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Community and Q&A

FPSF attached to existing home

bgibson11 | Posted in Green Building Techniques on

I am planning a new garage and would like to attach to my home. I am hoping to go with an FPSF rather than footers/frost wall, etc.

The following factors are impacting my decision.

Location: Upstate New York (Climate Zone 6)
Age: Home was built in 2013
Existing Conditions: 2,200 SF Home on 9’0″ Poured Basement Walls, Existing Basement Slab is 6’0″ below grade.
Site Conditions: Garage would not be placed on entirely virgin ground. Below the partial fill (approx. 1-2 ft. is clay-rich soil.

House is two stories for a 36′ x 28′ rectangular section, on the 28′ side is a 14′ x 14′ single-story room. The garage would be attached on the end of the 14′ x 14′ room.

Because my home is planted firmly in the ground and is not susceptible to frost movement what concerns should I have when it comes to ATTACHING a FPSF to my home?

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  1. GBA Editor
    Martin Holladay | | #1

    A short hallway or enclosed breezeway between your house and the new garage can help disguise any slight settlement problems.

    To minimize problems with your garage foundation, it makes sense to remove the existing soil -- the more, the better. Removing 12 to 24 inches isn't too much. Then compact the soil below, and add a very deep layer of crushed stone before installing your rigid foam and polyethylene.

  2. charlie_sullivan | | #2

    Putting it partially on the disturbed soil that is only 2 years old worries me. That part could settle, making the garage want to lean in toward the house. You might be able to do a hybrid, with stem walls in the 10' (?) closest to the house, going down to undisturbed soil, and the rest FPSF. But at that point, it seems like you might as well do stem walls all the way around.

    You'd also have to think through how the transition from the regular foundation to the FPSF works, probably putting in extra below-grade insulation to pay it safe.

    Note that unheated FPSF buildings need more frost protection insulation than heated FPSF buildings.

    Are you dead set on attaching it? It's very hard to eliminate the potential for VOCs and CO in garage air to get into the house. A covered breezeway can make it easy to get to the car in the garage without that problem. Or perhaps you plan on only storing bikes in the garage :-)?

  3. bgibson11 | | #3

    Thank you for your quick response.
    I have attached a quick sketch of my overall plan. Unfortunately, a short breezeway or hallway is not in the cards.

    Let me know your thoughts based on the sketch attached.

  4. peaceonearth | | #4


    I recently built a garage in northern Vt, partially attached to my house. I went with a frost wall; - part not so deep but starting from ledge.(This actually save some $ in less digging and fill). I was aware of slabs done with rigid foam and thickened edges as an alternative, but cement contractors here had at best heard of that method but were not comfortable doing it. Maybe just behind the times or entrenched habit. The highly regarded contractor I wanted to use had always done a frost wall, so I went with that. The excavator pulled out all the interior soil and it was refilled with gravel that would drain (the original fill was used elsewhere). Not sure this is in any way helpful, just fyi and my experience. Using foam for protection would seem cheaper, and since it is done in other cold climates (I understand in Alaska and Scandinavia this method is common), it should work if done right.

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