That’s where the frost-protected shallow foundation comes in. As we discussed [1], 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 [2]), then insulate its exterior with rigid foam insulation [3]. (Concrete on its own is not so good [4] on the insulation front.) Two problems with this approach should be obvious to RRR [4] 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 [4]. 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 [5] 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 [6]. 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 [7]. (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 [8]. 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 [8] is made out of them, a brand called Rastra [9]. 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 [9] approved. Cross those fingers.