Least Expensive Foundation/Floor
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What is the least expensive foundation/floor system appropriate for a small, single-story, superinsulated house given:
cold climate
level building site
gravely, well-drained soils
minimal building code (i.e, unusual ideas will be considered…)
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I'm in GA, on clay/gravel and the code/mores could be described more as "traditional" than minimal -- but I see no reason you shouldn't adjust what I'm planning for my site:an insulated monolithic slab. You'd probably want a frost-protected design and more insulation, but since you likely won't have to deal with termites (which inspectors DO pay attention to here), I'd think it would be somewhat more direct in the designing.
The slab will have integrated color and the floor will be polished concrete, since you're interested in cost effectiveness.
Joe
Jack,
If you're in termite territory, and you want a superinsulated foundation, you probably can't use foam.
That means you have to build a crawl space with very deep floor joists, so you can use blown-in fiberglass to insulate your floor.
Martin, too cold for termites around here :)
Joe, I've considered some variant of a concrete slab, and I can see how they would be a good choice for Georgia. Since I live in a cold climate I need both to insulate the floor from cold, plus either put in a deep foundation to get below frost levels (expensive), or put foam wing panels around a shallow frost-protected foundation to stop the frost from getting underneath the slab (also expensive). What I was hoping was that there was a less expensive alternative that I had overlooked.
Jack,
Superinsulated foundations aren't inexpensive. You could build an insulated raft foundation if you want high performance -- see Foam Under Footings.
Martin,
Thanks for the link to your blog. I found two statements very interesting. First, by you --
And next by Kevin Dickson, MSME, P.E. --
(followed by the rationale)
If your and Dickson's advise is correct, then maybe, with a properly reinforced slab, I don't need to worry about using any expensive external foam insulation at all! Why not just build a lightweight framed floor on top of the slab and insulate it with less expensive fiberglass or cellulose. No need to worry about thermal bridging through the concrete, as it will be outside of the thermal envelope. Do you see any of those pesky unanticipated problems with this sort of assembly?
Jack: I think we've all been where you are on this; groping for some kind of Grail. IMHO, skimp somewhere else, like using plywood for the kitchen/bath counter tops temporarily. You get one shot at a properly built foundation, and all the foam you can pour at it will likely pay for itself. If you put foam under the footers, make sure you read the fine print about what the compression factor is for various psi ratings. If you require 15 psi bearing, you may need "40 psi" foam to avoid "creep". You have to decide how much compression you can live with, and whether or not you want to push the elastic limits of the foam, then buy the foam accordingly.
Martin, you said:
"If you're in termite territory, and you want a superinsulated foundation, you probably can't use foam.
That means you have to build a crawl space with very deep floor joists, so you can use blown-in fiberglass to insulate your floor."
Just recently, the rater I've been speaking with says inspectors are willing to "consider" some of the new termite-treated rigid foam (Preventol is the brand name). Another option may also be, he thinks, some of the covered "fast form" insulation like Michael Chandler has reviewed (FHB) such as "EnergyEdge" or "PerformGuard."
If they're willing to do more than consider options, it could open the door to some new options.
Joe
Joe,
Thanks for sharing the good news. It's hard for those of us who live north of the termite line to keep ahead of local rulings by building inspectors in the South, so I appreciate hearing information from the field.
Martin,
I do live in termite territory (Eastern Tennessee) and I am building a house and I am very concerned about termites. What would be the appropriate foundation detail? If I opt for a sealed crawlspace, is it okay to put the insulation in the joists? I thought there should be no isolation between crawlspace and living space.
Is a slab on grade with insulation below the better option? (Forgive me if this should be a new thread; please let me know and I'll start one.)
My comments above are useful only if you can find a PE who believes the soil under your house remains dry.
"In developing the Frost Free Foundation solution, SBRA engaged the services of Paul
Hayman, PE. Explaining the dynamics of frost heave, Hayman emphasized that ground heave can
only occur if three conditions are met: the soil is frost susceptible, outdoor temperatures below
freezing are sustained for long periods and the soil has sufficiently high moisture content.
Paul Hayman wrote the report, and could perhaps recommend an engineer in your jurisdiction. You'd have trouble convincing your building dept. without someone stamping it.
Jack, I don't think the slab/floor system you describe is cheaper than just an insulated slab. Remember, if you go with concrete as the finished floor, you are saving about $6/sq.ft.
David,
Yes, it's possible to install crawl space insulation in the floor assembly (ceiling of the crawl space) rather than the walls -- as long as you live in a climate that is warm enough to keep your pipes from freezing.