Biochar as earthbag insulation

Anybody have any experience with this?

We're planning on building a strawbale house on an earthbag stem wall. Climate is temperate so nothing too extreme. Pumice, volcanic rock, scoria etc are way too expensive to be an option around here.

I've started making biochar out of the invasive trees we've been clearing. I made a char heavy clay plaster to give the pizza oven a fresh coat and got me thinking -- would the char work as insulation in a highly compacted earthbag stem wall?

What are the potential drawbacks? Char absorbs water, so moisture wicking would be a factor I imagine. Probably wouldn't work on it's own, but might be helpful as an additive? Thoughts?

From what I've seen, biochar absorbs moisture aggressively and holds it tenaciously. Once wet, it takes heroic measures to dry it out. That's part of why it's so awesome in gardens.

Given that, to use biochar for insulating purposes I think it would have to be enclosed in a seriously moisture resistant envelope. I imagine it could be done, but personally I would want the design to have room for Plan B.

Thanks Douglas. Moisture is my primary concern too -- especially considering the walls on top will be straw bale. I suppose a moisture barrier wrapping the char filled bags could work, but I'd like to reduce the plastic used in the build too.

Maybe I'll stick to experimenting with char in the plaster instead.

I am not so sure it would smell good either.

My fire[place in this old Victorian was taken out in the 1980's sometime. On damp days I can still smell the soot from wood fires kindled 40 years ago. I would think the bichar as insulation would continuously make your house small like woodsmoke as the moisture migrated inward.

I am not sure you would want a house permeated with that smell.

Of anyone who has had a partial house fire where the insurance company did not condemn the house afterwards, one of the complaints was always having that smoke smell on damp days.

The last thing I would want is to go out for a nice meal with the wife and have people in the restaurant sniffing the air because we smell like wood smoke and became nose blind to it. I think that would defeat an otherwise nice date night.

I'm putting biochar into light earth infill and so far it's going well. I'm using big chunky stuff, 1-5 cm mostly. It is bone dry to start with, but it's absorbing some of the water in the clay slurry as we mix and tamp the material.

Steve Zoma wrote:I am not so sure it would smell good either.

My fire[place in this old Victorian was taken out in the 1980's sometime. On damp days I can still smell the soot from wood fires kindled 40 years ago. I would think the bichar as insulation would continuously make your house small like woodsmoke as the moisture migrated inward.

I am not sure you would want a house permeated with that smell.

Of anyone who has had a partial house fire where the insurance company did not condemn the house afterwards, one of the complaints was always having that smoke smell on damp days.

The last thing I would want is to go out for a nice meal with the wife and have people in the restaurant sniffing the air because we smell like wood smoke and became nose blind to it. I think that would defeat an otherwise nice date night.

I've not noticed a smell from the biochar I've made, although I've kept it all dry until it's added to the plaster mix. Certainly something to consider. As far as I understand, properly charred char has all the smelly stuff charred away.

Phil Stevens wrote:I'm putting biochar into light earth infill and so far it's going well. I'm using big chunky stuff, 1-5 cm mostly. It is bone dry to start with, but it's absorbing some of the water in the clay slurry as we mix and tamp the material.

This was going to be my next experiment with char as building material. What were your reasons for adding char to your mix? Insulation, weight, or something else?

Mike Harris wrote:
This was going to be my next experiment with char as building material. What were your reasons for adding char to your mix? Insulation, weight, or something else?

All of the above, plus tensile strength, humidity control, and the principle of sequestering carbon within the structure. I've spent the past two days (with help) and have most of the interior wall packed and tamped, and so far we have used nearly half a cubic meter of biochar. When I have a few spare moments I'll start a thread detailing the project.

Steve, properly made biochar has no detectable odour and instead is good at absorbing and neutralising smells. When charcoal has a smoky aroma that is a sign that the production temperature was insufficient to convert all of the volatile hydrocarbons. I have a story about a biochar retort design gone wrong and the sweet smell of failure that I'll turn into a thread soon (one of my holiday projects is to document some of this stuff).

I'll add a link to an earlier thread of relevant discussion here:  https://permies.com/t/93192/Biochar-thermal-insulation

Interested in tracking more experimentation on this! I would also point out one potential issue is the degree of bonding between charcoal and soil/clay or other fill materials. For earth bag/rammed earth you need pretty good binding compaction, or else the strength of the wall is merely the strength of the thin plastic sack material it's packed in (plus any plaster coat). Charcoal can be pretty dusty/flakey and prone to de-stick-ifying earthen mixes. Definitely run some small experiments first and play with both the proportions and the sequencing of addition. I think it's possible to make it work, but be careful if the entire house is literally relying on it!

Phil Stevens wrote:

Mike Harris wrote:
This was going to be my next experiment with char as building material. What were your reasons for adding char to your mix? Insulation, weight, or something else?

All of the above, plus tensile strength, humidity control, and the principle of sequestering carbon within the structure. I've spent the past two days (with help) and have most of the interior wall packed and tamped, and so far we have used nearly half a cubic meter of biochar. When I have a few spare moments I'll start a thread detailing the project.

Steve, properly made biochar has no detectable odour and instead is good at absorbing and neutralising smells. When charcoal has a smoky aroma that is a sign that the production temperature was insufficient to convert all of the volatile hydrocarbons. I have a story about a biochar retort design gone wrong and the sweet smell of failure that I'll turn into a thread soon (one of my holiday projects is to document some of this stuff).

Interesting! Definitely keen to read about your experiments. Do you find the addition of char slows down the drying time significantly?

Ben Brownell wrote: I would also point out one potential issue is the degree of bonding between charcoal and soil/clay or other fill materials. For earth bag/rammed earth you need pretty good binding compaction, or else the strength of the wall is merely the strength of the thin plastic sack material it's packed in (plus any plaster coat). Charcoal can be pretty dusty/flakey and prone to de-stick-ifying earthen mixes. Definitely run some small experiments first and play with both the proportions and the sequencing of addition. I think it's possible to make it work, but be careful if the entire house is literally relying on it!

Good points Ben. Certainly wouldn't be putting too significant an amount in the earth bags due to structural concerns. Although char's superior water wicking ability rules it out for this application in my scenario anyway.

My first experiment with char as building material was giving the pizza oven a replaster. I have a fresh batch of site soil (some sand, some clay, mostly gravel) that I wanted to test anyway, so nice excuse to fling some mud.

I found that the more char I added (up to 50% char/sand + clay + water, no fiber) the fewer cracks in the plaster. I kept all other ratios the same for the experiment. It was a bit weird using such light (in weight, not colour) plaster.

Mike, I don't have a comparison section to assess any difference in drying time. It's been two days now and I'm just starting to see dry shavings on the surface of the wall. My friend who is an experienced earth builder and did his entire house with light-earth infill estimates 10-14 days to dry in this weather (early summer, and for now a dry spell). I do know that when I formed some test pucks with the subsoil and varying amounts of biochar that there was no appreciable difference in the time to fully dry.

The material I'm using is mostly dust free since I sieved it at the point of production to make three grades: big chunks for the infill, medium (2-10 mm) for the base coat plaster, and fines for underfloor. I think I will leave it out of the finish coat of plaster because I want a light colour. The chunks are irregular and have nice keying surfaces, so as the mix dries I'm expecting a good bond and contribution to overall strength.