We start by removing the topsoil from the building site and bringing in water, electric, and sewer lines as needed (see Figure 1). Around these, we place good-quality bank-run gravel, compacted in 6-inch lifts, to a minimum depth of 12 inches. The gravel layer extends on all sides beyond the footprint of the building by at least 2 feet. To prevent voids underneath the foam, which might cause it to deflect, we take extra care to level the compacted gravel. Alternatively, we’ve used a 1 1/2-inch layer of flowable fill — a thin, aerated concrete mix with sand aggregate — to true the surface. We form the area with leveled 2x3s, then screed the fill flat. This is a less forgiving method than moving gravel around, but it produces excellent, void-free results.
Installing foam. We then lay the rigid foam over the leveled base (Figure 2), extending the sheets about 2 feet beyond the foundation footprint. The sheets are substantial enough to stay put until we’re ready to place the concrete forms for either a monolithic slab or a perimeter grade-beam foundation.
Forming a Monolithic Slab
The more conventional of our foundation designs is the monolithic slab, which has an edge that’s 16 inches thick and at least 12 inches wide at the bottom. Working on top of the foam base, we form the slab with stacked 2x6 framing lumber connected with 2x4 gussets (Figure 3). We line the inside face of the forms with 4 inches of foam. To make sure the foam will be secured to the concrete, we push long panel screws with plastic washers through from the back.
We stake and brace the forms at no more than 6 feet on-center. The closer, the better; wet concrete is heavy stuff and the last thing we want is a blowout. To prevent the forms from moving laterally at the bottom, we drive steel stakes along their edges down through the foam base into the compacted gravel.
Air-vapor barrier. On top of the foam, we place a continuous polyethylene vapor barrier, running it up the sides of the forms and bonding it to the foam with acoustical sealant. This prevents air and water vapor from rising through the seams between the foam panels. You can actually get quite a lot of air movement through the soil beneath a foundation. Air can find its way into the living space around utility and plumbing stubs, in some cases bringing radon gas with it.
We seal all the penetrations with tape, caulk, and additional pieces of poly. As an added precaution against radon, we also install a length of perforated PVC pipe on top of the foam, with an ell that brings it up through the slab. Then, if testing later reveals an unhealthy level of radon in the home, we’ll vent the PVC through the roof to mitigate.
To make up the difference between the thickened edge and the 5-inch-thick slab, we place compacted bank gravel inside the forms (see illustration). Detailing the gravel to form the inner wall of the thickened edge is a pain; the slope has to be shallow enough to permit compacting, but if you make it too shallow, you have to make up the difference with a lot more concrete.
Forming a Perimeter Grade Beam With ICFs
I like monolithic slabs for their structural integrity and because they’re easy to place — one pour and you’re done. But forming and bracing the thickened edge can be time-consuming. So recently we’ve gone to a two-phase process using insulated concrete forms (ICFs) to create a perimeter grade beam and then a separate pour for the slab (Figure 4). While ICFs are typically used to form 8- to 10-inch-thick walls, our design requires a 12-inch-deep, 20-inch-wide grade-beam footing. We found an ICF manufacturer, Arxx Corp. (arxxbuild.com), that offers form ties to accommodate thicker wall designs along with a variety of ICF sizes and configurations to suit our needs.
To keep the forms straight, we run lengths of framing lumber along the bottoms, secured through the foam base into the gravel layer beneath with steel stakes. Corners call for extra bracing. To support interior column point-loads, we form 3-foot-by-3-foot pads using 2x12 lumber. The pads get three rows of #4 rebar 3 inches up from the bottom and three more 3 inches down from the top.
As soon as the footing concrete hardens, we install the vapor barrier and radon vent. In some cases, we’ll add another layer of 2-inch foam board over the vapor barrier, increasing the nominal R-value by 10. We fill the slab area to the top of the footings with compacted gravel, and we’re ready to frame the building (Figure 5). Once we’re closed in, the plumber roughs in the waste and supply lines, and we place the rebar or welded wire mesh to get ready for the concrete. Up to this point, the work has been done by my carpenters and our excavation sub, but we turn concrete placement and slab finishing over to a concrete sub.
Overall, the two-step grade-beam method actually takes less time and labor than the monolithic slab approach. While I can’t make a completely accurate cost comparison, my sense is that the labor savings offset the added cost of the ICFs. The method offers another significant advantage, too: Pouring the slab inside a dried-in building minimizes the adverse effects of weather and temperature. Should a future job find us pouring the slab after winter sets in, it’ll be relatively easy to heat the building above freezing until the slab is poured and has cured.
Before backfilling, we add 2 inches of foam to the outside edge of the foundation, over the ICF forms, which are only 2 inches thick. Here in Maine, carpenter ants find easy nesting in buried foam, so we do what we can to discourage them. We fold ice and water membrane into the corner between horizontal and vertical foam around the entire foundation (Figure 6). Later, after we frame and sheathe the walls, we cover the sill joint with another piece of membrane that overlaps the foundation strip. By code, a minimum 6 inches of vertical foundation edge must remain exposed between the wood line and the final grade. We cover it with Galvalume sheet metal, a neat and durable solution that’s easy to repair if it becomes necessary.
The advantages of an FPSF are considerable: lower cost, faster installation, less disturbance to the site, fewer subcontractors to schedule, and simpler detailing and finishing. Its ready applicability to super-insulated passive-solar design only adds to the list. But because it’s an unfamiliar foundation, it’s critical to resolve all structural questions and to address the clients’ concerns about durability and aesthetics. We find that once these issues are covered, however, a frost-protected shallow foundation is a nearly ideal base.
Alan Gibson co-owns G-O Logic, a design-build company in Belfast, Maine. His partner is architect Matthew O’Malia.