cobwood greenhouse?

Hi,
I am wanting to build a 300-400 ft2 passive greenhouse, and I'd love some insight.

I am at 7300' in northern Arizona, so I get a ton of sunshine, wind, and large daily temperature swings. Winters are in the teens at night and 30s-50s in the day. Summers are 40s at night and 80s during the day. Along with extending my growing season, I want to create a space where I can moderate the large temperature swings.

My land is flat and at the bottom of a bowl. I have 6" of beautiful loamy soil and then it's pure red clay. No rocks. Because of this, we get some flooding each spring along with snowmelt. By "flooding", I mean places with standing water, deep mud, and water flowing through all the gopher tunnels that plague my yard. This is more a result of the impermeable clay soil than the water table actually getting this high. Either way - I don't think a walipini is a good option at this site.

Here are my questions, and I'd appreciate to hear thoughts on this!
1. What foundation issues do I need to consider? My county considers the frostline at 30" depth, but with the clay and the seasonal water saturation, is this going to be a problem?

2. Given the local, abundant resources, I was thinking of making a cobwood north wall to provide insulation and thermal mass. Obviously I have all the pure clay I want, and there are huge piles of Ponderosa Pine from all the forest service thinning projects. However, I have found 0 examples of greenhouses made from cobwood. There must be a reason for this - is this a bad idea?

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I think the hot moist environment of a greenhouse could wreak havoc on any kind of structure that included cob.
I have also heard keeping cobwood walls sealed  is difficult, as the wood and cob expand and contract at different rates.
Finally, cob structures need a good roof overhang, something not usually built into a greenhouse.

All that being said, a cobwood wall could offer a lot of mass.
So if you have one on a proper foundation of stone, maybe cover it in plastic on the southside and build your greenhouse against it.
Bring that same plastic sheet over the top of the wall to become the roof of a canopy, so the wall is open to the air, but protected from precipitation and you can extend the canopy as much as you like to form a place for drying firewood , for instance.

William Bronson wrote:I think the hot moist environment of a greenhouse could wreak havoc on any kind of structure that included cob.
I have also heard keeping cobwood walls sealed  is difficult, as the wood and cob expand and contract at different rates.
Finally, cob structures need a good roof overhang, something not usually built into a greenhouse.

All that being said, a cobwood wall could offer a lot of mass.
So if you have one on a proper foundation of stone, maybe cover it in plastic on the southside and build your greenhouse against it.
Bring that same plastic sheet over the top of the wall to become the roof of a canopy, so the wall is open to the air, but protected from precipitation and you can extend the canopy as much as you like to form a place for drying firewood , for instance.

Thank you for your thoughts! This was really helpful and echoed some of our concerns as well. We were planning a substantial roof anyway, and something key to the design would be to ensure that condensation forming on the inside of the roof drains away from the cob wall. I was wondering about the effects of humidity, but I also need to check with people around my area to see if their greenhouses even get that humid. Our climate is so dry that it is hard to imagine a situation where humidity actually becomes a problem, unless I happen to build a particularly airtight structure.

Would a cordwood wall with just cement mortar perform better, as far as sealing goes? Or same problem?

Looking into the sealing question more, it seems most cordwood walls are made with a portland /lime/ sawdust mix.
Cordwood differs from cobwood in this respect.
I think the sealing/uneven expansion contraction problems were probably seen in them.
Here is link to a cobwood cabin build that is pretty informative:
Cobwood Hut - Ran Prieur

Thank you. I wish their blog had an update on how the cobwood hut is faring, beyond the initial build!

I appreciate the multitude of reasons they felt that going with straight cob would have been easier, though several of them don't apply to me (I have an unlimited supply of wood) but I will keep it in mind.

I think I will make a mini cobwood test wall and see how it fares this winter, and I'll ensure it's in the sun and gets the full range of daily temperature swings.