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Cob - Does it work in central TN? Does it mold?

Posts: 95
Location: Central TN
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Trying to find a low cost natural way to build, I've read good stuff about Cob and am working on a book now, but I've also ran into local homesteaders who swear its a bad idea and have firsthand experience. I know they might have done it badly but on the offchance they did not i want to discuss and hopefully learn from others who have done cob preferably even in my area.

One person said "t will dry slowly and re-absorb moisture any time the cob is at or below the dew point. Indoor relative humidity above 60% for any length of time is sufficient to support mold growth, so the cob need not be "wet". Surface moisture percentage will remain at around 15% in cob or wood here without HVAC (these are my empirical figures, but you can plot them out on a psychrometric chart to see what that means for the moisture exchange happening in your home at given temps (http://web.uconn.edu/.../NewFiles/psychrometric_inset.html) . In our experience, the cob sported mold before the timbers. Since borate treatment, mold has abated on those surfaces and taken up residence in books, furniture, clothing, cardboard, stone, mold resistant drywall (ha!), etc. There's a MUCH more dynamic process conspiring to rot your home and sicken your air quality than thinking about it in terms of one component not drying summarizes. It's a cyclical thing requiring constant vigilance. That's why modern HVAC systems are king. They are on constantly. What power source do you have in mind for these buildings?

When you say cob, what wall technique specifically? If you mean solid, I see a host of issues, not least of which is the sheer labor of excavating, sifting, testing, and moving that many tons of labor intensive material, let alone the many hours of skilled labor to apply and finish with a good air barrier while achieving properly engineered bearing.

So much literature on these techniques comes from the desert southwest and the Rockies. We're from Colorado, and please, take my word for it, most current info is wrong for this climate. In addition, much literature is simply wishful thinking (check out air quality, temperature control and real-world durability of Michael Reynolds' Earthships. We've lived there. It isn't pretty. Marketing looked great, though. He now claims he's adopted contemporary techniques for reasons of code compliance and lending standards. It's because he got sued by seriously dissatisfied occupants.

People have been using light framing with insulative fill in climates like ours here for centuries, however. Tall, voluminous houses are a time honored technique for hot humid zones, a design largely impossible with what you might be referring to as cob. Timber will create greater moisture latency than masonry or clay. Thermal mass achieved with higher density inorganics like the latter will give some thermal latency, but a late July heatwave will pretty much neutralize the effect or worse for the remaining two months. Likewise, winters here are significant (and damp) enough to necessitate planning latent stability there, too. In our climate, vapor pressure switches direction back and forth several times a year, hence another reason southwestern, high mountain, or even midwestern designs struggle in TN. Likewise, tropical solutions don't fare well here, either. Light framing with omni-directional vapor permeability seems to be a good route here. Alternatively, closed cell non-permeable, and/or non convective solid fill is good, too (spray foam, polyiso, adhesive treated pressure blown cellulose, etc.). Fiberglass? Not so much. Vapor barriers over fill? Nope! Thermal mass centered well inside the building envelope works pretty well here, as opposed to building it into walls, or even floors.

We built a 200sqft cabin to start, with five kids, for $1900 plus $1300 in PV gear and batteries. Water catchment from that structure met our domestic needs completely. We made a few mistakes in that structure that wouldn't have cost more to get right the first time, we just needed more info. All in all, a pretty comfortable little place, though. We occupied it for 2 1/2 years while we built the $10k 720sqft mansion we live in now. The cabin is our attached solarium, so 920sqft, really. With a good plan, no phase has to go to waste, even if it's not what you want in the end. Take comfort there! It doesn't have to be perfect now, but the plan should be well understood and each phase should teach you to adapt to new info. I hate to say it, but if you commit to understanding just what I've said in these comments, you'll be leaps ahead of the people writing books and blogs about owner-built houses."

The other said he had to repair it often and it wasn't performing well or as expected.

I'm curious and want to get more experienced advice on this topic! Don't have much funding and need to get multiple structures built for a full intentional community with 5+ families.

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Location: Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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First off, since cob was used for many houses in England, Scotland and Ireland, and these houses are still being occupied today, If your stated hypothesis is correct, how can this be so?
All houses require regular maintenance, without it the modern house tends to remain standing for less than 10 years.
Where I live there are five abandoned houses, all modern, fairly well built, with the five years of total neglect they have endured, they are falling down, roofs are caved in and fungi have started the dismantling process quite well already.
It is also good to note that rain forests are not good places for either cob or straw bale construction, and the US North West is indeed rain forest, just not the tropical kind.

Yes, cob takes time to dry, however it rarely goes up at a steady, day by day pace.
The normal pace of build is lay cob for a day or two then a two to five day rest, sometimes it will be weeks before the next laying of cob occurs.

In the southern US, our main obstacle is the high humidity, that is why the first settlers here mostly built plank wall cabins, some of which are still standing and occupied.
The modern desires obstacle is harder to leave behind than the building techniques are, we all desire our comforts, what ever those might be.
Most of the people I hear about having problems with decay (all sorts) have built too tall, too fast, which slows the drying time.
Many have neglected to use a proper double coat of plaster inside and out as well, which leads to disaster.
If you live in a humid environment it is wise to add borax to your cob mix, that will stymie the molds, also you might need a bit more straw in the mix than other places for better strength.

Any alternative building method will be labor intensive, these are methods mostly from the middle ages, where labor was neighbor helping neighbor.
This also means they are not up to "Modern man standards of comfort for living". Usually I end up telling people who think a cob or straw bale house will be "neat to live in" that they really need to sit down and think hard about living in a tent for a year first.
Then they will know what they could be facing to follow their "dream".
Anyone having huge amounts of doubt has either gotten into the overthinking pitfall or the modern is far better pitfall, both are going to lead to dissatisfaction with the end product produced.

Cob and straw bale can work in most regions of the world when properly thought out and possible issues prepared for.
These are build it your self projects which can bring loads of either satisfaction or anguish to the builder.
Both can be quite comfortable when done right. Cob houses built in England and Scotland are usually built and the fire lit for weeks prior to the interior finish plaster application, eaves are very long, overhanging enough that rain hardly ever hits the plastered exterior.
fire places are large and in winter they are kept hot enough for the interior to be quite comfortable. And this is in a region that sees a lot of rainy weather both summer and winter along with the high humidity of island living.

For AR, TN, MS, GA and the other southern states, it can work but you do have to plan every step accordingly. Get that roof up, the walls and fireplace built and soon as you can get the fire going to drive the moisture out before you start the plastering.

I hope you find the method best for your personality and desires.

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http://www.cobcottage.com/ Cob Cottage Company is in Coquille Oregon, which gets 55-60 inches of a rain a year, and plenty of humidity. They developed the Oregon Cob mix, and have been living in their cob structure for 15-20 years. They have a book called The Hand-Sculpted House which explains how they are built and covers issues with weather and how to account for it.

Cob isn't the answer for every site/conditions but it works very well in many areas, not just arid southwest. My guess is that if a person does a bad job building it, they will have lots of problems down the road, just like poorly built stick houses.
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