Biochar as thermal insulation

I didn't see much of anything here about using biochar as thermal insulation in natural building, so here's a link...enjoy!

The Use of Biochar as Building Material:  https://www.biochar-journal.org/en/ct/3

Great stuff, Greg.  I had been contemplating methods for building a shop, at some point, using a rammed clay-straw infill in the timber-framed or post and beam walls.  I think biochar would be a great addition to the mix, particularly considering the insulation that is needed in my climate, the need to regulate high and low humidity at certain times, and the greater need to sink more carbon into permanent stores.

Correct me if I'm wrong, but wouldn't charcoal work just as well and you'd avoid having to introduce all the "bio" to the char?

You are correct Mike.  I keep forgetting that different people have different definitions for biochar.  For some biochar is charcoal that is made under conditions such that the charcoal is ready to accept biological inoculation (tars burned out, high surface area, good balance of pH and CEC, etc) which comes from getting to the correct temperature when forming it.  For others biochar is not biochar until after the inoculation.  Thank you for that clarification.  I think inoculating it for this application would likely be counterproductive.  

I go with the IBI definition which does not indicate inoculation has occurred, but they do indicate that it is produced with the intent of adding it to the soil.  Since houses will ultimately break back down then use in the wall will ultimately end up in the soil, so I thought it wasn't too bad of me to use that term here :)

This seems like a very good idea to me.

If you go about halfway down the page on this link so, you will see some ideas on the subject.

https://permies.com/mobile/t/19303/Dale-Marvellous-Inventions-Adaptations

Thank you for that link Dale....great stuff!

Based on the article and discussion seen at the link provided.
It would seem that it is best for odor control and humidity control.
And is wonderful for internal walls but not so much for insulating external walls.
It can be added to the outside of a house as a "facade" that stores carbon vs letting it enter the atmosphere to become a greenhouse gas.

To me seems to be as insulating as cob, not horrible but not the best for wet, humid cold Maine.
If the walls are thick enough say 24inches. It is still doable.

Based on the article and discussion seen at the link provided.
It would seem that it is best for odor control and humidity control.
And is wonderful for internal walls but not so much for insulating external walls.
It can be added to the outside of a house as a "facade" that stores carbon vs letting it enter the atmosphere to become a greenhouse gas.

To me seems to be as insulating as cob, not horrible but not the best for wet, humid cold Maine.
If the walls are thick enough say 24inches. It is still doable.

That's not what I got from Greg's linked article, at all.  Here's a couple quotes from the early part of the article:

These properties mean that biochar is just the right material for insulating buildings and regulating humidity.  

 

Biochar can be applied to the outside walls of a building using standard plaster spraying or rendering equipment. Applied at a thickness of up to 20 cm, it can be a substitute for Styrofoam. Through the use of biochar-based insulation material, houses can become very long-term carbon sinks,

It might not be the best insulation for really wet climates... I'm not sure.  They do recommend a slip coat of something without the char to seal the outside of exterior char/clay walls.  I think that it would be a great insulator, considering the extremely large internal surface area that these particles have.  This means that the char particles have isolated air pockets, which in insulation terms is gold.

I read through Dale's article as well.  He seemed to waver a bit on its insulating properties.  

Maybe I'm getting this part wrong.... Dale, at one point you said (paraphrased) that a char/clay brick would be a waste of time and energy when it would not be any different than wood.  I doubt that.  Wood, even light airy wood like cedar, is thousands of times denser than biochar type charcoal, and even when the char was compressed with a clay slip, I think that it is going to be extremely light.  Of course this would assume a certain ratio of char to clay.

Also in your article, you talk about the thermal conductivity and other properties of carbon, mentioning graphite and diamonds...  But those materials are again, many many times denser than charcoal.  To say that a diamond isn't insulating and then to correlate that char isn't insulating because they are both made of carbon seems rather strange to me.

I actually think that the char house would actually work in a damp cold climate.  I'm assuming here, that a house that has char in its walls for insulating from external temperatures and moisture likely will also have an internal heat source in a climate like New England.  The dampness might try to go in the walls but the char's qualities seem, as indicated by Greg's linked article, to result in the moisture being drawn from the walls when the interior of the space is dry (like when it is heated by a wood stove or RMH, for instance).

These walls are cob walls not strawbale walls that have been blacken/pyrolyzed/enhanced.

