I devoted spare time this past year to building a new small office for our architecture practice. I thought of a simple way to use insulating concrete forms (ICF’s) to make a frost-protected, shallow slab foundation. It worked out well, so I’m sharing the idea, with hopes that other folks will improve upon it.
Here in Maine, foundation walls typically extend 4 to 5 feet (122 to 152 cm.) below finish grade to be safe from freezing soil. That uses a lot of concrete, an energy-intensive, carbon-intensive building material. Don’t get me wrong, concrete is great: plastic and cast-able when wet, strong and durable when cured. Treated correctly, a concrete slab can be a finished floor, and that floor can provide significant thermal mass within a structure.
Frost protected slabs help us use concrete for real advantages and avoid using it just to deal with freezing soil. The technique has been used for decades in the cold climates of Alaska and Scandinavia. The National Association of Homebuilders has researched and endorsed the technique. The idea is simple in principle, although the details are tricky. First, use well-drained material below and around the slab to keep water away. (Think about it: railroad tracks don’t “frost-heave” because they rest on about 3 feet (1 meter) of stone ballast.) Second, place a nearly-horizontal layer of rigid insulation around the perimeter of the slab, extending outward at least as far as the frost depth in your region. This helps prevent freezing at the slab perimeter.
When making a frost-protected slab, one of the trickiest parts is forming the slab “haunch,” or the perimeter edge requiring thickening and reinforcement for structural loads. Another challenge is placing and leveling formwork on uneven ground. And a further challenge is providing ample insulation on outside of the slab edge – energy modeling reveals that slab edges are a large source of building heat loss.
That’s where ICF’s come in. They make handy, pre-insulated formwork. They’re easy to handle and place, and they have hard plastic splines embedded in them that can take fasteners – making it really easy to attach additional rigid insulation, expanded metal lath for stucco, and so on.
Here’s what I did. First, I had my neighbor Jim place clean, bank-run gravel to make a well-drained building pad. I compacted the gravel in 15-inch high “lifts,” then allowed a winter for the gravel to settle.
My buddy Stephen and I poured a footing, taking great care to make it as level as possible.
I coated the top with a cementitious “capillary break.” Then I snapped chalk lines for the slab perimeter and nailed steel track to the footing while the concrete was still “green.”
The next step was perhaps the most innovative. ICF’s usually come as 2 “panels” that are assembled using plastic connectors that double as holders for steel reinforcing bars in the haunch. I cut one of each pair of “panels” on a table saw, lowering it by about 4 1/2 inches (11.4 cm.), or the depth of the slab (except the haunch). Then I connected the panels, with the shorter ones to the inside. (The photo shows the corners placed first.) I could level the ICF’s and fasten them to the footing by screwing into their plastic splines through the steel track. Because we were careful with the footing, we only needed to level the ICF’s by 1/8 inch (3 mm.) over the whole perimeter.
The idea is then to place under-slab compacted fill and rigid insulation to the top of the interior “panel.” Then you’ve formed the interior and haunch of your slab. Here’s an illustration:
You’ll notice that you only benefit from half of the ICF’s insulation at the outside perimeter. The ICF’s I used provided 2 1/2 inches (6.4 cm.) of expanded polystyrene on the outside edge, with an R-value of about 9.5 (RSI-1.7). I added 3 inches of extruded polystyrene insulation (R-15, RSI-2.6) to the perimeter, fastening it with screws driven into the plastic splines. The total slab edge R-value is 24.5 (RSI-4.3). I also placed 3 inches of extruded polystyrene below the slab.
With the ICF’s all in place, it was time for under-slab fill, under-slab plumbing, insulation, vapor retarder and steel-bar reinforcement.
I have no skill with a screed or trowel, so I hired my friends Keith and Howard to pour the slab – they did a great job. Once they finished, I sawed control joints to let the slab crack in a controlled way – I placed the saw joints in a pattern that mimics large tiles.
Later, I added the extra insulation at the slab edge. I protected the insulation by attaching expanded metal lath, using long screws to reach the plastic splines in the ICF’s, and applying two coats of cement stucco to the lath.
When the slab had cured, I etched it with acid and applied a concrete stain to make the slab the finish floor of our office. (I “embellished” the stain color by adding mineral pigments of my own.) Several coats of sealer and buffed paste wax gave the floor a nice patina. After several months in the office, the floor is warm, durable and attractive.
If I were doing the job over, I’d calculate the loads coming through the bottom of the slab haunch onto the footing – if a dense foam insulation could support those loads without compressing, I would add it to the top of the footing, eliminating a significant path for heat conduction. I placed a 2-inch (5 cm.) layer of rigid polystyrene insulation horizontally 4 feet (122 cm.) around the slab perimeter. That’s an R-value of 10 (RSI-1.8). If I were doing it over, I’d use 4 inches of foam for an R-value of 20 (RSI-3.6).
It was fun to figure this out and do the work. Through the years, the inspiring work being done by folks in the NESEA community, and the great presentations I’ve attended at NESEA conferences, have given me the gumption to try new ideas. If someone with my meager skill levels can do this, then a lot of you skilled folks out there will be able to really improve upon the idea!