FPSF Pricing
ranson
| Posted in General Questions on
Anyone who has built an frost protected shallow foundation with an insulated slab, do you know how much your foundation cost per square foot, not including sitework?
I got a quote for a FPSF, and it’s quite high, but potentially within reason.
Thanks,
John
Zone 5, NY
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John, there are so many variables in a FPSF that comparing numbers would be meaningless without more details. How thick is the slab and how much foam do you need? Does your location and situation require frost wings? Are you spec'ing the concrete to be polished and/or finished? Reinforcing schedule? My FPSF projects usually have so much foam under the slab that we need frost wings, even when they are not prescriptively required for heated spaces. I've used 2" foam and 4" slabs, to 12" foam and 8" slabs, with or without perimeter grade beams, either site-formed or built from ICFs. For unheated spaces, the frost wings can get huge--nearly 5' wide in my zone 6 location.
I've found that most concrete companies really, really do not like working on anything other than what they are familiar with, so you may be getting a hefty PIA factor, especially if your details and specs are not fully developed. On the other end of the spectrum, with a preformed system like Ecocor's raft slab, carpenters can set the insulating forms--then placing the concrete is like any other flatwork.
A design like figure 7 below looks simpler than an insulated stem wall and slab design. And has less chance of differential movement between the walls and floor.
http://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf
Jon, in theory I agree with you, but the order of operations gets weird with that type of design. The most clever approach I've seen is GO Logic's raft slab system, where they set the foam and then pump in flowable fill before placing the concrete. Otherwise it's hard to get the fill placed on top of the foam without destroying the foam. Forming up tall-ish forms for thickened-edge slabs is one of those things that concrete guys don't like to do, at least in my experience. I'm not a fan of thickened-edge slabs because the perimeter sinks a bit as it dries, leaving a slight depression. Not enough to worry about it you're covering the slab, but if it's to be exposed it catches the light differently.
Perhaps the thickened edge could be poured separately (similar to a footing) and the slab could be poured over that. Maybe that helps - don't tell contractors that it's a FPSF - call it very thick but shallow footings with insulation underneath them and no stem walls.
That's one of my go-to details, called a grade beam slab-on-grade. I've done it with site-built forms and with ICFs for the grade beams. On one recent job, in MA, the builder could not find a concrete contractor to do it so they ended up doing a frost wall instead. But as a rule that's how I like to do slab-on-grade foundations. Unless they have to meet Passivhaus standards, in which case Ecocor's raft slab is the way to go. (I used to work there so I may be biased.)