Cob/pumice

Hello
After much research, and falling in love with cob, I have decided to begin building a cob/ pumice/ sand / straw mixture. That being said in another post about ponds in greenhouses I took Dale off the tracks of the discussion, and I am reposting our discussion here. He brought up information about cob/ pumice that I thought everyone interested might want to know.
So here is our discussion:
" Suzanne Cornell wrote:
Hello,
I am a relative newbe, so forgive me if my manners are poor. I have a question for Dale re heat capacity of cob. If this is not the place to ask this please let me know and I will repost. I was thinking others might like to know... Dale, do you know what the heat capacity of cob made with 1/3 pumice, 1/3 sand and 1/3 clay? Does the pumice raise or lower the heat capacity?
Thanks
love the idea of a pond in a green house. My old boyfriend had a pond in the kitchen. It was great.
Suzanne


Suzanne, I think questions are totally acceptable and yours is well within the context of this topic. By dealing with it in the thread instead of as a private message, others may benefit or if I was to make some error, I'm sure it would be pointed out in short order.

--- It all has to do with weight. The vast majority of earthen materials have a rating of about .2 which is 1/5 that of water. With pumice, the density is lower, so there would be a drop in heat capacity per cubic foot. The capacity per pound is unchanged. This sounds like a mix that would come in at 70-75 lb per cubic foot as compared to regular cob at 95 lb.

Pumice has insulating properties and would affect the speed of heat absorption and transfer. For walls this is a definite positive. The story is different if the cob is for a RMH. It should slow heat transfer which may not be a desired result. Also with pumice there is a greater chance of encapsulating air. At regular temperatures, this would probably have no ill effect, but if the riser were cobbed over as is sometimes the case, this air could expand and cause an explosion. Not an A bomb explosion or even as powerful as a steam explosion, but still enough to do damage. "


So I wanted to ask the forum how do I transition the pumice clay on the out side of my building into the traditional heat absorbing/ releasing cob on the inside? The weight, and therefore the dense ness would be different I would think they will pull apart as they settle and dry. I was thinking gradualy adding less and less pumice twards the interior might keep that from happening... Any opinions?
Thank you all for being here
Suzanne

This topic was placed here so that we don't highjack the other thread. Suzanne has learned very early to stay on topic.
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I can't see a problem in transitioning from one mix to another, if the materials aren't too wet. It's going to come down to shrinkage rates and a dryer mix will shrink less. Some sort of exterior form work would help on placing the pumice mix.

Do you have a free or almost free supply of pumice? Just a few inches of straw clay encasing your walls, would go a long way to improving heat retention. If the pumice is free and abundant it could be used throughout the entire wall.

I'm doing some experiments with charcoal insulation and will now paste in a little from that thread. Our discussion about your insulating needs led to some head scratching where I went through every insulative natural material in the hope of finding an efficient substitute for pumice.

Here it is --- brought over from the thread entitled --- Dale's Marvellous Inventions and Adaptations. https://permies.com/t/19303/green-building/Dale-Marvellous-Inventions-Adaptations

Charcoal in Green Building - slip coated insulation, cob insulation, infrared resistant plaster, plaster pigment ...

I'm trying to find uses for charcoal or bio char as an insulating material. Charcoal is one of the lightest natural solid materials and it has excellent thermal and hygroscopic properties. Based on this, I've got several ideas brewing. The idea first struck a couple hours ago, so I'm quite excited about the possibilities.

1. Granular charcoal insulation for attics. Charcoal is one of the most insulative materials on Earth. It would need to have a clay slip coating for fire protection as is done with straw clay. Fine and coarse materials would be mixed for maximum effect. I would expect convective currents to be more of a problem than with blown cellulose. This problem can be alleviated with a thin cap of blown cellulose. I've done this with other granular insulation (chunk fibreglass, redwood bark, vermiculite, wood chips). A thin cap fills voids and causes an improvement in performance greater than would be expected based on the R value of the cellulose. --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

2. Granular charcoal mixed with cob for lightness and insulating. By putting a greater proportion on the outer few inches, we could have insulation but still retain the benefit of the thermal mass of a pure cob inner wall. The hygroscopic nature(water absorbing) of charcoal should help moisture migrate through the wall. Charcoal conducts far less heat than does pumice and it has the infrared property to boot. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

I SAVED THE MOST PROMISING ONE UNTIL LAST
3. Ground charcoal/clay paste to give cob walls an infrared and conductive barrier.
(High end foam insulations use an admixture of carbon black in order to impart better infrared properties.) (Asbestos worked well for this but caused health issues.)I imagine using the charcoal paste product in the scratch coat of interior plaster. Regular plaster would be used in the finish coat since we don't want a black interior. Trailers and other light buildings often have a shiny paper product under the panelling to reflect infrared. A layer of charcoal should work even better than this.
A black exterior might be desirable in certain situations. A cob bench in the greenhouse might look and perform well with a shiny black finish.

