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Community and Q&A

Frost-protected shallow foundation

GBA Editor | Posted in Green Building Techniques on

I’m still not clear how a heated building frost-protected shallow foundation (FPSF) and unheated building FPSF differ in design and theory. I’m about to try to build myself a small home/shop and I don’t know how far I’ll be able to get before the dead of winter here in Vermont. The cold doom and a potentially unfinished home puts me in the unheated classification for a month or however long it takes me to get insulated but back to heated when I’m finished.

I’m looking to use 3.75″ EPS under slab and 2″ XPS for perimeter. How balanced is the FPSF foundation system, can I add extra under slab or will it stop too much heat loss and freeze below?

I’ve found plenty of construction details but not enough information to understand this aspect.

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  1. Expert Member
    Michael Maines | | #1

    David, I think you mean FPSF, "frost protected shallow foundation."

    You are balancing the natural heat of the earth against cold air freezing the ground. In a heated building you technically don't need as much help keeping cold air from getting to the dirt below the slab.

    In an unheated building you need more insulation to keep the ground below the slab from freezing.

    You can add as much insulation under the slab as you want. The ground WANTS to be un-frozen--think of the molten core of the earth. The prescriptive details are minimums to keep things from freezing, more insulation is better.

  2. user-1140980 | | #2

    yeah FPSF, thanks...

    I'm looking at 3.75 eps under slab for a r15. I wonder if 2 layers of 2" is better than one at 4" due to staggering the seams...

    There an obvious point of cutting off heat loss to the ground below with more slab insulation. At a certain point you'd almost consider a heated building unheated because theres isn't enough heat loss. I'll model this when i learn how!

    Can the slab survive a winter if i'm not able to finish the building in time?

  3. John Klingel | | #3 has info on fpsfs, if interested. in your neighborhood, i would think the insulation thickness you are talking about is a minimum. martin h is near you, so he sure has a better handle on that. i've become big on ground insulation after reading about passivhaus, thorsten chlupp, and others. do you have any info on your average ground temps? something like 45-50 F or so? unless the ground is up to room temp, it is taking heat all year long.

  4. Expert Member
    Michael Maines | | #4

    David, if you have enough insulation under the slab, and if it extends out far enough from the building (or out-and-down), then your slab will survive the winter just fine. What those dimensions are I can't say. IRC 2009 does not allow unheated FPSF buildings without an engineer's approval, but they can work. As long as the concrete has cured (30 days is enough) it does not mind being frozen, as long as the ground below does not freeze and heave.

    I'm not sure I understand your comment, "At a certain point you'd almost consider a heated building unheated because there isn't enough heat loss." You want sub-slab insulation for two reasons, to prevent heat loss to the ground for the sake of energy efficiency, and to keep the ground from freezing in case the building is not heated.

    Here's an anecdote that might help. Two years ago a neighbor gave me a bag of tulip bulbs. They are supposed to be planted in late fall or early spring, before the ground freezes. I didn't get to it before the ground had frozen to a depth of about 6". In January I shoveled some snow out of the way and laid down a 2" sheet of foam. After two weeks I removed the foam and the ground had thawed, except for about 6" around the perimeter.

  5. user-1140980 | | #5

    We're getting at the same point michael,

    if the heat isn't getting to the earth through the slab, the earth underneath doesn't care if the building is heated or not because it isn't gaining any warmth from the building.

  6. GBA Editor
    Martin Holladay | | #6

    As I wrote in my Fine Homebuilding article on frost-protected shallow foundations, "Fortunately, these shallow foundations don’t depend on leaking building heat to keep the soil warm. Instead, horizontal wing insulation extending from the bottom edge of the slab helps to retain the natural warmth of the earth."

    For more details and design guidelines, see Frost-Protected Shallow Foundations.

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