Building a cob house in Montana

Hello. I currently live in South Dakota and my girlfriend and I are looking to move to Montana and build a cob house. I've never been around cob and I'm trying to start building a plan. The big questions I have are: how did cob stand up to the cold? What do I need to make sure I don't have any mild issues? What else do I need to know before buying land to build on?

Hi Luke, so long as you don't seal the cob, it is able to "breathe" and moisture levels shouldn't get so high as to cause mold on its own. Perhaps you build in a location that is super damp all the time though, then like any other building material your results may vary. As for cold locations such as a Montana winter, cob alone is not a very good option as it's a very poor insulator. Instead cob alone is a good thermal mass, and with a good roof and foundation (good hat and boots) to prevent rain and groundwater from damaging it, cob is a great building material.

In a location where it stays consistently cold, a combination of cob, balecob, and light straw clay could work very well. Strawbale walls when you have the space for the thickness provide great insulation, and can be finished with cob and a breathable plaster. If you need the walls to be thinner, say to fit an existing stick framed house, then light straw clay lets you pack insulating straw covered in clay slip into whatever thickness you have.

With either option you can then add several inches of cob to the interior surface to add thermal mass if you like. You can also sculpt the interior cob surface to add more art to the surface. Strawbale with a layer of interior cob for thermal mass was called "balecob" at the Cob Cottage Company in Oregon when I visited them, maybe there's another term for it. You'd add say 3-4" of cob to the inside then finish the surface.

As far as land, the biggest first step I'd take is talking to the building code departments of each county to see what they allow. Montana probably has a minimum level of code, and then each county might have adjustments from there. For example you might ask what the code is for a pole structure (round wood timber framed) and the "infill" permitted, in this case earthen infill for cob. If a department gives you a "what is that?" response perhaps have a few very commercial looking web sites, ideally with some technical, engineering stats included so they understand it's legit. They should have a good idea though. Then it really depends how far from the official code you want to stray.

Luke, if you research this site, there are good notes about buying land.
Basically you ned to be aware of the following points;
- preferably south facing
- possible water supply across land to fill a pond
- neighbours
- potential pollution
- access
- flooding
- water supply , where and what
- soil quality

Putting insulation on the out side in cold places is a good idea, cobbauge (clay mixed with hemp shiv) or hempcrete (lime and hemp shiv) is a good way to keep the heat in the cob walls. Allowing solar gain and keeping it in the house.
Plymouth uni partnered with a French uni to test cobbauge to come up with one of the most if not the lowest embodied co2 building material that meets building regs ever.