Cob is made from clay and straw/biochar.
And while strawbale/biochar is a great insulator, sadly cob is not, In fact 12inches of cob has a R3 value.

These walls that are being explained are cob walls were the plant fiber has been replaced with 'biochar'.
Clay/Cement is not an insulator, and the water that is trapped in the 'spongy' biochar is not an insulator.

Even strawbale walls which are 'good insulator' have a thickness of 18inches.
So yes a 18inch or 24 inch biochar-COB might be okay. But we are still talking about 4x normal thickness wall.

I also find it very telling that the examples that they gave was
(No temperature difference) "unheated' basements/cellar filled with wine.
(No temperature difference) interior wall that are not exposed to colder outside environment.

At best the walls used in the example given are
1) great thermal storage structures, just like water or stones or dirt.
2) great air purifiers, similar to how activated charcoal purifier tap water.
3) great humidifier that will absorb and release water to moderate humidity.
4) it is possible that in a low humidity environment  (desert) it can be a type of evaporative cooler

So I agree with Dale that cob-bricks made with biochar fiber is so-so.
I do like Dale's idea of using pure biochar as an insulator covered by drywall/plaster.

All that said I REALLY REALLY love the idea of taking more greenhouse carbon out of the air and storing it long term.

A couple quotes from S.Bengi and one from the article and then another from S Bengi with comments

These walls are cob walls not strawbale walls that have been blacken.

Did anyone mention straw bale walls blackened with char?  I'm not sure what you are referring to there.  

Cob is made from clay and straw/biochar.
And while strawbale is a great insulator, sadly cob is not, In fact 12inches of cob has a R3 value.

 Cob is usually made with straw, clay, and sand.  Sand is not porous.  It's quite dense, and when the spaces between sand particles are taken up by clay, then any air pockets that might exist in a given volume of straight sand are eliminated.  Cob generally is dense and as you mention, S Bengi, is not a very good insulator at all.  The amount of trapped air is minimal.  Cob is a great thermal mass.  

Cob made with char as a sand replacement is very different.  The char is very porous and thus has air pockets within it.  These char particles will take on water when the wall is created (wet), as the clay dries, however, I think that the water will be drawn naturally from it.  Char is not dense, or massive.  Char based cob, in my thinking, would perform much to the opposite of sand based cob.  A block of regular cob will sink in water.  Even when combined with concrete and lime, the blocks containing char float.  This would require a high ratio of char in the mix, as can be noted in this quote from the article:  

In combination with clay, but also with lime and cement mortar, biochar can be used as an additive for plaster or for bricks and concrete elements at a ratio of up to 80%.

 Until studies can conclude the moisture/insulation conundrum that is discussed only briefly after the article, I don't know if we can really discuss the merits or detriments of char wall insulation with any degree of accuracy/certainty.

Even strawbale walls which are 'good insulator' have a thickness of 18inches.
So yes a 18inch or 24 inch biochar-COB might be okay. But we are still talking about 4x normal thickness wall.

Strawbale itself is an alright insulator, but it is a better combination material that has attributes of thermal mass and insulation that create great thermal qualities.  Straw bales are dense and quite massive, particularly if they are packed with tighter bindings-as is recommended when using them for constructing buildings.  I've had a hand in building a few.  While having thick walls can be a problem with eating up floor area, I don't see what other problems are associated with the thick walls beside the need to accommodate that in your design.  A Char wall with 80% char in a cob type mix, I figure is going to be lighter and a lot better insulation (R value) than a straw bale, but it might not have the same thermal mass potential.  I don't know.  That's just how I see it.


 

Cob Wall = 10% straw, 40% sand, 50% clay (no one ever gets to make perfectly ratio clay soil or cob)
Biochar Cob Wall = 50% biochar, 20% sand, 30% clay
Biochar Cement Wall = 80% biochar, 20% portland cement
Lumber/Timber Wall = 100% wood
Strawbale Wall = 100% straw
Real 100% BioChar Wall = 100% pyrolyzed strawbale/wood

In my head the above is listed in order of increasing R-value (and decreasing density too)


My gut feeling is that a bale of straw/timber/lumber will float down a river even more readily than Biochar Cement, aka less dense
I don't think that 50% Biochar Cob will even float, in the articled linked they only showed 80% Biochar Cement floating, and obviously regular cob/cement will not float.

