Jim,

A few things. Reflective type insulation needs an air space to work, it would be useless under a slab. As for the need for insulation under your slab. What is the slab being used for? House, shop, what? What part of the country are you in? Did you insulate your foundation and if so what is you frost line and how deep did you insulate?

ICF Contractor

In my humble opinion this stuff is one of the biggest scams in the building industry.....Understand that heat is transmitted in several ways, conduction, radiation, and indirectly via convection of a fluid (either liquid or gaseous). This stuff might have a minor role in slowing down radiant heat ransfer IF and only IF installed so that it can do that - ie with an air space on each side of it. Problem is, the air space is a prime candidate for developing convection losses. My advice, forget this stuff even exists - it's virtually worthless and the manufacturers claims are pure fiction. Use good old styrofoam under your slab in the thickness appropriate to your local climate demands. This site has lots of good information on EPS vs XPS. One caution, do not use polyisocyanurate under a slab as it will absorb water which reduces it's insulation value.

Sam Bird, AIA

I would agree with the Birdman.
A layer of 6 mill poly and then foam board are the best method I have found for insulating slabs.

Jim,

When the concrete is poured, the Tekfoil will be 100% compressed and any R-Value will be "squeezed out".  If you need insulation and a vapor barrier, use a vapor retarding plastic and foam. 

Jim

I would recommend at least 3/8" of this: http://www.thebarrier.com/

I agree, a poly and a foam board is the best option. However, I often see disagreements on what type of foam board should be used. There is the perception that the extruded styrene (blue or pink board) is required for under slabs, yet the white board (expanded EPS) is often significantly less expensive for such projects. Other than the slightly better R-value on the extruded board - is there any reason that expanded EPS whiteboard should not be used underslabs? I would appreciate any definitive answer with reference back to an industry or research detail that backs up the answer.

The purpose of underslab insulation is to create a thermal break between the ground and the slab. Foam board insulation does this admirably as long as the board does not absorb water. I don't feel that it takes several inches of foam to create an effective thermal break. One of the advantages of THE BARRIER, in addition to cost, is that it also incorporates a vapor barrier to keep moisture out of the slab.

Heat goes to cold. Insulation interferes with that transfer in direct relation to the R value of the insulation. R value of EPS or XPS is directly related to thickness. The Barrier will work as insulation up to the limits of its R value, so any prodct with 1/2" of EPS (R-2) will insulate to the value of R-2.

Jimbo should add a minimum of 2" of EPS or XPS; 4" would be better since it will insulate better. While THE BARRIER may be a good product (I'm not familiar with it) it should be used in combination with sheets of foam.
Bob

NONE of these foils or thin insulation is worth looking at. Period. As stated above, at least 2" of EPS or XPS and (debatable part) 6 mil poly. My own belief is that a proper taped foam is good enough but some codes call for poly regardless.

Thermal break is not the only reason for insulation

This topic falls right in line with where my current thoughts are directed for the design of my future mountain "cabin".

First, there are different density EPS products for foundations. Some SIP mfgs do make flooring SIPs. IIRC they can be 8'x20'. Can they eliminate the slab and be in contact with the soil?

Second, why cement slab? Since cement has such a high thermal mass, high heat transfer rate, and is spread completely across the bottom of the structure, would it not be bass ackwards? Isn't it the weak point, from a solar collection perspective?

Third, what about radon? Radon will penetrate through a cement slab. The cement must be on top of a plastic barrier with a duct from beneath the slab to the roof, and labeled. You cannot test for radon until the structure is complete.

Y'all realize you're responding to a 3 year old thread, right?

Start a new one if you have a related issue.

Thanks Dana.

As a fairly new member I did not realize how old the thread is. I saw the most recent reply prior to mine and did not look at the other dates.

The hardest part for me regarding starting a new thread is that it takes more work to ask the right question than to understand the answer.

I will start new threads, but not immediately.

Also a new member. Perhaps helpful: See the step by step insulation photos. Under the footing is Formular 400 (pink XPS) with a compression strength of 40 PSI. http://www.newhudsonvalley.com/2012/04/14/footing-forms-in-place-with-rebar/ Under the slab we have 10 inches of white EPS. http://www.newhudsonvalley.com/2012/05/09/installing-the-under-slab-eps-foam-insulation/ Then Neopor rigid foam (EPS with graphite added) surrounds the basement. http://www.newhudsonvalley.com/2012/09/13/attaching-neopor-foam-insulation-to-the-foundation/ It's like a giant coffee cup that will last for the life of this home.

Shaker- I am in the process of laying 6 mil poly and 2" XPS 250 prior to a slab pour as we speak. Poly down first over the rock, returns up the wall 4" and is taped to the ICF forms. I understand the PH concepts in general, but have never understood the massive amounts of sub-slab insulation. Do you have any information on energy savings/reasonings behind 10" of EPS below the slab especially when compared to the more typical r10-15.