Do I need to use "Highload" slab insulation?

[ScaldedDog]

I'm about to have the final grade done in our 32x34 garage with a floating slab. The slab will be 6" thick, with in in-ground lift in the (more or less) middle, and hydronic radiant heat. I'll routinely park an 8000lb Excursion on it as well as other 5000lb vehicles.

So, do I need the Dow Highload 40, 60 or 100, or is the 25psi stuff from Dow or Owens Corning good enough? This whole "styrofoam"-under-a-concrete-slab thing seems so wrong to me... I'll happily do whatever is right.

Thanks,

Mark

 

[Junkman]

I have 2" blue Dow insulation under the garage slab that was placed there in 1982. It hasn't failed me yet, that I know of. I believe that the higher rated insulation is actually for commercial use where heavy machinery is going to be installed. By heavy, I mean 10+ tons. You might want to call Dow customer service and see what they recommend. Your 8000 pound excursion is only exerting 2000 pound over the foot print of the tire, so 2500 PSI concrete would be fine for that alone. Don't get too worked up over ratings, since it is actually the proper instillation that is actually the important part. The highest grade of insulation or concrete if improperly installed is wasted money.

 

[5wndwcpe]

Seems to me the compaction of your sub grade is more critical. Even if the EPS did compress some, it should compress at the same rate everywhere. If your sub grade develops a low spot due to poor preparation, no foam is going to support the slab regardless of it's rating.

 

[ScaldedDog]

So I did a little math tonight:

Concrete floor - 3900lbs per cubic yard, which translates to .083lbs per cubic inch, or .5lb PSI for a 6" floor. Pretty much nothing.

Ford Excursion - Contact patch is roughly 54 square inches, less open spaces in the tread, which I'm ignoring. That area supports 2000lbs. (It's an 8000lb truck, though probably somewhat front heavy.) That's 37lbs PSI.

I need a little more convincing that 25psi EPS/XPS is the right load rating for this under slab application. Is my math right and, if so, is this a problem? I suppose that weight gets dissipated by the concrete, but I have no way of measuring or estimating that.

Your thoughts?

Mark

 

[rburke65]

The 25lbs. PSI.....per square inch. If you take a square foot or 144 square inches this would be 25 X 144= 3600 lbs. per square foot. Times this by 4 ( for your 4 tires ) and ths is equal to 14,400 lbs. Your truck doesn't weigh that much. As always, I could be wrong....My wife tells me so everyday!