Design
Designing and Pouring an Insulated Slab
See how to detail the footings, sub-slab insulation, vapor barrier, and sill plates for a simple, cost-effective slab-on-grade foundation.
Tim Biebel of Prudent Living explains the design considerations for the foundation of the 2017 FHB House project: “We chose a slab over a traditional basement for several reasons: One was cost. Another was water drainage issues. But mostly it was due to cost.”
The build also features continuous insulation under the 4-in. slab . A 15-mil vapor barrier also serves as an air barrier, since 15 mil is very thick. The finished flooring is applied directly to the slab. With no foam insulation on the outside, once the foundation is poured, it’s done.
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I dont trust the 4 inches of foam to support the interior
load bearing walls. I dont care how the engineering calcs works out... 40 or 50 years down the road I would want a firm footing under the walls. My knee jerk replacement would be 6x6 pressure treated posts laid on their sides below the load walls. I doubt the thermal
conductance through the floor 6x6 posts is even measurable.
One of the disadvantages of a slab on grade home involves chasing plumbing issues years later. All drainage and water supply plumbing will be underneath the slab and essentially inaccessible without major expense.
On initially reading the tagline: "Using a slab over an excavated basement...", I was looking for the excavated basement with a slab on top. I'd only seen that in commercial / industrial construction.
I'm not sure I agree with the 'cost saving' aspect of slab on grade vs. basement. I find it hard to believe that the 800-1000 sq ft of basement space can be replicated above ground at a price competitive with a basement. Especially since you're paying for footings and insulated stem walls anyway. A 9 ft poured basement is a fabulous place for a home theater Is an above ground 16' x 20' space really cheaper? Even if were minimally finished?
A well thought out unfinished basement provides real value at modest cost. I think what he's really saying is: It costs less to build a smaller house, making it easier maintain margins while hitting a price point.
Where do people without basements put their stuff? Table saws 'n' drill presses 'n' workbenches 'n' suchlike?
“Slab over excavated basement” – sounded like a great thing in tornado territory. I prefer a house with basement too. Good place for the home workshop and cool in summer. You do need a dehumidifier though. Basement or slab foundation – that’s a discussion that can go on and on.
Unable to open or view video???????
I agree with scottperk. How long will the foam last? Will there be uneven shrinkage or deterioration? There seems to be to much emphasis on energy efficiency over longevity.
I think it's a great foundation. Every building site will have different foundation requirements, pros, cons... it's easy to make up scenarios why it's a poor choice.
If we're talking about 50 years from now, how about a full basement with retaining walls that needed rusted out rebar to hold back the unbalanced dirt leaning on the walls? Or a sump pump that went out during a really bad storm, flooded the basement and ruined everything electrical.
I'll take PVC and XPS or EPS lasting 50-100-150 years any day. Foam generally has much greater compressive strength than the soil it is sitting on anyhow.
No wire mesh or rebar in the slab?
Over here total foundation and load bearing slab is left cold" and rough (not floated).
outer walls and sometimes also inner walls, start with an about 8" high Autoclaved aerated concrete block wall on wich the sill for framing will be fixed. after the house is framed and roofed, insulation is sprayed on the foundation slab between the walls, and then finished with a cement screed of 2.5" on which floor finish is put.
Most utility pipes just run on the foundation slab and are sprayed in with the foam.
We care the same way about energy efficiency and cost. But it seems the way we build still differ oh so much (not sure which way is better).
Regards, Gert from Mol, Belgium, Europe