What's your soil? there's no need to have more PSI in your foam than your soil. Sedimentary rock is 6000 pounds per square foot - roughly 41.3 PSI - so if you have lesser soil than that....

That being said - we used 40 PSI foam under both our footings and our slab.

Hi Just_janni,

our soil is clay, need to do some research to find the density.

so the base is clay, 4" of coarse granular fill, insulation board and 3" concrete floor.

Thanks,

I thought I had to be looking at the concrete floor weight and not the base. Thanks.

If I follow just_janni explanation, a 16 psi should be sufficient and a 25 psi ( usually used on walls) is more than plenty. So, if this is the case, why do the manufacter has the highload products (40 and even 60 psi).?

There is a major cost diference between lower to higher PSI ( ~100%).

Does it mean I could use a 25 PSI without any concern?.

Thanks

So, if this is the case, why do the manufacter has the highload products (40 and even 60 psi).?

Airport runways, industrial buildings.

You can also consider high-density EPS or recycled XPS.

mqcola, I put Owen Corning foamular 250 25 Psi XPS under my full 4" thick slab in my garage which I built 15 years ago and have had no problems.

We've parked two cars, some times a car and full side pick up and at times I'd park my farm tractors in it, I didn't use rebar or the wire re-mesh in the concrete and used only reinforcing fiber mixed in by the concrete company, so I didn't go hog wild on reinforcement to spread out the load of the concrete just so you know. There is no clay or rock under my slab, just sand, and sugar sand and it's not a floating structure, the building has footings down 42" and everything was inspected by the city. (Not a DIY hack job.)

Lucky you, I wish I had the stuff under my basement slab. You'll have no problem going with the 25 psi XPS and is whats commonly used by most people. I just checked Home depot and Menards prices and both show the foamular 250 (25 PSI) is cheaper then the foamular 150 (15 Psi) for some reason, so it's a no brainer which one I'd use, the 25 psi.

The high PSI products are used when big load are involved. If I was going to build a heated garage for a semi truck or ran bigger tractors I'd need the denser foam to support the load without crushing.

I don't get the soil type as a factor but I'm not a structural engineer, but I do know I could take a sheet of high density foam and throw over quick sand and walk across it, but using a very light density foam I'd crush and break through from my weight and I'd sink. I've seen bridge builders use huge blocks of high density Expanded polystyrene foam (EPS) under bridge wing walls and piers when they can't pile drive pipe to rock to support the structures, so again I don't get the soil type as a factor. It's about the load above and spreading the load.

the soil discussion is simply one that you don't need a higher PSI rating on the foam than you have in the soil underneath it. We used the high PSI foam UNDER the footings, and we had compacted the soil, so we went a little overboard and used the same foam under everything

Under the footing I can understand.

Hello all, thanks for all comments. After reading your experiences and recommendations we are going with 2" DOW 25 psi. Underslab and 1" for walls ( before batten insulation).

Thanks.

Still another dilema: should I install the 1" foam before concrete floor is poured or should install 1" board after floor is poured... I guess topic for another post. :) :)

Thanks again..

Hi Dean Wilson, what do you mean by foam? DOW is a xps board. And which cases foam would be better?

Underslab insulation is not as much an energy saver as a comfort creator. By keeping the slab warm, it prevents moisture condensation during the summer.

You can create the same effect by placing the foamboards above the concrete. But then you'll have to add layers of plywood as a surface for flooring. Much greater cost and a loss of height. And if you want to add a hard surface flooring such as tile or stone, even more prep work is needed.

Hi worthy, this is the goal in having the underslab insulation, to have a comfort level as the house is a bungalow and we will be using the basement as a living space.

thanks you all. now is time to get the product secured before it increases in price again... :) :)