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How to Pour a Rock-Solid, Well-Insulated Garage Slab

comments (3) December 28th, 2012 in Blogs
ScottG Scott Gibson, contributing writer

But don't worry too much about the strength of the concrete under the car lift, says clewless1. The manufacturer's call for 4 in. of concrete had seemed skimpy to greggo, which is why he had considered placing as much as 7 in. of concrete in that bay.

"Concrete at 3,000 psi distributed across a metal plate say 8x8 gives you a lot of capacity when you think about it," clewless writes. "Also, if in doubt, simply cut away the insulation around your mounting area and thicken the slab ...standard detail for column support in the center of a slab."

Insulating the slab
Using rigid foam insulation around the perimeter of the slab to minimize heat loss seems like an obvious step, but whether to place insulation under the slab is another story. That decision may hinge on whether the slab is heated, but there's no agreement on what building codes require, if anything.

"Are you planning to heat the garage now or eventually?

Then insulating under the slab is not the way to go," writes manhattan42. "The foundation walls should be insulated instead, either outside or inside, from the sill plate to the top of the footing or frost depth whichever comes first. Otherwise you will lose energy through the edge of the slab and any masonry exposed above grade."

Not so, says davidmeiland. "Energy code around here requires R-10 under the slab unless the building is unheated," he says. "You are correct in pointing out that the foundation walls should be insulated as well. Here that would be R-10 inside the walls to a depth of 24 in. or more."

But manhattan42 insists: "Underslab insulation is not required under most energy codes(like the IECC) and in fact is prohibited from being used instead of foundation insulation by such codes," he writes. "Of course, there may be some local variations of these national requirements.

"Generally speaking, however, the only place underslab insulation is required is when the slab itself is heated.

Otherwise the majority of heat loss is through the upper portions of the building, not through the soil or slab, and adding insulation under the slab provides only added cost while offering little or no energy savings benefit at all."

And clewless1's retort? "The slab and ground beneath are unlimited heat sinks that will draw energy out of your heated space. In radiant slabs, the slab temperature is much higher and it makes most sense to insulate because of that. But insulation should benefit either condition...albeit maybe fairly small in the unheated slab condition...but to imply it provides NO benefit and is in fact PROHIBITED by an energy code is IMO incorrect."

An expert opinion
We asked Peter Yost, technical director at GreenBuildingAdvisor, for his thoughts. Here's what he had to say:

First, absolutely nuke the sand layer and go with free-draining 3/4-inch, no-fines gravel as a capillary break.

Second, use XPS insulation. It is non-porous, so it can act as a capillary break as well.

Finally, ground-coupled heat flow is really complex so figuring out the heat loss difference between a shallow frost-protected foundation and sub slab insulation is hard, BUT if any sort of heating is going into the slab, definitely insulate under and around the perimeter of the slab, decoupling the slab entirely from the ground.



posted in: Blogs

Comments (3)

user-4908160 user-4908160 writes:
Not having the insulated slab "sweating" with seasonal changes is enough reason for me.
I always follow Mr. Yosts recommendations when building in the north.
Posted: 6:04 pm on September 1st

BonnieR BonnieR writes: What will that building be doing in twenty years?
It's a heck of a lot easier and cheaper to insulate under the slab now!
Posted: 3:58 pm on October 3rd

clanow clanow writes: I live in florida...the state is a sand-bar........the moisture break is the visqueen or plastic............
Posted: 11:35 am on December 31st

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