I live in a moderately cool climate, southcentral Alaska, where it rarely goes below 0 deg F for very long. I plan on building a one story house (about 1250 sq ft with an attached garage of about 750 sq ft) on a mono pad. The locally accepted practice is to place 2″ EXP below the pad and also on the outside of the footing, and extend another layer just under the grade, from the footing outward and on a downward angle. Since I plan to use “in the pour” plastic tubing for heating, I am concerned with heat loss through the sides of the footing. Would it be feasible to pour a floating slab inside the footing, isolated from the footing by, say, 2″ of EXP? Would the energy savings be significant? Would I be “shooting myself in the foot” by keeping enough heat from the footing to allow the sub soil to freeze?
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Greeting, I too live in SC AK (Anchorage) and am eventually planning on building in Wasilla. If I don't go with a basement I will strongly look at the "Frost Protected Shallow Foundation" method building with ICF's. I took a look at one near Wasilla Lake that was being built a couple of years ago, looks almost too easy. Check with the Reward distributer in Wasilla for locations that are building with standard foundations and FPSF, he should be able to put you in touch with owners/builders that have used both.
Jim
I am not from your area, so I don't know the excavation issues, but I personally would wonder what would happen if for some reason you were unable to heat the house, and the shallow foundation would be unprotected. Or does the insulation keep the frost heaves away on its own?
What depth are frost footers required to be in your area? Why not bury a CMU or ICF wall on a footer to that depth, and pour your slab over that (with edges insulated)?
I like frost walls built with ICFs with a slab floating inside for situations like yours.
You would still have to insulate under the slab, but the sidewalls will be well insulated and the labor savings in using the ICFs adds up if you intend to do this yourself.
Brian and BoJangles:
I want to avoid the labor and expense of below-frost footings and ICF. Also, with a monoslab on grade I wouldn't have to worry about footing drains. I think the insulation alone might prevent frost, but once I get the slab warm it should never get cool enough to freeze.
Are you saying that the house and garage are going to be poured on the same slab and both are heated?
The perimeter and bottom of the footing on a floating slab is definitely a weak spot when using radiant floor heat.
Yes, I plan to build both on adjacent slabs, but may separate the garage slab from the house slab. The garage will probably not be heated separately, but should stay above freezing from the heat received from the long wall of the house.
When you say that the perimeter and bottom of the footing is a weak spot with radiant floor heat, do you mean because the floating slab will not transfer enough heat to the footing to prevent frost? I would think that with proper insulation of the footing, including insulation placed horizontally out from the footing, enough of the earth's heat would be retained.
I'm just saying that you get a large amount of heat loss around the thickened edges of the slab. It's not anywhere near as energy efficient as using frostwalls with an isolated slab inside.
We would not even be allowed to build as you have described, but you may have much different soil conditions than we would experience.
I was concerned about part of the attached structure being heated and the other part not being heated.
Thanks. That was really my original question: would it pay to isolate the slab from the footing with EXP. I quess you agree that it would.
I still can't picture what you are trying to pour. A "mono slab" , to me, means a continuous concrete slab with integral thickened edges that act as the footing.
I don't know how you can have a "monoslab" with the floor separated from the footings???
Or is there a different version in Alaska??
Originally I was going to use a mono slab, then thought of keeping the thermal bridging down by separating the slab from the footing, so you're right; it's no longer a mono slab.
I got some good information and calculation formulas for insulation on a FPSF from The Revised Builder's Guide to Frost Protected Shallow Foundations from NAHB and ASCE 32-01 from the Society of Civil Engineers.