ICF House - 3500 sq feet - Cleveland, OH - R 38 cellulose in attic - no cans in second floor ceiling. The builder put a product called "slab shield" from www.low-e.com under the slab. We did this in part because it is also a radon barrier. However the listed R value is only 2.9. The basement is about 9 feet maybe slightly more. So I was thinking of placing rigid foam on top of the basement concrete floor followed by OSB followed by whatever flooring I decide to go with. Wish I'd put rigid foam under the slab but too late now and thinking I might be able to do fine as above anyway. Will this work well?

Are you planning on radiant floor in your finished basement?

If not, your heat loss out the basement slab will already be pretty low with ~R3 under the slab. (I looked it up- it's closed cell foam and tested to ASTM 518, not some crummy bubble pack tested in a totally inappropriate manner like some of them. The foil in the center lamination basically the vapor & radon barrier and not pretending to be much else.)

It may not be cost-effective or necessary to add an inch or two of XPS to bring that up to R8-R13, but it won't hurt, and it would allow you to lay down even thickest plushest carpets with out fear of the bottom of the carpetor sub-floor reaching mold-enhancing relative humidiities. If you're planning on wood/laminate/etc, the low R-value of flooring itself are low enough that with a 60%RH room and a foil-sealed R3 slab you won't reach 70% RH on the sub flooring. If you're headed for for Net Zero Energy or something, go ahead, but from a total heat-loss POV you may be better off spending the money elsewhere.

It'll cut your heat load some, but it'll raise your cooling load some- might be a wash. I'd probably only go there if the long term plan was to super-insulate the place at some point. DEFINITELY go there if you're planning a radiant floor, where the flloor will be more than 30F warmer than the sub-soil for months on end. But a 60F floor with R3 between it and 55F subsoil isn't a huge heat loss. If I did the math right in my head this late in the aftenoon, it's ~1700btu/h for a 1000 square foot basement. If you keep it a very toasty 70-72F down there, the floor might hit 65F, doubling that heat loss. An inch of XPS would cut it by more than half. That's a very different payback scenario than if heating the slab up to 75F or more as a means of heating the room.

Thanks Dana1 - very helpful info.

I do not have radiant floor and wont be getting it. We keep the basement about 66F but it is about 1800 sq foot down there.

Decreasing the heating but increasing the cooling might be good for me as we have geothermal and I would prefer to have a balanced heat/cool load so that the ground temp on the vertical loops does not drift over the years.

As far as the payback - I priced out the EPS board and it is 26 bucks a board for one inch thick stuff that is tough enough to walk on (with OSB on top).  It would take about 20 boards to cover the basement - so about 500 or 600 dollars. 

An inch of EPS with nailer decking is reasonable here. It's probably good for ~R4.5 which will cut the heat loss by more than half.

It'll be slightly cheaper in materials (but slightly more labor) to go with 3/4" XPS between widely spaced 1x sleepers for nailing 1/2", or alternatively, 1" XPS applied with adhesive (no sleepers) + a few through-screws per sheet of OSB into the concrete For a similar or higher R value. XPS will likely have a somewhat higher compressive load rating, but that shouldn't be much of an issue either way unless you're storing pallets of bricks or parking your bulldozer down there.

IIRC, in order to meet fire codes the decking has to be 1/2" or greater. This would be true for thecomposite EPS sheets too. Stuff sold as roofing insulation with pre-applied nailer deck may be on the thin side (or not, but pay attention or you may have to do an over-layer to meet the thermal barrier requirements.)

Check out this link:

The product provides R18 insulation and typically replaces a concrete foundation floor, but could also be used on top of an existing slab. There are attachment point @12" o/c for your T7G plywood sheeting.

Feel free to PM me if you would like more information.