The purpose of a footing is to distribute the weight of the house so that it doesn't sink in to the ground ("settling"). Settling causes cracks in foundations, in drywall especially around openings, and doors that stick.
I would like this house to last for a long, long time, so getting the footing right is important.
A rule of thumb I've often read is to make the thickness of the footing equal to the width of the wall, and twice as wide at is it thick. A common insulated cordwood wall thickness is 16" (4" mortar, 8" insulation, 4" mortar). In Manitoba they build cordwood walls at 24" thick (4" mortar on either side, and 16" insulation, wow!). I want an energy-efficient house, but I need to balance that against how much wood I have, how heavy the cordwood logs will be, how much concrete goes in to the foundation, how much excavation the footing requires, and the fact that wall thickness is included in the official square footage for tax purposes. (Stupid, why charge people for good insulation? It incentivizes the wrong behavior.) A 16" cordwood wall is about R-20, which is a common code requirement (although a well-built 16" cordwood wall will far outperform an R-20 conventional studded wall, because the studs transmit heat but are typically ignored in the calculations.)
Anyway...
A 16" x 32" footing, over the perimeter of a 40' x 40' structure requires about 21 (cubic) yards of concrete. At $75/yd, it's $1600. That's just for the concrete, not for excavating or installing forms or anything else. I plan to have a concrete slab subfloor, as well. If you assume 4" thick, that's another 20 yards, or $1500 more. It's adding up, and we're not off the ground.
I like the slab for its high thermal mass for solar gain and comfort. It's also good for radiant floor heat, which is supposed to be very comfortable. I also don't have to build a crawlspace, so it keeps things simpler. (Unfortunately, the systems you might put in a crawlspace, like waste plumbing, have to be built in to the slab, so that's a bit of work.)
If I'm going to all the trouble of pouring 21 yards of concrete in a footing, I'd like to take advantage of it for thermal mass. That means insulating the outside of the footing. I'll actually put Dow Styrofoam Blueboard panels down before pouring the footings.
In many places, the footings need to be deep, to protect against frost heaves. (As the ground freezes, it can move houses; it's very powerful.) I don't know what the frost depth is here, but it can't be much. It just doesn't get that cold, and not for very long. Still, a 16" thick footing will start off 16" below the frost long, so we're off to a good start. If the frost line is lower, I'll have to adjust.
Conventional thinking is that cordwood should start a foot or so above the ground. Codes often require 8" between grade and wood. The idea is that rain can splash up and invite rot. I could just pour a concrete wall there, but it's not that simple. I need a layer of insulation splitting that wall, and separating one side of it from the foundation.
One option is to use concrete blocks, which are 8" x 8" x 16" after 3/8" of mortar. (You can skip the mortar, and use surface-bonding compound, which is stronger and simpler for the novice, but the math is harder so I went with mortar.) I drew 3 courses of blocks, getting us 2' above the footing. I could go with 2' if I make the outside of the footing flush with the exterior wall, but in this drawing I filled in over the footing. Hmm...To make the lower exterior wall more pretty, I drew stone masonry. When I first started thinking about house building, I read a lot about slipform stone masonry. It's absolutely gorgeous, and you can use found stones instead of paying for siding or whatever. But there are some issues: how to the the thermal mass on the inside of the insulation without building two walls? How to attach things to the walls on the inside? Could we find a sufficient supply of beautiful rocks in this area, where it's mostly glacial till? But those issues are all resolved in this design, so I went ahead and included an 18" course of stone masonry on the exterior.
I could replace the interior concrete block with stone masonry as well. Or I could use concrete block inside and out, for simplicity, if I make the wall 18" instead of 16" (8" blocks + 2" blueboard insulation).
In this drawing I know I ignored are drains to remove water from under the slab and around the footing. I don't know if they're required around here, but they seem like a good idea.
I also ignored the vapor barrier. It's so thin that modeling it seemed like more trouble than it was worth. Still, getting it right is non-trivial, so I'll have to think about it more later.
Doing this drawing was a good exercise. It forced me to answer some questions that I hadn't hit before, because some ideas are impossible to draw.
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