So I agree that in terms of R-value straight BioChar is superior to Cob wall or Strawbale wall.
But we aren't doing 100% "biohar' walls we are doing 50% cob+50%biochar walls and with the 50% cob (or 20% portland cement) it's R-value is cut by more than 80%

S Bengi wrote:
Real 100% BioChar Wall = 100% pyrolyzed strawbale/wood

I've been looking forward to trying this out, either between 2 masonry walls or with an outer masonry wall and an inner rammed earth wall for a greenhouse insulated thermal mass north wall.  I've always worried that strawbales would mold and or become a mouse hotel here in Maine, but charred strawbales should address at least the mold issue and probably the mouse worry as well.  Long fibrous biomass like straw should not break up due to shrinkage stresses like wood does and should therefore handle pretty well as "bricks".  Fire danger should be addressable by sealing in the layers within the wall with a layer of clay "mortar" to create air blocks, though presoaking the straw in borate before drying and bailing should make the resulting biochar resistant to fire, though less friendly to ultimate addition to the soil a century later (though I'd love to build structures that would last for thousands of years).


Thank you for bringing up pyrolyzed strawbales S Bengi.

Also, very sorry for the multiple "should" use in that last post....I should not have done that!

Just wondering--has anyone tried a rocket oven for making char? Since it is closed off and the fire is not actually inside the oven, it seems that you could cram the whole oven full of material and make a large amount of biochar all at once very efficiently. Having never used a rocket oven (I only heard about it with the recent kickstarter), I do not know if it is air tight enough to do this, nor do I know if it would harm the oven in some fashion. Thoughts?

Has anyone here successfully pyrolised a strawbale? Not saying it's impossible, mind you, but think for a moment about the logistics.

Has anyone here successfully pyrolised a strawbale? Not saying it's impossible, mind you, but think for a moment about the logistics.

I imagine that a specialized system would have to be built such that a straw bale fit pretty much solidly (except for a few slots going down the side for gasses to move) into the inner chamber of a retort, and or, in a space for a certain amount of other material on top of a 'TLUD such that a bed of coals and downward movement of gasses was established before the coal began to form on the bale.  Not sure.  Just guessing. It could definitely be done in an industrial system.  

What are the people doing to eliminate the smell of the char (minus the bio part)?

The reason I ask is, I know of people who have had partially burned houses, had the insurance company say they must rebuild, and when they left the charred walls, on humid or wet damp days, the house was always filled with the smell of charcoal and smoke years later.

But what a house is sheathed with has more to do with mositure problems then char. I say that because I have a few houses, one was made in 1995 and I have to really pay attention to moisture migration because the sheathing is made from plywood and oriented strand board. At only a half inch thick, any water reaching it will degrade the adhesives and quickly render the sheathing useless.

Another home I have though, built in 1922, is sheathed with (2) layers of 1 inch board, then 3/4 of an inch of cedar shingles, and 3/4 of an inch of eastern hemlock clapboards. That is 3-1/2 inches of sheathing before you even get to the studs. We could not get enough rain in a hurricane to saturate all that wood, and even if we did get that much rain, it would dry out when the sun and wind returned with no ill effect.

With the first house, I have a moisture barrier that allows internal moisture to pass out through, but not back in through (called Typar). In my older house, there is no need for it. Interestingly enough, both homes have the same R-value in their walls, R-20.

Hi Travis.  Partially charred wood has that smell, but fully and properly produced biochar/charcoal has no odor.  Properly produced biochar reaches a temperature of almost 1000F.  At those temperatures there is nothing volatile left to produce smells.  But partially charred wood still has lots of volatiles in it that were produced during the burn.  Due to it's absorptive high internal surface area biochar will actually absorb odors, not emit them.

Kevin Young wrote:Just wondering--has anyone tried a rocket oven for making char? Since it is closed off and the fire is not actually inside the oven, it seems that you could cram the whole oven full of material and make a large amount of biochar all at once very efficiently. Having never used a rocket oven (I only heard about it with the recent kickstarter), I do not know if it is air tight enough to do this, nor do I know if it would harm the oven in some fashion. Thoughts?

When you're making char in a low oxygen environment you are driving off the volatile gasses.  In the rocket oven case, I think those gasses would exit the gaps between the door and the oven.  That might be totally fine.  But if they were ignited, it could give a nice bit of ambient heat, or burn your house down, not sure which?

