Cob house, insulation and TN

Hello! So I’m starting my journey of preparing to build my own house with self labor. It’s gonna be a while. Before I ask questions tho, let me tell you the plan. Next year I plan on buying land and living in a travel trailer while I build. The whole first year of living there, I’m not really gonna build and just try to source as much material as possible. I’m gonna try to find as much reclaimed as I can and what not and prob do a few small experimental structures to test out the feel and all that.

I was interested in Cob at first tho my dad says he wouldn’t bc if the climate. I’m from Tennessee and it get very humid there. It also gets hot in summer and cold enough in the winter to want insulation. Would con be something y’all would consider in that kind of climate and would adding the extra work of sandwiching cob wall with insulation be worth it?

I’ve also considered earth bermed  earth bag building. However, I would still want insulation, even in the bermed walls. What would be the best insulation for those buildings? Could I maybe add loose cellulose to the earth bag mix? Would that effect the integrity of the mix?

I’m looking in building my cob house in Tennessee as well I want a two story home with a nine sided Centeral area but. Everyone around me keeps saying no to everything I want to do I just want my home 😞  

For cob, I don't think the Tennessee climate would be a big problem - cob is an ancient tradition in England where winters are chilly and damp. Just follow the adage of "a good hat and boots" - wide roof overhangs and a high foundation to keep splashes away from the cob. What is the typical winter day/night temperature range? If you seldom get prolonged cold spells below freezing, cob would probably work fine with the use of a rocket mass heater to offset the average. The thick mass walls with their moisture-moderating effect would likely be beneficial for hot humid summers, as long as it is not typically near 100% humidity. Nighttime cooling would carry over to the daytime and make the space very inviting.

I think cellulose in earth bags would either compress so that gave no real insulation, or be squishy and poor structural performance. You would want something uncompressible like perlite or pumice.

We have built several houses in Tennessee with cob. It works well, but is about as far north as I would go without insulating at least the north side with straw bale.
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Hey Krystal! I'm Jen, moving to Tennessee this summer and thinking the same thing. Let's rap! You can email me at julietmikeecho.com if you'd like!

Doing cob in TN. should be just fine. May want to consider 2" rigid foam panels in the middle of the cob for insulation. I've just finished building a cottage size cob house. I used 2" and 3" rigid foam panels sandwiched in the middle of the cob. Then built a small rocket mass heater inside for added warmth in winter. I cranked up the stove 3 or 4 hours before bedtime and it did just fine all night. Now getting into summer temps. and it stays 5 to 10 degrees cooler inside all day. This is middle Ga. weather. So we'll get several days in the 100's. Curious to see how it does this summer.

What is ..middle Ga? please

GA is Georgia (the US state). TN is Tennessee.

Same here! I'm heading up next week to evaluate the land I've purchased. I'll remain in Florida with monthly weeklong visits up there to build.

Not one that minds roughing it and tent camping, but given the frequent planned trips I have decided to build a small little hut to stay in for my visits while I build everything else out. Would love to swap ideas! I've got a pretty good plan for my initial build and will go from there.

Regarding insulation, I plan on using straw bale but not the full sizes many use for the combo houses but more slices as insulation as I still want cob to be the primary construct.

Ray Sackett wrote:Doing cob in TN. should be just fine. May want to consider 2" rigid foam panels in the middle of the cob for insulation. I've just finished building a cottage size cob house. I used 2" and 3" rigid foam panels sandwiched in the middle of the cob. Then built a small rocket mass heater inside for added warmth in winter. I cranked up the stove 3 or 4 hours before bedtime and it did just fine all night. Now getting into summer temps. and it stays 5 to 10 degrees cooler inside all day. This is middle Ga. weather. So we'll get several days in the 100's. Curious to see how it does this summer.

How did you get the cob to stick to rigid foam?  I thought about that, instead of bales, but keep figuring it would just fall away from it.  How thick were the walls on either side of the rigid?

Mary, if I understand Ray correctly, he didn't worry about getting the cob to adhere to the foam because he essentially built two separate walls. That would probably work, but it seems like a lot of labor. Also, although foam has a fantastic R-value, it sure is toxic to produce.

I agree with the other posters (if the Original Poster is still following) that the TN climate would probably be fine with plain cob, but insulation remains one of the greater challenges for the technique. I'm a lot further north than TN and scratching my head about it. It doesn't seem there's a natural and affordable way to stick together a highly insulative aggregate for a thick plaster, not without resorting to cement or some other chemical binder. Foam, either rigid or spray, has toxicity issues, and also rigid foam plays poorly with curvy walls. At the moment, I'm considering Rockwool batts, and what I'm thinking of would go something like this:

*Bed rust-resistant wire ties in the cob every foot or so horizontally, at the height of the width of the batts. So if the batts were 2' wide, I'd put these ties every 2' up in the wall.
*Roll out the batts horizontally around the building and affix them back to the walls with thin vertical members (saplings, bamboo). So you'd have a sandwich of cob wall -- Rockwool batt -- vertical sticks holding the batts in place.
*Plaster. Someone on a different forum suggested it was possible to plaster directly to Rockwool, but I suspect he was speaking of the rigid boards, not the flexible batt form, which is what you'd need to deal with curvy walls. So in all probability, you'd need to wattle between your vertical members with more saplings/lath/Phragmites/bamboo strips, or else use chicken wire or metal lath, though metal is prone to rust, high embodied energy, and, at least here in the US, recently much inflated in price. You'd carve out the insulation around the windows and that would probably have to be chicken wire wrapped. But at any rate, you'd be making yourself some sort of base for the plaster. Since you'd want to plaster anyway, the wattle frame would be the only extra work added this way.

