I am building a 30' x 30' garage on a concrete slab with an insulated floor with pex piping for future use of geothermal heating.The slab will have 10" thick (20" wide edges and 5 " slab in the middle)I want to use 2" rigid foam.Can I install the foam under the complete slab including the footing area and then insulate up the outer edges of the 10" sides to keep the heat into the floors.Will the 2" foam support the load of the building.Also , can I insulate the entire area on the flat compacted gravel and then put gravel on the top of the centre area where it will fo from 10" on the sides to 5" in the middle.This means the insulation will be under the gravel in the centre of the pad.This would save me trying to slope the rifid insulation to the edge of the footing area.The slag is being build just above bedrock and already has a compacted fravel surface to build on.Thanks for any help. Dennis Rennie

This is a very small load for Geo.

You need a heat load, and available fuel comparison to get started.

Insulation depends on climate, inside and outside design temperatures and dynamic loads i.e. intended purpose. You may need to consider indoor air quality, humidity for man and machine, makeup air for hoods etc.

If it is just a heated shed, you are trying too hard.

Have your structural engineer or architect work out the loading on the slab. There are DOW and Foamular XPS foams availible from 40psi to 100psi ratings depending on the load and safety factor required.

Be forewarned that the higher compression foams get pretty expensive. We have 4" of 60psi foam under our slab, and there was definitly some sticker shock.

XPS foam or Type IX EPS foam have compressive strength ratings of 25 psi. That's 3600 pounds per square foot. That's equal to or greater than the load bearing capability of many soils.

As far as costs go, I recently purchased some Type IX EPS foam sheets with termite inhibitor, 1" x 4' x 16', for $0.35 per sq ft. Figure on 2" thick sheets would costing approximately twice that per sq ft.

Posted By arkie6 on 20 Aug 2010 03:49 PM
XPS foam or Type IX EPS foam have compressive strength ratings of 25 psi. That's 3600 pounds per square foot. That's equal to or greater than the load bearing capability of many soils.

Correct, but that is the ultimate load at either 5% or 10% deflection. Once you factor for 3:1 static or 5:1 for dynamic load such as DOW recommends, your now down in the range of 700-1200 PSF.

Talk to a structural engineer for loadings, or a geotechnical engineer for any underslab fill materials.

Yes, you can keep the foam flat (ie, put it under the center fill). Unlikely that the foam won't be strong enough on a thickened edge slab, but it never hurts to check.