Homestead scale biochar makers (55gallon metal drum) have a exhaust and it is hot, the air coming out is hotter than 450F. So you can cook or bake whatever you want

Mike Jay wrote:When you're making char in a low oxygen environment you are driving off the volatile gasses.  In the rocket oven case, I think those gasses would exit the gaps between the door and the oven.  That might be totally fine.  But if they were ignited, it could give a nice bit of ambient heat, or burn your house down, not sure which?

So, if doing this it would be best to specially design the oven to have an air-tight door, but to have an exit tube for gasses. It would have to be a metal tube, but once away from the oven it could have some cooling fins or other apparatus and then hook onto a flexible tube and taken to a wood gas collection tank (thinking of this)

Yup, now you're on to something.  As long as you plan for the gasses to go somewhere (or help with the heating of the biochar) you'd be good to go.

Might be better to start a new thread to discuss the oven?  But, my two cents for the time being.  In addition to storing the gasses for other uses, you could have a metal pipe exit the sealed oven, removing these hot gasses from the char, and then pipe these into the burn tunnel, if you knew what you were doing so that it entered at the right place, thus firing your charring project up even more.

Greg Martin wrote:Hi Travis.  Partially charred wood has that smell, but fully and properly produced biochar/charcoal has no odor.  Properly produced biochar reaches a temperature of almost 1000F.  At those temperatures there is nothing volatile left to produce smells.  But partially charred wood still has lots of volatiles in it that were produced during the burn.  Due to it's absorptive high internal surface area biochar will actually absorb odors, not emit them.

Hey thanks for schooling me on this; that makes perfect sense.

My attempt at making biochar last year was so pitiful and disappointing that I lost all interest in the project though I got tons (literally) of logging debris to burn.

It was so disheartening that while I was working with a local historical preservation society on charcoal making due to its big impact for this region years ago, that too stopped. So all in all, I have little to add.

Travis, the open pit method has been posted on here a few times.  It's my favorite method and I use it all the time with great results.  Here's a link describing the method:
https://pacificbiochar.com/how-to-make-biochar-with-only-a-match/

If you have any questions I'm happy to help....cheers.

I remember seeing Travis' photos and description. I think the main problem with Travis's method was that he used large log rounds (like firewood rounds) piled up in a heap in a pit, which would, in my way of thinking, have too much air gaps between the rounds.  There was too much oxygen in his pit, and the wood.  Then when he buried it, the excess oxygen burned off the rest to ash.  Is that accurate to your method, and the way you might see it, Travis?  I can't remember exactly what you had written.  I find this youtube link on turning forest waste wood into char helpful :  

(this guy also has an open heap method without the trench,if you search for it)  This is hopefully the method that I will be trying this year. If I can get my trenches dug before the ground freezes hard.



Edited for spelling.

I wonder what the actual R-value of 80% Biochar Cement is?
And what would it take to make it structural concrete (2500psi)
1) Reduce water to min = 1part water for 1 part cement by volume
2) Silica fumes/fly ash
3) Fibers

Mix by Volume for a 80% Biochar+20% Cement (in brackets is the corresponding ratio by weight)
Biochar = 4 (1.5)
Water = 1 (1)
Cement = 1 (3)

Just for kicks I looked into 80% biochar by weight. That turned out to be 1 bag of cement for 32 bag of biochar, so I am sure they were not going by weight.

Mix by Weight (in brackets is the corresponding ratio by volume)
Biochar = 12 (32)
Water =  1 (1)
Cement = 3 (1)

Density
Biochar = .37 g/cm3
Water = 1 g/cm3
Cement = 3 g/cm3

Roberto pokachinni wrote:I remember seeing Travis' photos and description. I think the main problem with Travis's method was that he used large log rounds (like firewood rounds) piled up in a heap in a pit, which would, in my way of thinking, have too much air gaps between the rounds.  There was too much oxygen in his pit, and the wood.  Then when he buried it, the excess oxygen burned off the rest to ash.  Is that accurate to your method, and the way you might see it, Travis?  I can't remember exactly what you had written.

Yeah that is very accurate. I have a host of photos somewhere of the event. It was also White Pine that I used and not hardwood.

I got 100 tons of logging debris right now half buried and might touch it off this winter after snow comes and see what I have left over in the Spring. It is on the edge of a field/forest so I want to get rid of the wood debris anyway, but maybe it would make good biochar.

I still think it would be very hard to have homemade biochar that had all the volitiles burned off though to keep it from smelling. Maybe mixed with concrete it would be okay, but I still am not sure.