Since the batts are relatively soft, I suspect you'd still get more cracking in the plaster than you would by plastering direct to the cob, but it's the best I can think of. Anyone comes up with a better idea, do tell!

I would consider a layer of light sraw-clay for modest insulation needs. Build a cob wall as thick as structurally necessary but no more for the inside, then apply a layer of LSC which dries rigid, then plaster over that. The LSC can be sculpted as desired for the outside form.

Ye-esss, but that would be for quite modest insulation, I'd think. To get a sculptable LSC, you'd need quite a lot of slip, no? Which is going to come out fairly low R-value. Low-slip LSC falls apart without compaction in forms, and by the time you're building forms and compacting, it doesn't seem like any less work than a wattle plaster panel with infill. And while either (or neither) would probably be adequate for a warm-temperate humid climate like TN, NC, VA or that region, once you get into a bit more winter ... I'd think you'd be burning quite a lot of wood. Vermont's Residential Energy Code wants to see wall insulation of at least R-25 (or R-15 with interior mass; ceilings R-49). Low-density LSC such as would require compaction and forms to install comes out around R-1.6 to R-1.8. So that's a minimum of 8", and only if we were talking fluffy low-slip LSC, which I've found cumbersome to work with. The kind of high-clay LSC you suggest for sculptural ability is closer to R-1 ( http://www.designcoalition.org/StrawClay/research/rvalue.htm ). An additional 15" of material gets overwhelming pretty quickly. And in a climate so humid as the US East, I'm still not convinced that any high-straw material won't have later issues with mold, especially one applied so thickly as to slow drying. Clay helps with that, but only to a point.

I *want* straw to have a better R-value! The same issue seems to arise with perlite or pumice aggregate plasters: adhesion requires a high proportion of binder, which lowers the R-value prohibitively in cold areas. I don't like the embodied energy of Rockwool either ... but fire-proof, moisture-proof, bug-resistant, and less toxic than foam or fiberglass, that seems to add up to the best combination I've seen. Straw of course would beat it hollow on embodied energy, but does that make sense if your heating energy costs are greater every year the building is inhabited?

Have you tried LSC in upstate NY, Glenn? Any performance results you can share?

I agree about the toxicity and would prefer to avoid at all costs.

Ran across a site the other day that had hemp (hurd/fiber) batting available.  Still a tad expensive but worth keeping an eye out for.  A good possibility for roof insulation.

I haven't used LSC yet; I am building an addition where I have plans to use it. That may or may not happen, as my wife is pushing for faster construction and to be done with projects.

Oh, well, you know the adage:
"Man finishes house. Man dies."

LSC can work as infill in cold climates, I think, but it needs good airflow on both sides to dry in the humid East, or else you get the black fuzzies. Probably there's a thickness limitation as well, exactly what I don't know. I've never heard of anyone putting in more than 6" (150 mm) at a time, and that between studs with both faces drying clear. As a plaster on cob, with only one side exposed, I don't think it would work more than about 3" thick, in a droughthy summer. Which would be fine for TN, as the original poster questioned.

The stuff really does like to spring apart as you apply it unless you whack it good though. One still might need a form to compact against. A rough chop might help too, which isn't necessary when it's smushed between forms. But last time I used it, when we got the forms to the top few inches of panel and had to cram it in sideways without being able to compress from above, we had trouble getting an even application.

Good luck with it and let us know how it goes!

Angel Richardson wrote:I’m looking in building my cob house in Tennessee as well I want a two story home with a nine sided Centeral area but. Everyone around me keeps saying no to everything I want to do I just want my home 😞  

I know, sweetie. It's frustrating. I'm right there. N I know it'll get more obnoxious as I actually start to get into it and am told I can't do anything where I'm living. By ignorant people. And I totally anticipate it. So I'm considering moving where I can do it. You know northern TN has a workshop for cob. But I dislike that area. 🙄 I'm willing to hybridize my house to a degree that works for me and whoever is overseeing the build. But it depends. If something doesn't make sense to me I'd rather stay in a trailer until I can get what I want. Like a nicer airstream or something. But tbh those aren't safe. Even n a valley.

OK what is LSC?

John C Daley wrote:OK what is LSC?

LSC I believe is Light Straw Clay.
Meaning straw that has been lightly coated with a clay slurry so that when rammed together in a form will retain its shape and act as insulation.

yes LSC is light straw clay, which is a very useful building method with like ten different names, but then again also at least  ten distinct ways of using it to build.

slip straw, light clay straw, light straw clay, slip form with straw, these are also common names for this.
it is also related to slip form masonry, wattle and daub, or some people have experimented with using wood chips, wood, or smaller gravel and stone, using the basic slip form method of building.

in light straw clay building, you use plywood boards as a movable form, generally on either side of framed in wall, interior walls, or between posts...and then stuff in straw covered with some slip clay, tamped down real good. after some time to dry, you remove the form and continue upwards and along...

more info ---> https://www.greenhomebuilding.com/QandA/cob/strawclay.htm
or -> https://theyearofmud.com/2016/09/28/light-clay-straw-wall-building/