paper on the surface of vermiculite and place the lightest weight that you have on the surface of the paper. You will observe that paper deforms badly and the area under even this light weight settles badly into the vermiculite. This occurs because the structure has insufficient strength to share the load and evenly distribute it to the vermiculite. This is why civil engineers first need to strengthen the road substrate when building roads across poor soil.

Now place the corrugated cardboard on the surface of vermiculite and again place the lightest weight that you have on the surface of the corrugated cardboard. You will observe that corrugated cardboard does NOT deform and that it does NOT settle into the vermiculite. The corrugated cardboard did NOT deform because it has sufficient strength to share the load and evenly distribute it to vermiculite. The corrugated cardboard did NOT settle into the vermiculite because the total applied weight divided by 1 square foot did NOT exceed the psi rating of the vermiculite.

Now keep adding weights until you observe that the corrugated cardboard finally does settle into the vermiculite. The corrugated cardboard finally settles into the vermiculite because the total applied weight divided by 1 square foot finally does exceed the psi rating of the vermiculite.

Now remove these weights, double up on the corrugated cardboard, and then place these same weights back on this thicker/stronger corrugated cardboard structure. You will observe that this thicker/stronger structure settles exactly the same amount into the vermiculite as it did previously. This occurs because the total applied weight divided by 1 square foot exceeded the psi rating of the vermiculite by the exactly the same amount as previously. Making the structure thicker or stronger does NOT do anything magical to alter the basic physics/math of this reality.

The article you referenced correctly states that you can install foam under footings if the foam has a sufficiently high psi to support the building load. The article does not say anything about foam having the magical ability to allow one to build on soil that does NOT have a sufficiently high psi to support the building load as you and Jonr appear to be suggesting. The article does not appear to provide any new engineering information that a competent contractor/engineer would not already know and already be adhering to...namely that BOTH the foam and the soil each have to be of a sufficiently high psi in order to be capable of supporting the building load. Sorry, but I am not understanding the point that you are trying to make with this article?

Flotation is the principal behind being able to build on soils that can't support the weight on a psi basis. Good engineers don't consider it magic.

Yes, we have very nice house boats and floating bridges where I live. One could even excavate the soil within the entire footprint that a structure will be constructed, fill this excavation with EPS blocks, construct a reinforced concrete slab on top of the EPS blocks, and then construct the structure on top of the slab. If the weight of soil excavated is such that it is equal to the total weight of the EPS, slab, and structure, the foundation created by this approach is called fully compensating and the structure will indeed float. This approach is commonly used in civil engineering construction projects to address very poor soil conditions as previously described. The amount of EPS block required to accomplish this feat is staggering...just for a relatively light road project.

This approach would also have to address the issue of dealing with variable soil density over time. For example, if the soil gets saturated with water, this floating foundation and structure will float to a different height. Furthermore, the structure would have to generate symmetrical loading so as to place the center of gravity at the centroid of the footprint to avoid having the structure tilt over with time.

I am not aware of anyone using this approach for residential or commercial building construction. I am also not aware of any building code or engineering guidance that would legally allow us to take ANY credit for using foam to allow us to exceed the soil psi requirement. I have seen first-hand what happens to contractors/engineers who did exceed the soil psi requirement and then received court assistance to remedy the problems they created.

Posted By sailawayrb on 29 Sep 2013 09:56 AM
...snip...
So lets perform a simple experiment that I have found will convince even those folks who are not comfortable with physics/math. Fill a box ...

I would call that a clear and persuasive argument!