High-Performance Raft Slab

For many years, my company built conventional poured-concrete foundations, always aiming to be below frost depth (4 ft. in our area). But as we got into high-performance building and changed the way we built walls, our foundation system had to evolve. We started with double-stud walls about six years ago, and were able to pair those with 4-ft. frost walls and maintain a thermal break between the interior slab and the outer frost wall. But as we strived for even higher levels of performance, we began looking at exterior insulation and Larsen truss assemblies that necessitated a different type of foundation. Further driving this evolution, many of our customers started requesting houses without basements.

Our standard high-performance house uses a 2×6 structural wall with Zip System sheathing and 12 in. of exterior insulation supported by vertically mounted I-joists. For optimal performance, the insulation layer needs to be continuous from the walls to beneath the slab and over the building (a separate discussion). A frost-protected shallow foundation (FPSF) is an efficient way to handle the underslab part of this. The approach has been around for years, and there are a handful of off-the-shelf systems that can work well.

The problem we encountered was finding an FPSF system that would work with our various wall assemblies. No two clients want exactly the same thing, so we need the ability to vary the level of performance of the foundation system along with the walls. We looked all over, and talked to a number of suppliers of ready-made systems. Some had some of the features we wanted, but none touched every base. We may be picky, but I have to stand face-to-face with our customers and answer the hard questions when something isn’t just right. Accepting a foundation system that doesn’t check all the boxes isn’t an option.

That led us to develop our own system. We found out we could order EPS foam in different sizes, thicknesses, and densities through our local building-supply center and have it on-site in about a week. From there it was a matter of designing a simple yet effective system we can assemble on-site and customize for various projects. Five houses later, we like what we’ve come up with—and our excavator likes it, our concrete subcontractor likes it, and our homeowners like it too.

Two-way heat shield

This type of frost-protected shallow foundation relies on geothermal energy to keep the soil around and below the building from freezing. Our thickened-edge slab sits in a raft of EPS foam insulation, which itself is ringed with a skirt of insulation that extends beyond the building perimeter. The underslab foam slows heat loss through the floor and, along with the skirt, traps heat in the soil below the building. This, along with a thick gravel base and a perimeter drain to move water away from the structure, prevents frost heave while keeping the heat produced by the mechanical systems inside the house.

Different approach, different considerations

Of course, building a raft slab is a different animal from a conventional foundation. Within the world of slab-on-grade options, FPSF systems have their own set of considerations. Beyond the calculations (which you can find in the American Society of Civil Engineers’ ASCE Standard 32-01), there are some critical considerations to keep in mind when selecting or developing an FPSF system:

The first is prep. How easy is the system to prep from an excavator’s point of view? We sub out excavation and are fortunate to work with an experienced operator, but we need to consider his work in addition to ours. If it’s too complex or too confusing, the project will suffer.

Next is height. How much of the stem wall shows above ground and how much is below ground? Many of these systems rely on the backfill outside the foundation to keep it in place. We like the ability to have 1 ft. showing and 1 ft. below grade. Code also plays a role in determining the depth requirements.

Perimeter foam is key. Minimum requirements for the thickness and projection of this frost skirt are determined by local code, but we go a bit above minimum for ease of mind and use 3-in.-thick EPS foam frost wings that extend 4 ft. from the building. Each inch of EPS has the insulation equivalent of 4 ft. of soil. We may be able to get away with less, but it all comes back to risk assessment. The incremental cost of 3-in. foam vs. 2-in. foam is minimal, but a foundation issue down the road may be irreparable, both for the physical building and our reputation.

Stem-wall thickness is a big deal. We like to have some flexibility. One house may have 12 in. of exterior insulation, but another may have 6 in. We need to have the option for both within the same foundation system.

Another consideration is finish and protection. There are lots of ways to protect the foam above grade (and below if you’re in termite country), and this is what people are going to see in the finished product.

Finally, think about the inner foam. Some systems come with inner foam and some require you to supply your own. Some details can complicate its placement. For example, is a grade beam necessary, or even possible with the chosen foundation system? To read the full Fine Homebuilding article, click here.

PDF Image of raft slab 2


Josh Edmonds is a Certified Passive House Consultant and managing partner at Simple Integrity LLC.

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