What is the proper way to build a Slab on Grade Foundation?
Building in WY area Zone 4b. I would like to build a Slab on Grade foundation. Reinforced concrete perimeter footings approximately 12" x 40" deep, and a 4" thick concrete slab on top of 2" rigid insulation. This will be a monolithic pour. The top of slab will be approx 12" above the soil. I plan to isulate the outside face of the perimeter footings with 2" rigid foam board.
Does all this sound typical? When I discuss with local concrete contractors, they seem confused of this "unusual" appraoch. The norm seems to be "stem walls" with a basement. Am I missing something here? Can you please confirm I am on the right track with my SOG approach. I have done much concrete work in the Western warmer areas and a SOG is the norm.
Slab on grade foudations in cold climates are unusual, but they are very safe and a great way to build. A lot of builders have just never worked on one and so they don't quite get it. we are so used to building frost walls or stem walls to get below the frost line that we seem to have forgotten why we do that, which is simply using 'dirt' as insulation. A frost protected shallow foundation uses 'insulation' as insulation instead, read more here on our slab guide page -
How to build a slab on grade foundation in a cold climate and prevent frost heave
Wyoming is a cold climate for sure, but there are much colder climates with slabs that never had a problem. what is missing from the equation in what you wrote above is a horizonal skirt insulation surrounding the perimeter. There is a diagram on the page link above showing a perimeter skirt for reference.
I cannot advise you on how much skirting is needed to prevent your foundation from heaving in your specific climate, so I would recommend having that determined by an engineer, but there is no reason whatsover that makes a stem wall 'safer' than an insuation skirt.
Whether you insulate with rigid foam insulation or with dirt, it needs to be planned for your climate. A slab may heave if it doesn't have sufficient skirt insulation, and a stem wall may fail if it isn't deep enough (as in it doesn't have enough dirt insulation).
There is at least one firm in our professionals directory that engineers slab on grade foundations all across the US and you may be able to find others. Having a firm that specializes in slabs design it for you takes all the guess work out of it, and worry, as it comes with an engineers stamp which is effectively a guarantee for life.
Excellent, thank you. Yes, I would plan to insulate the outside face of the perimeter footing from the top of soil to the bottom of the footing, about 40" in this case. Plan to use something recommended like a 2" rigid foam. I believe this would be considered a skirt.
Glad to hear you've got a skirt planned for it John. Be sure to slope it away from the house, even just a 2% grade is enough to be sure water drains away and not underneath.
For added long-term security its smart to lay something like a geotextile or tyvek on top in case someone tries to plant something close to the house long into the future. That will prevent a shovel from going through it and someone pulling it out thinking its just lefteover waste.
Just so we are on the same page, here is a 2-foot wide 2-inch deep perimieter skirt insulation. The red mesh is to do the job I mentioned about the tyvek to protect it from getting damaged or dug up.