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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?

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    If going to all the work to insulate why not put some poly line for hydronic heating, if you work in your garage in the winter it is really nice to have a warm floor. I did that with 1 bay in my last home and it almost kept the shop warm if I would have done all 3 I would not have needed the heater. But with hydronic heat it needs to be on for a long time so I used to keep mine at 50 and turn it up to 70 while I was in the shop. – Ed Beal May 30 at 22:03
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    I forgot to mention i'm going for this, good call – Jeffyx May 31 at 12:09
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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.

  • That geofoam is amazing stuff. Also thanks for showing me the math! – Jeffyx May 31 at 12:12
  • You don't need to do any hard math to turn an automobile's weight or distribution into earth-crush PSI... just stick a $5 tire gauge on it and measure the PSI of your tires. Yeah, it really is that simple. – Harper Jun 1 at 6:14
  • @Harper Assuming that you start with atmospheric pressure in the tires or what? – Alex Jun 1 at 16:31
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    @Harper That would give you a maximum possible value in that case. A lightweight racing bicycle with 110psi tires would obviously not give you 110 psi earth crushing power, and it would roll over the styrofoam just fine. – Alex Jun 2 at 0:27
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    @RossMillikan Real tires, in contrast to theoreticall ones, have rubber between the compressed air and the ground. Rubber is soft and deforms on its own, even if the airpocket inside would have been filled with hard solid steel. The contact area is not linear with the internal pressure when you get up to high pressures. – Alex Jun 2 at 7:39
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Foam insulation is designed with a loading value per unit area and as long as that loading is not exceeded it will not deform.

When we designed the insulation for the base of our property we specified an 8” thickness of a particular grade of foam board. The builder ordered a different board and we had to check to see if it was sufficiently strong - luckily it was...

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XPS and EPS rigid insulation is used under concrete slabs.

They both come in various rated compressive strengths from 10 psi to about 80 psi.

Say you use an average (and relatively inexpensive) rating of 40 psi, then the insulation will support: 40 psi x 8” wide tire x 12” long tire print = 3,840 lbs. at each tire. (Total load supported would be: 3,840 lbs. x 4 tires = 15,360 lbs. )

Actually, the maximum load would be significantly more, because the load would be transferred through the slab at a 45 degree angle. So, the actual “footprint “ on the insulation would be about double the area...thus double the allowable load.

Here’s a site that explains the use of rigid insulation under slabs. https://www.concreteconstruction.net/how-to/site-prep/choosing-between-eps-and-xps-rigid-insulation_o

There is some logic to not using insulation, unless the garage is being heated.

BTW, I don’t recommend the use of wire mesh in garage slabs. They make them crack.

  • I worked with concrete for years as a foreman, I would tell customers two guarantees about concrete: 1) It will harden 2) It will crack. You add control joints in concrete to control where it will crack. If you don't, it will crack along the line of least resistance, which will be along any rebar or along any wire mesh. Adding of that "BTW" weakens this answer. – Keeta May 31 at 12:01
  • I'm guessing you're used to working with heavy trucks if you think an 8"x12" contact patch is what springs to mind! – FreeMan May 31 at 13:22
  • @Keeta Yes, control joints are a must, but wire mesh “creates” cracks because it’s too small and gets pushed to the bottom of the slab and is allowed to extend through the control joints. Rebar in slabs is fine if installed in the middle of the slab. – Lee Sam May 31 at 14:45

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