With some work, I may become the "Henry Ford" of charcoal insulating, and as Henry would say, "You can have your charcoal plaster in any colour you want --- as long as it's black."

Dale,
I love the idea of charcoal, you know much more about thermal mass than I will ever. Do I understand correctly that a coating of charcoal would insulate like reflective insulation, but still allow the cob to act as a mass heat holder/releaser? I know your still thinking, but do you have a sense of how much charcoal would be needed? If you could get enough charcoal, what I wonder would a mix 1/3 sand 1/3 charcoal and 1/3 clay be like? I think I'm going to be making more test bricks really soon.
I don't know how i would make enough charcoal, i have to work so hard to get my firewood for the winter, the idea of using it for charcoal makes me sigh.

I thought pumice would be a great option, as I could pour a layer of it under the foundation and not have to use that foam sheeting. I guess charcoal would work the same way. I realize that I would have to pay for shipping, but pumice is prety cheep by the ton, i think if i am rembering correctly $300,00, and it's carbon footprint is relatively small compared to other insulative additives. But charcoal, ... it's a neet idea. Tell me more. Also tell me why not pumice? What happens if charcoal is mixed with lime?
Just when I thought I had the building figured out, you bring up these great points! Well I have until spring to decide........

Suzanne Cornell wrote:Dale,
I love the idea of charcoal...

The charcoal idea was brand new to me when I wrote that blurb on the idea. I expect that It would behave like those reflective products but have not tested it. Sand is a good conductor of heat so it would more than likely reduce efficiency. The idea of the barrier is to reflect heat back toward the interior.

In order to still have the thermal mass work the way you want it to, the insulating layer should be on the exterior. This might require something fairly thick in your climate to give adequate insulation.

I still think that the best option would be a substantial layer of straw clay covering the entire exterior of your building. A pure cob wall could be built with no plastering. After you experience a winter without insulation, you can add the straw clay house wrap and then give it a coat of plaster. This would allow the job to be done in two stages so that there would be absolutely no doubt that insulating is needed in your area. I'll post a test results for charcoal as they are available.

You don't have to convince me that I need insulation! Although as I write it's like 50 degrees out. So why not pumice? I can pour a layer of it in the ground as frost heave insulation, seems nifty to me. No trouble with the damp that we usually have too much of. No foam insulation.

I've done some reading on pumice. Althoug claims for R value vary widely, it is a very viable option. In pumice concrete and stucco applications, there are plenty of air holes and just enough binder to hold it together. Mixed with cob, there would be a major loss in efficency due to the tightness which eliminates the air spaces and the sand which is a very poor insulator. If a clay binder were used, it should out perform one which includes sand. I would only use pumice stucco with the types of cement that have proven effective in the past.

If a regular cob wall were constructed, I could see it being very well insulated by covering the exterior with a pumice crete stucco. This material would usually require a grout type finish but some sort of earthen plaster might work. I've found many sites that mention pumice but none where it is used with cob. Here's one. http://books.google.ca/books?id=It_JCv6MWFwC&pg=PA93&dq=pumice+crete&hl=en&sa=X&ei=yM28UMWnIerWiwLPiIDgDA&ved=0CDUQ6AEwAQ#v=onepage&q=pumice%20crete&f=false

I found this one !
http://www.islandnet.com/~anngord/builders.html

Suzanne, your situation has inspired me to adapt my gabion pebble wall system to accommodate pumice and to create a thread regarding that. I've linked it to here but also reproduced the first posting below. The thread is called "Pumice Gabion System – Insulates Cob and Concrete Walls – Stucco Over Wire. " https://permies.com/t/19421/cob/Pumice-Gabion-System-Insulates-Cob ---------------------------------------------------------------------------------------------------------------------------------------------------

This idea struck while trying to figure out how Suzanne Cornell could effectively use pumice to insulate a cob house without resorting to the use of portland cement.

