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

Dustin Nemos
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 ( . 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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