Anyway, I almost feel like somebody is trying to tell me something as I was planning on building a cob home already, but was a little worried about the low insulation value. Kind of a odd coincidence.
I have read online a few places where they recommend adding Vermiculite to boost cobs R value but I haven't seen any info on someone actually doing it. Does anyone here know what the ratio of Vermiculite/cob should be? Would I replace the sand for Vermiculite, or still add the sand as well?
Think I'm gonna make up some test batches this weekend and see if it affects the strength of the mix, since I plan on my walls being load bearing.
Thanks for any info!
I would venture a guess that you might have issues with the vermi-cob attracting and/or holding moisture once it is cured as well. If you have a lot of moisture in your climate the cob may weaken. The clay component of the cob is raw (not fired as brick) so if the vermiculite still absorbs moisture it could affect the clay as well. But only some testing of vermi-cob will tell.
And Pahanna also touched on another good point, cob (or any mass for that matter) is better completely surround by an envelope of insulation. I get the appeal of adding insulation right in the cob mix, as I still have trouble visualizing a completely natural, superior in dealing with moisture and pests, envelope of insulation myself.... but I'm still hopefully looking.
After you do your experiments please post back so we can all learn along side you
Bulk Up & Add Mass
Plan to incorporate some kind of exterior insulation into your cob home. Near the end of our inspection process, our inspector questioned the insulation value of our 12- to 16- inch-thick (30–40 cm) cob walls. I had been sure they would be adequate, so I was shocked when I discovered their total R-value to be only R-3 or R-4 (about R-0.25 per inch). This was unacceptable, so we had to find a way to insulate the outside of our cob home if we wanted to get our final certificate of occupancy. To help boost the R-value of our home, we ended up painting the outside with a new-fangled coating called Nansulate, which uses itty-bitty ceramic tubes that effectively trap air to slow heat transfer. Neither my wife nor I were thrilled with the idea of painting the gorgeous exterior of our home, but the practical results were eye-opening. The combination of our high thermal mass walls now surrounded with insulation remarkably improved the energy efficiency of our home. http://www.cobprojects.info/Projects/NC/Hren_112_Final.pdf
Vermiculite is heat treated mica. It "explodes" into little crumbly accordian-pleated bits. As someone has reminded me, vermiculite soaks up something like 3 times its weight in water, so it would be a Very Bad Idea to add it to a cob slurry, I would imagine.
In fact, given its propensity for acting like a sponge, I'm not sure it would be all that good an idea to try to use it even as cavity fill. You'd have to be extra certain-sure to protect it from moisture encroachment.
However, perlite, the white styrofoam-like stuff, is just about waterproof. You could use it as a cob additive, but you would have to mix it pretty thouroughly as it has a tendency to "float out". It is very, very lightweight.
> What about this? A full cob wall 2-3' thick - straight cob. Add a 6" layer of vermiculite laden cob to the outside as a very thick outer coat? or even a foot if you like...
Make it perlite, and do some testing with it as both an inner plaster and an outer layer. See how it adheres to the rest of the wall.
I don't know what difference it might make to put it on the inner wall and the outer - does someone have an idea and an explanation of how it might make a difference, heat/cooling-and-perceived-comfortwise?
Sounds to me like an idea worth investigating.
Things to be considered include how it adheres to the rest of the wall, the effects of weathering (if you slather it on outside), how well the perlite will "stay in solution", how much perlite to use per wheel-barrow load of cob (or whatever quantity you are mixing up at a time), how expensive is perlite in your area and can you buy it bulk? If you put it on inside, is perlite at all flammable and if so what sort of gases might it release? (What IS perlite, anyway? I know where vermiculite comes from but not perlite, and I've used both in potting soil mixes . . .) http://www.deatech.com/pipermail/coblist/1999/002368.html
In a former life I was a plasterers mate It got me through university.