When we pour a slab on grade we are basically pouring concrete directly onto the earth. Sure, there is a 4” layer of foam under the concrete so we don’t lose our heat, and yes, there is 12” of sand under the foam so the foam sits perfectly flat and the plumber can easily locate drainpipes in the sand… but under that sand is the compacted native earth.
No crawl space, rats, trash or mold, just a crisp concrete slab sitting on the earth, insulated so the slab holds in heat (or cold), and keeps the temperature inside the house consistently stable.
Slab-day is stressful, because many of the affordable Net-Zero houses from TC Legend use the slab-on-grade as the finished-floor.
Concrete slab as a finished floor provides a cost-effective, durable, aesthetically modern floor. However, the concrete needs to be finished perfectly flat on “Slab Day.”
Last week at the Lake Stevens house, the second concrete truck was late and the guys had to scramble to pour and screed the last truck-load before a permanent seam became inevitable!
Disaster was fortunately averted, the 1,500 square foot (sf) floor had the seam massaged-out, was troweled flat, covered in plastic and will be scrubbed and sealed in a few months before the trim goes up.
There are other finish options for concrete floors; they can be ground down, acid etched, and stain can be added to the concrete in the yard.
The following photos are gathered from a couple other already-built jobs. Dan was too busy at Lake Stevens last week, shoveling, troweling and de-stressing the crew to run a camera.
The black slab under the mountain was poured using black dye within the concrete.
The yellow Legos are a layout grid for in-floor radiant hydronic heating, which until recently was one common heating option deployed within our Net-Zero homes (more on heating systems soon).
The TC Legend Homes fellas put up the Lake Stevens house’s insulated concrete form (ICF) walls in the rain last week over the course of just (2) days! Dan said it went well since the soil had a good consistency to seat the Form-a-Drain footing, and the blocks went up fast, as per usual, since everyone likes stacking up Lego blocks!
A Net-Zero house is insulated on all (6) sides, and TC Legend builds with structural insulated panels (SIPs), so we have R-29 SIPs walls all around, R-49 SIPs roof above, and 4” R-20 foam under the slab-on-grade.
The edge of the slab also needs insulation, otherwise there would be a weak spot and heat would leak out through the slab perimeter, into the stem-walls (which are exposed to the weather).
We solve this problem by pouring the stem-walls into R-24 ICF forms which complete the insulted shell of this affordable Net-Zero home.
Concrete has been poured into foam wall-forms since the ‘80s. The hollow blocks go together like Legos with the required re-bar clipped inside, and once full of concrete, the Styrofoam block remains in place performing the job of insulation, and containing concrete while wet. The process is fast and clean, and has lower labor costs compared to traditional concrete forming.
We use NuDura R23.8 ICF blocks to form the stem walls, and retaining walls when we need them. The NuDura blocks are great since they concertina-down to transport. ICF blocks are available in many thicknesses with many variants for different construction requirements. We use 6” walls and usually only order inside-corners, outside-corners and 8’ straight blocks. The NuDura blocks have hard plastic structural strips that accept screws for siding and drywall.
The blocks sit atop the hat-channel that spans the footing form. At Lake Stevens we used Form-a-Drain footing form which, like the ICF’s does not need a form-board to be stripped-down after concrete is poured. The Form-a-Drain replaces 8” footer boards, is hollow, and drains water away from the foundation foot.
The Lake Stevens house has a 4’ stepped retaining wall up-slope and NuDura supply a very, very sticky vapor barrier to waterproof the stem-wall. TC Legend Homes has a metal cap/ cladding system system that wraps the exterior ICF foam from the sill plate downwards below-grade with roofing metal, leaving no visually, or thermally exposed concrete.
ICF’s are great for us! They perform two functions: (1) insulation, and (2) concrete form, they are a smart solution for our thriving Net-Zero, and increasingly Net-Positive SIPs housing in Washington state!