Cob Building is Not Appropriate For This Cold Climate

There’s something I’ve had to come to terms with living in our cob house, Gobcobatron. Cob is not the most appropriate, responsible building material for this cold climate of northeastern Missouri. This applies to similarly cold climates as well. Sad to say, but that is the reality.

Here are my thoughts on why cob building is not appropriate for cold climates, based on personal experiences and understanding more about the thermal properties of cob.

Cob is Not Insulative, it’s Massive

Let’s get this out at the forefront: applying an R-value (insulation value) to cob doesn’t even make sense — it doesn’t register. It is not an insulative material — it is thermally massive. And by the way, thick walls do NOT automatically equal warmth or insulation. Cob itself will never be insulative.

Think of thermal mass like a storage battery — a massive wall is always working to achieve thermal equilibrium. Have you ever walked barefoot on hot pavement in the summer sun? That’s the pavement’s ability to absorb thermal energy at work. Cob works the same way. And in the cold, a cob wall will eventually conform to the ambient air temperature and have a hard time budging from that.

On the other hand, insulation is the reduction of heat transfer from one side of a wall to another. Insulative materials are the opposite of massive materials — they’re lightweight and have dead air space contained within them, and they serve as a buffer to heat and cold.

In essence, my cob house has effectively zero insulation. The roof (if you don’t count the soil, which I think is fair) does not have insulation, nor does the foundation. The walls are 100% cob. Practically speaking, it’s pure mass from floor to ceiling. However, that does not mean that a cob house can’t be built without insulation.

You can include insulation in an all-cob house in the floor, foundation, and roof. So wouldn’t that give cob houses more of an edge in a cold climate? Sure. But I’m still very doubtful that cob is actually a responsible building material in locations with cold, cloudy winters. As a note, where I’m located (northeastern Missouri) is USDA planting zone 5, with max winter lows clocking in at -20º. Although that is not common, it can and does happen. Our typical low temperatures in the coldest months hover between 10-20º.

Heating a Cold Cob House = Heating a Fridge

Even with insulation in the floor, roof, and foundation, heating a cob house is a constant battle against cold outdoor temperatures. The mass is totally exposed to ambient cold air temperatures more than it is exposed to warm indoor air. There is a great imbalance. It’s like trying to heat a refrigerator — the cob wants to be cold because that is what it is most exposed to. (Think of all of that surface area of the exterior face of the walls.) To make matters worse, if winter days are frequently cloudy (as they are here), the cob will have little exposure to the warming sun. This means that you’re not going to be warm without firing a wood stove very regularly (basically, constantly).

Yes, passive solar design helps, but it only really helps if you have insulation in your walls to slow down the transfer of heat/cold. And abundant sunshine. You won’t really be capturing heat because the walls/floor are likely so cold to begin with that the sun will only perhaps prevent temperatures from dropping further. And again, if it’s cloudy half the time, the sun will only be able to work with you half the time to warm your home.

Plain and simple: heating a cob house in a cold climate with insufficient sun takes too much wood to be considered ecologically responsible. My original theory was that building my home with as little embodied energy as possible in the first place would give me something of an edge as far as the total energy required to build and then maintain the building.

I also thought that because the house was so small a large mass heater would make all the difference. Now I know that to be very flawed thinking. Using little embodied energy in construction is crucial, but I think you can still do that AND create a building that requires less energy during its lifetime of usage.

When warm indoor air meets a cold wall, you get this: condensation. In this case, the walls are so cold the condensation froze… which is more or less impossible to get rid of until it warms up. It also molds after enough time. Problem? Yes, big time.

Cold and Condensation

Oh, and that’s not the only issue either. Condensation is a major concern in situations where you have warm air hitting a cold surface. In this case, warm air generated by a wood stove that makes contact with cold walls is an extremely favorable situation for condensation to form. We have suffered greatly from condensation issues in two of our winters living in Gobcobatron. The walls on the north and west, just above the foundation registered as low as 35 degrees Fahrenheit most of the time!

As we generated warm air with the wood stove, moisture formed on the surface of the wall, and because it was so damp, it never had opportunity to dry out. Then the mold came in… and it needed to be constantly wiped down with vinegar. That is not a healthy living situation by any stretch of the imagination. Believe me when I say this was an intensely stressful living situation.

Cob = Not For Cold Climates

It just doesn’t make sense. Cob is better suited to mild climates, where it either rarely (if ever) freezes, or places with abundant and reliable sunshine in the wintertime (like the southwest). Northeast Missouri is not that place. I strongly advise against building with cob in climates like this. And huge swaths of the US fall into that category.

This is a reality check — there are better choices for building in cold climates. Straw bale is a much wiser choice, for one. You’ll be burning a lot less wood during your building’s lifetime if you have adequate insulation, and you’ll be able to maintain a healthy living environment free from condensation, moisture, and mold, too.

(Updated: 10/2013)