Biochar as thermal insulation

Ben Brownell wrote:Reviving this to see if any of the above experiments and hypotheses have advanced...and to chime in with my own pleasant surprise/success forming a couple of test blocks out of char grit (roughly 1/2" minus sized) and clay slip, inspired by the established chip-clay method for natural wall fill.

I haven't gotten into measuring, testing, refining any further yet before seeing what others might have learned, but have a good superficial result with about 90% char to 10% clay, lightly tamped into a simple 8x8x4" wood form. Looks and feels like it would perform well as form-able fill material with likely good insulation value, and potentially some moderate load bearing if adequately designed. Very intriguing!

Feels like this must have been explored somewhere already? I can readily produce yards of the stuff, and it seems tantalizing like it *could* be an ideal wall system for temperate climate "SIP" building. How wrong am I?!

I'm in the process of enclosing part of a garage to make a sleepout and studio space, and will be using lots of biochar. The interior wall will be slip plus shavings with chunks of biochar, and there will be an earthen plaster with biochar over that. On the inside of the exterior walls I'll just do a thick layer of the plaster for thermal mass. I did a few test pucks with my on-site subsoil and the simple addition of biochar chunks made a massive difference to the density and strength of the cob. Very little shrinkage and cracking with the addition of 1/3 to 2/3 biochar by volume.

I'll start a thread documenting the build as the next phase of the project gets underway. I have all the framing in place and need to get the ceiling finished, then the earth building part will kick off to coincide with warmer weather.

I’m keen to experiment further and think through the supplemental systems that could make this high-char mix work in a few different climates and construction settings, it’s quite promising and fills a void (lol!) in the frugal DIY materials palette. Should play well with a few other standard methodologies too.

Im in a Mediterranean climate and decent insulation is important both winter and summer, plus moderate interior thermal mass, so outside lime and inside earth plasters over 8” of this would probably perform well. Might have to put that to the test next year!

This is an intriguing idea. We're planning a cordwood wall for our cabin, don't have a good supply of sand on site, but can easily make large amounts of char. The one thing I'm wondering is whether the char will do in the cob what it does in soil, and retain water? More than would "normal" cob, I mean. And if it does, what would that mean for the longevity of the wood in contact with the char-cob? Would the risk of rot be bigger or smaller than with ordinary cob? Our place is in a humid climate, but the bottom of the wall will be quite a bit above ground level, and have a decent amount of roof overhang above it.

I'd treat it like any natural building material, Eino. Good hat and boots. If your foundation is well above grade and there's a sufficient roof overhang, then you should have no problems as long as the entire wall system is breathable.

Yep, that's what I was thinking. Thanks! Oh, all of that sand we now won't have to carry uphill! Lovely...

I think the behavior might be rather the opposite, if char has higher affinity for moisture it will draw it from adjacent materials rather than conducting it to them. It will only accept what it can hold, so don't think of it as a source of moisture, just a temporary store. As long as the wall isn't saturated beyond what the char can absorb, nothing else will have to deal with excess. So yes, normal protection from direct water exposure. The only other caution would be to avoid introducing condensation points at or near char-wood contact, such as metal or glass features, flashing etc which could amplify the dampening effect of increased 'breathing' (humidity movement in/out of wall) from adjacent char volume.

That makes perfect sense. Thank you! And good point about avoiding condensation points. It fits with our plans, since we're using treenails, and the window glass won't be in direct contact with the cob.

I agree with Roberto on the recommendation of that book. He goes into a lot of detail about using biochar in cement, insulation and in many other processes.

JohN S
PDX OR