So maybe I wasn’t very clear in that last post. Here’s a little recap: I have a 350-square-foot work studio that’s infested with carpenter ants; I’m going to tear it down and rebuild it with straw bale walls and solar thermal panels and a frost-protected shallow foundation. (Well, there’ll be a little more to it than that, of course. But let’s keep things basic for the moment.) The current building sits on an uninsulated four-inch slab of concrete, which is a no-no for two reasons: One, that’s not a good enough foundation for Boulder’s building code; and two, leaving it uninsulated means up to 50 percent of the studio’s heat is being sucked through the concrete and into the ground during the cold months. More heat loss not only means cold feets, it means more natural gas burned and electricity consumed by the gas/electric heater I’ve got in there. (It ‘s 95 degrees in Boulder right now, by the way. Slightly challenging to remember the cruelties of winter in this heat.)

That’s where the frost-protected shallow foundation comes in. As we discussed, an FPSF basically insulates the ground under your structure. What we’re going to do is lay a new insulated foundation around our existing slab, effectively building an insulated, underground wall around the earth directly below the slab. Make sense? We’ll dig an 18-inch wide, 18-inch deep trench around the perimeter of the existing slab; that’s where the underground wall – the new foundation – will go. While the weight of the current building is supported by the slab, the weight of the new building will be supported by that underground wall.

There are a number of ways to build that new foundation. One is to just pour straight concrete (along with some rebar), then insulate its exterior with rigid foam insulation. (Concrete on its own is not so good on the insulation front.) Two problems with this approach should be obvious to RRR regulars: One, while concrete is a super durable material—a key consideration in green building—it’s also super energy intensive to produce and ship. And building a three-foot high, 12-inch wide wall in the ground will use a lot of it. Not more than, say, a new, 4,000-prisoner, all-concrete jail would be, but a lot relative to the size of our studio. The other problem is the insulation. What do we know about rigid foam insulation? Our little ant friends love it. So a three-foot wall of insulation—installed underground—would be a nice new housing project for them. The more foam they chew through, the less R-value that foam has, the more useless it becomes, and pretty soon we’re back to where we started.

A second way of building the new foundation is to use ICFs – insulated concrete forms. When you pour a concrete wall, you need something to hold it together in wall-form until the concrete cures. Usually, that’s a plywood or steel “form.” After concrete cures, the forms come down (and usually get reused). ICFs on the other hand are rigid foam forms that stay intact: so the concrete remains sandwiched between two layers of insulation. Useful and handy: no need for plywood forms means fewer resources used.


[That’s not my photo, obviously – we’re quite a ways from pouring a new foundation – it belongs to these guys. (Shout out to green builders in my hometown Albuquerque!)]

That’s one kind of ICF – the kind we can’t use. (See above: “ants.”) Another kind comes basically in block form – very much like a cinder block – but made out of (usually recycled) polystyrene beads mixed with cement. As I said, like a cinder block, but a very light cinder block. These you can stack like Legos. The finished wall has narrow-ish channels running vertically and horizontally within that get filled with concrete and rebar. You can build entire houses with these things – in fact, one of the additions to our house is made out of them, a brand called Rastra. They have crazy insulating properties, something like R-45. And you use a lot less concrete, like 70 percent less.

The biggest thing about foundations is you can only talk about them for so long. And I think I’ve reached that point for today. But don’t fret, dear reader: On Tuesday I’ll talk more. Right now I’ve got to go files some forms with the county so we can get our construction drawings approved. Cross those fingers.