concreteinsulation
I plan on making a garage out of a large carport, and I've watched and I understood how to properly insulate a new concrete slab. 4 inches of gravel, vapor barrier, 2 inch foam board, 2 inches of sand and then 4-6 inches of poured concrete with mesh rebar in it.
My question is, how in the world will 2 inch foam board support all the concrete? Especially when I park both my trucks on top of it! Won't it crush in some parts and be uneven underground. Why does it make a good base?
Yes, Rigid foam (XPS and ISO) boards are frequently used in exterior applications, and can be a great option for improving efficiency. There are a couple of issues with your proposed approach, though:
On a wood frame house, you'd want to use this order:
Here's a visual from Fine Homebuilding, which has a couple of good articles on this:
Note that adding foam on the exterior of your house complicates some things. You will likely need to redo the trim on your windows and doors, which will become more in-set. These inset areas will need to be flashed properly or you will have leaks, which have more potential to do damage when you've got multi-layered, thick walls that could trap moisture. If done well, you can get very good insulation. But make sure to work with someone who knows what they're doing!
Quoting from "Common Questions and Answers about Frost-Protected Shallow Foundations" on page 29 of the Federal HUD guide on constructing frost proof shallow foundations dated June, 1994:
Question No. 1: How does insulation stop frost heave from occurring?
Frost heave can only occur when all of the following three conditions are preset:
1) the soil is frost susceptible (large silt fraction),
2) sufficient moisture is available (soil is above approximately 80 percent saturation), and
3) sub-freezing temperatures are penetrating the soil. Removing one of these factors will negate the possibility of fromst damage.
Insulation as required in this design guide will prevent underlying soil from freezing (an inch of polystyrene insulation, R4.5, has an equivalent R-Value of about 4 feet of soil on average). The use of insulation is particularly effective on a building foundation for several reasons. First, heat loss is minimized while storing and directing heat into the foundation soil -- not out through the vertical face of the foundation wall. Second, horizontal insulation projecting outward will shed moisture away from the foundation further minimizing the risk of frost damage. Finally, because of the insulation, the frost line will rise as it approaches the foundation.
Since frost heave forces act perpendicular to the frost line, heave forces, if present, will act in a horizontal direction and not upwards.
So you are OK as long as your non-susceptible fill does not contain much silt.
Best Answer
By way of example, the pink Owens-Corning FOAMULAR 250 product and the blue DOW STYROFOAM Brand SM product rated for 25 and 30 psi compressive strength respectively. If weight is distributed and applied evenly a square foot of FOAMULAR 250 could support 25*12*12=3600 pounds at its limit while the STYROFOAM could support 4320 pounds. A footnote in the data sheet qualifies this as "at yield or 10% deflection, whichever occurs first."
The weight of concrete varies but 150 pounds per cubic foot is a common approximation. Then a square foot of concrete, half a foot thick, should weigh about 75 pounds. That leaves at least 3500 pounds of weight carrying capacity before the foam below the concrete would fail.
The weight of a vehicle sits on the contact patch of its tires. If we consider a hypothetical 8000 pound pickup truck with 70% of its weight on the front tires, that's 2800 pounds per tire. (Maybe a 70/30 distribution of weight is extreme; I don't know.)
The tire contact patch is much smaller than the square foot we've been thinking about, so it would seem that the load might be too concentrated and would crush the foam. Indeed if the tire rolled directly over the foam it likely would be crushed. Fortunately the reinforced concrete does a pretty good job of spreading the load. The weight on that tire contact patch is spread over a larger area of foam and it all works out.
On a related note, enormous foam blocks can be used instead of soil as fill in construction projects like bridge abutments, roadways, etc. See Geofoam.