I am building a 78' x 38' (garage/shop) building in Maine. 14' x 38' will be an apartment. I have a 4' frost wall down to a footing, and all but a bout 6" will be backfilled. My plans are to put 2" of Dow type insulation on the exterior of the poured walls and at least 4' of the same insul. under the slab around the perimeter. My soil is sandy gravel and well above the water table. After reading some of the stuff here on similar topics I wonder if complete insulation would be better in the long run? The building will have a radiant heated slab with a wood fired - Alternate Heating Systems (AHS) "wood gun" unit. Thanks for any help

Frost is the main worry. Insulation below the slab will afford you better response time, but if you set and forget your thermostat, you will store more heat below a slab lacking insulation, an advantage if you heat source is intermittent.

I strongly disagree with morgan, and strongly recommend you fully insulate this particular slab.

I also agree, insulate the whole slab. The only purpose to not insulate would be if you had some sort of solar or free heat that could be stored under the slab. If not you are just allowing the heat you put into the slab to leak away, complete waste of money.

The ground has its own insulative value and many commercial system past and present work perfectly without insulation below the slab. Mean ground temperature is the main number to plug into any radiant slab formula. Insulation is usuallly a given but it literally pays to do the math. This is science not religion.

Show us the math then.

Prove your point.

Where is our slab?

"I am building a 78' x 38' (garage/shop) building in Maine."

the "math" is horrendously complicated and impossible to project without knowing site conditions. but morgan is right that many commercial slabs operate without continuous underslab insulation and do so fine.

but I do not consider a slab 38 feet wide "commercial" in its scope. and in maine, you better know your job site before you risk pouring a commercial slab without insulation as well.

Axiom vs. math.

I agree, it is easier to insulate than investigate. However, the $3000. our friend will pay (for insulation materials) will take a long time coming back to him.

I would insulate any living space, but a warehouse with intermittent heat source may be better served by using the ground below the slab as a heat sink.

I would call that nearly opposite morgan. if only *intermittantly* heated, the ground sink is a massive detriment to system operation (during startup conditions is the worst case scenario for ground tied systems). do I misunderstand what you're saying?

Also, to use the ground as any kind of efficent heat sink, you would need to
prevent any water close to the wall, that would draw away heat.

A heat sink needs some type of cost effective heat source and a way to draw that heat back out when you need it.

Call this to simple, but if you intend to keep the space above the median ground temp of the area, insulate it. I know in Northern WI a well insulated (foamed) garage with insulation down 4 feet on the perimeter, but none under the slab will stay above freezing even in winter from the ground heat. Now If I wanted that space to be 70F year round I would insulated under the slab.

And how far above ambient ground temps makes a difference as well. If it averages 65F over a year and you want it 70F, the insulation will take a while to pay for itself. If the ground temps are 50F and you want 70F, obviously insulation will pay off much sooner.

Of course water flowing or being really wet under the slab throws all this way off and would cause me to drain and insulate for sure.

Brock is exactly right. It is all about delta T.

Intermit heat sources such as solar, wood etc. may benefit from the heat sink which the ground represents. As you know the ground will absorb considerable energy and reverse its flow, albeit uncontrolled, as the space cools. We have used this in simple commercial and agricultural designs where response time and perfect temperature control was not a concern.

Many of our earliest designs both residential and commercial followed Mr. Ebo's simple and effective frost barrier insulation strategy. In warmer climes and/or colder indoor design temperatures, insulation is simply not mandatory or even a good investment. I would rather have a ModCon boiler...but you knew that Rob.

Still I concede, when in doubt insulate and hope the cost of insulation doesn't drive your customer to gas-fired unit heaters.

hehehehee

Thanks for all the input! To add to my prev. question, Do I need to insulate down to the footer (vertically) on the exterior 8"poured wall or would or would 2' be ok? and would I loose much by placing this insulation on the interior surface, so it is not seen or damaged on the outside. It's only a work shop and I'd be very happy to keep it at about 50 - 55 deg.

Hi all, first time poster here. I, too, am building a heated garage in the St. Paul area of MN. My garage will be 34x28, and slab on grade with a floating slab. If I am only planning to keep the shop heated to 50 or 55, which is not too far above ground temp, I'm now second guessing my plan to insulate the entire floor. Perhaps it would be better to just to the walls and the interior perimeter about 8 ft in from the walls. Any thoughts? Thanks, Boris

It is all about temperature differential and return on investment. You are correct, as your desired room temperature is very close to our year round ground temperature. Whereas it is absolutely critical to insulate against frost-capping the ends of the slab and insulating four feet down, in or out from the foundations edge, interior insulation is less critical except in the rare instance where ground water is an issue (a condition that usually precludes the construction of a building in the first place).

It will take longer to heat up the slab on initial startup and you will not have the luxury of fast response times, but you will save a dollar a square foot on materials and use very little more fuel.

It should be noted that many of my colleagues disagree on this question while they and I continue to install the vast majority of commercial systems without wall-to-wall insulation.

this is a small slab. insulate continuously. If you have to drop to 1" to save some bucks under the center, go ahead. concrete is nearly forever, and while you might only want it at 50 now, that's now. What about the next owner? What about the ground getting very wet during early spring melts?

Yes, I agree. It's not the cost savings I'm looking for in terms of insulation--a few dollars for foam board is not going to break the budget. What I'm getting at is the concept of the earth as thermal mass versus the earth as an unsatisfiable heat sink. Regards, Boris