The foundation for this idea was laid out in this thread that I produced last February. --- "Dry Stone Pebble Wall. Stone Siding for Wooden Buildings - Requires no mortar or masonry skills." You must view the link for any of the following to make sense. https://permies.com/t/12592/green-building/Dry-Stone-Pebble-Wall-Stone

This type of wall could be filled with pumice which would give an insulation R value of about 1.5 per inch according to some optimistic folks selling pumice-crete. R value drops as the proportion of portland cement rises. By using the correct gauge of mesh to contain the pumice, all cement would be eliminated. This should maximize insulation value. Rather than building concrete forms, we build the gabion wall to the existing wall. A suitable stucco wire would be included along with whatever gauge mesh contains the pumice. Cover it all with stucco and you're done. I suppose a clay based stucco could be tried as well. Tie wires would produce a very small amount of thermal bridging.

Now I have to find a source of pumice and a wall to test this on. I could see this being used as a heat shield near a wood stove as well. With a good quality water proof stucco, this might be the best idea yet for insulating a cob hot tub. But , it would then look like a stucco hot tub.

Thank you, the mad inventor : Dale Hodgins.

This site gives a good overview of the various application of pumice in green building. Note that walls in New Mexico are typically 14 inches thick. http://www.pumicecrete.com/ I love their disclaimer "These designs can be, or may not be, appropriate to your locality. Local conditions and markets determine how a building should be designed and what materials should be used.

This seems to be the only YouTube video on the subject. The guy is not a natural film director but he's built a nice shop. He could put someone on Red Bull to sleep.

This is an upscale home built from pumicecrete. http://www.youtube.com/watch?v=BY7a5E7XRQ0 The narrator has a very deliberate and precise way of speaking common to upper class Britons of Indian decent. She's easy on the ears, and I'm guessing that's why she's the narrator.

This is the first time I've linked to something off of this site. Is there a way to prevent the reader from going off on a YouTube flight of fancy by limiting the link to just that video ?

Dale,
your a genius, can't wait to see how your experiments turn out!
I'm thinking of building a small cob/ pumice 14' x 14' "shed"( ie no code needed ) to hold up my solar panels and store the batteries and converter. A smallish rocket stove in traditional cob with 3" of traditional cob on the inside walls and at lest 6" on the floor. that way I'll see how it works. Promise to post pictures. If this weather stays warm I might use my break in January to start it. I'll have the needed addition of 5 more sets of hands. Dale you might be interested in the fact that It's always been my intent to build about 2' off ground level because it flooded here for the first time since the 1700 a few years ago, seems like it may be the start of a trend. I planed on using Gabion baskets to build the foundation wall, and infilling with rubble. That all will go over a few inches of just pumice to prevent frost heave.

Suzanne Cornell wrote:Dale,
your a genius, can't wait to see how your experiments turn out!

Some people (my kids and friends and random people I meet) say that I'm quite fond or my own voice and a little too in love with my own ideas and opinions. Statements like yours will only stoke the fire and cause my head to over inflate . But keep it coming, I'm willing to risk it. ==============================================================================

I'm glad you mentioned the risk of flooding. The gabion baskets are a great way to support the load and a rubble trench is a great way to support the basket(s) and to provide drainage. In one of the articles regarding pumicecrete they mentioned that there were hybrid pumicecrete straw bale homes where the bale section starts at the height of the lowest window and all beneath was pumicecrete. This is done where flooding or melting snow could cause damage to lower bales. The pumicecrete could be coated with a water proof grout as is used to coat concrete water tanks.Suppose that your lowest window is 2 ft. off the floor. You have 2 ft. of gabion followed by 2 ft. of pumicecrete for a total of 4 ft. of flood protection. The window openings and all other areas above this line could still be hand sculpted as is usual, so that your house would still look very much like a cob house. Remember the cob building mantra of a good hat and a good pair of boots ? This treatment of the lower walls means that your house would have a good pair of hip waders.

In a worst case scenerio where the water goes say a foot above the gabion, and rushes in the door, the clay portion of the walls would be saved. Damage would be limited to the floor and the RMH assuming that you're home and move everything to upper shelves for the flood. It might be wise to get some fishing net and attach ropes and pulleys to the ceiling joists or rafters so that everything can be hoisted skyward should the need arise. I guarantee that in a small house, you'll put those ropes and pulleys to work on a regular basis whether it's to dry herbs or laundry or to sling out of season items up and out of the way.

Now, about the flood risk. Is it from a river or lake ? During a flood, is the water moving quickly past or is it more like a giant puddle ? For your sake , I hope it's number two. If there is flow, would logs, ice or other floating debris have the potential to be swept against the house ?