Hello,
I am an Alabama resident looking into the possibility of building my first home with cob, and I am very excited about the idea of doing so. However, I have several questions and concerns. Firstly, as Alabama is humid and hot, is cob (or perhaps straw bale cob) feasible? I’m concerned about curing time as well as general retention of heat and difficulty cooling. This leads into my next question: can conventional AC be used in a cob home? I am a very warm natured person and unfortunately require air conditioning in the warmer months. To accelerate curing time, could dehumidifiers be used, or could this cause the walls to dry too fast and crack/reduce structural integrity in some way? Lastly, I have read that cob must be able to breathe or it will disintegrate. Does this permeability result in a damp/humid interior space, damp/musty smell, and/or encourage mold growth? Thank you for your time, and I look forward to hearing from you.
Warm regards,
--Nick
Dear Nick,
Earthen homes have been the norm in most of the world for most of history. In the cold wet Brittish Isles where my ancestors come from most of the homes were made of cob up until the commercial marketing push that came with industrialization shamed everyone into abandoning their "primitive" but much more comfortable and economical earthen homes for the more fashionable (and breezy and cold) brick homes. Up until the advent of modern insulation cob homes were among the most comfortable just because the monolithic walls didn't let the wind through all the cracks like log or brick walls did.
Clay walls also slowly absorb and release humidity as air passes through them, which tends to protect buildings from mold and rot and the health problems they cause. Since we are almost always making indoor humidity artificially low with a woodstove or adding excess humidity with a pot boiling or a shower going or just breathing, permeability tends to even out humidity, which most people find more comfortable.
Cob works. Straw bale works better in the southeast. Cob works as a battery to stabilize the building temperatures and humidity toward something more comfortable to humans than the extremes we experience in Nature. In the southwest you get regular almost daily sunshine and cool nights so your thermal battery only has to keep things temperate through a twelve hour swing or at most a few cloudy days. In the east where you and I live it can go weeks or a month some times without a summer heat or winter cold spell breaking, so the thermal battery works against you so that even when you finally get more temperate weather your house will take a long time to heat up or cool down.
For a small house this doesn't matter much if you have grid power. The thermal battery tends to keep things comfortable with a minisplit or small woodstove and air conditioner. We have come into my friend's little cob house when it was 113 degrees in the shade to find her little window air conditioner chugging along sucking the warmth and humidity out of the walls keeping it about 80 inside for a few bucks a month. In the winter she used less than a tenth the firewood her neighbors in new modern singe-wide trailers used.
With a larger house the thermal bleed through the mass walls is more telling, especially if you are offgrid. It will still always be more comfortable to have a few inches of thermal mass slowing temperature changes through the day or between wood stove firings or even just when you leave the door open to bring in groceries. Straw bale or balecob or some other breathable insulation helps slow down the thermal transfer and humidity transfer between indoors and outdoors saving you a lot of energy over time. The larger the building envelope the larger the savings.
Good passive heating and cooling techniques also make a big difference--Shade trees that lose their leaves in summer, coniferous trees that block the north wind through the winter, porches and hanging vines and carefully calculated eaves that block the summer heat but let in the winter sun, even buried air tubes that use the earth's more steady temperatures six feet down to temper and and dehumidify the air that comes into the house. Just building with the long face of your house within 30 degrees of south is supposed to save you an average of 13% on annual energy bills all other things being equal.
Regarding speeding up the building process, a dehumidifier could be used, but it wouldn't save you much time. When I am in a hurry I rent a big "torpedo" construction heater and place it a few inches from a cob wall (a few feet from plastered straw bale) and watch it "turn to rock" in front of me. Cracks are normal in the drying process however naturally it happens. They get filled in with plaster.
We have been in Alabama plastering the inside of a straw bale building over this last winter and I must say your winter has its moments but it is much kinder than ours in Cleveland. Best of luck. I look forward to seeing what you build. Be sure to send pictures.
--Uncle Mud (aka Chris McClellan)
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