I have often wondered though if there might be a lot easier way to make thermal insulation with concrete panels that did not involve the making of biochar...leave that stuff in the garden where it belongs.

When my Grandfather built his chicken barn, he needed concrete floors but had a 5 story timber frame barn. To match the two up, he used lightweight concrete derived from the University of Maine Cooperative Extension. It did not use aggregate for a filler, but rather sawdust. Wood only has a R-factor of 1.7 per inch of depth, but does have mass. I wonder if using sawdustcrete could have other building uses other then lightweight concrete???

I am not sure if I should view/present this building material as 2500psi concrete or as 650psi cob.
I just don't see it having a 2500psi so I will view it as cob.
Structural and Insulating Cob.

Has anyone considered using bio char as loose fill insulation? Not powderized but in chuncks? Seems like thatvwould have a lot of pore space. But maybe the density is the concern?

I would personally be concerned with flammability. Ignoring that, it would probably work similar to vermiculite, loose fill chunks which seems to have fallen out of favor for other reasons.

Assuming conventional construction and depending on the R value you are trying to achieve, the weight per square foot becomes important at a certain point to prevent overloading your ceiling.

This is a fascinating discussion.  I'm trying to think of a way to cost-effectively and time-efficiently insulate an earthbag dome. For the purpose of discussion let's say the default is to hire a company to come out and spray blown cellulose all over the dome and cap that with plaster.  Which has it's own problems.

So one valid approach is to make paper-pulp-based insulation from newspaper, water, and borax for fire/insect resistance.  This means a car-mounted mixer with a lawnmower blade and a garage full of newspapers which will need to be kept dry until the moment they are needed.  Doable, if a pain.

This conversation suggests another approach might be biochar.  Either using large hunks as dead space then capping with a biochar-based plaster, then capping that with normal plaster.  Sandwiched against a 2 foot clay wall in a relatively oxygen-poor environment, my guess is fire would not be as much of a concern, but then again charcoal is highly flammable.  Would borax offset this?

Another option might be crushing the biochar, mixing it with clay and water into a slurry, then spraying that over the earthbag dome, then covering that with plaster.

If either of those last two are reasonable options, I'd have no problem with making a permanent 1000 gallon biochar TLUD and doing a burn every couple days over a period of months to create the biochar needed.  But not if this is going to burn down my house.

Can anyone make biochar bricks??

Greg Martin wrote:I didn't see much of anything here about using biochar as thermal insulation in natural building, so here's a link...enjoy!

The Use of Biochar as Building Material:  https://www.biochar-journal.org/en/ct/3

From that link:

First tests to produce biochar bricks using cement, lime or mud as a binder are very promising. With wet bulk densities under 1.2 g/cm3 and partly under 1 g/cm3 and a compressive strengths around 20 N/mm2, it would seem that a very exciting, highly functional lightweight material will soon be making its appearance. When using cement and lime, sand can be completely replaced by biochar reducing the weight of the material by factor 5 (see picture with the swimming bricks).

Ok, even if there were no other advantages whatsoever, the fact that char could be used in place of sand is HUGE!!!

In some places, it can be very hard to get hold of the right kind of sand for making concrete. Large portions of the Middle East have this problem, where they have tons of sand, but the shape and texture is wrong for concrete. In fact, there are even such things as "sand bandits", who steal sand by the truckload to sell on the black market for use in construction, doing terrible damage to beaches and seafloors in the process.

If char really can be used to replace 100% of the sand in concrete, without sacrificing strength and integrity, then this information needs to be spread far and wide! Char can be made from anything organic, so even desert regions should be able to come up with some waste product they could burn to produce it. This would ease pressure on sand mines, and cut the legs off the illegal sand market, while helping countries be more self-sufficient at the same time!

I have no idea who to send this information to. Does anybody have contacts in the construction industry in some area where they have trouble getting the right kind of sand?

This is exciting because my sand is too fine for cob but I have a ton of trees. And there’s a lot of waste material when prepping them for cordwood. I will be experimenting with this soon!

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?!

Hi Ben.  Thanks for your post and welcome to permies!  I personally haven't experimented yet with this idea.  I do think it has a lot of merit/potential.  


THIS BOOK goes into a lot of different ideas about using char in building and construction materials, including replacing sand in concrete.  



Sounds like you are well on your way to some interesting discoveries.  Thanks for reviving this cool thread.