Mass and Insulation with Adobe
Quentin Wilson and Associates, specializes in solar adobe design and construction. He grew up in the South Valley of Albuquerque, New Mexico where he watched adobe bricks being made. In the fifth grade, he made miniature adobes on cookie sheets in his mothers oven in order to construct house models for a class assignment. By age thirteen he made full-sized adobes in the back yard and ruined the grass. Later, he traveled a bit, went through the Army, and graduated eventually from the University of New Mexico with a major in physics, minors in math, chemistry, and education in 1970. After teaching high school two years and community college math for three more, Quentin moved into professional solar adobe construction in 1976 as the Project Manager and Instructor for the Sundwellings Demonstration Project at Ghost Ranch, Abiquiu, NM. He became a licensed general contractor in the State of New Mexico in 1982. He has been building homes and teaching seminars and workshops ever since. In the fall of 1995 he established and taught the full-time Adobe Construction Program at Northern New Mexico Community College. His website, quentinwilson.com, lists the course schedule and many other resources related to working with adobe.
Q: I am involved in the design for a passive solar home in Maryland. We are investigating materials to use for interior thermal wall mass, and adobe seems like an excellent choice.
A: Adobe would indeed be a good choice. Besides the fact that it has a specific heat of 0.2 which is the same as concrete, stone, brick and some concrete blocks, adobe has a wonderful coefficient of heat diffusiveness which results in its releasing heat more slowly that the other materials which all have higher diffusively.
Q: There seems to be some discrepancy as to whether adobe is an effective insulator. Adobe is a tremendous capacitor of heat. Can you clarify this for me?
A: In non-steady state situations - such as our planet- capacity mimics insulation. Insulation cannot mimic capacity. Some sources cite it as a good thermal mass, but poor insulator.
Q: Im grad student currently working on modeling the energy efficiency of a passive-solar adobe home. I been having a lot of trouble trying to find the physical properties of adobe, thermal conductivity, absorptance and specific heat. Can you guide me on this matter? I would really like to find a reliable (quotable) source for these parameters.
A: Specific Heat: 0.20 BTU/DEG F/LB Source: Passive Solar Home Book by Edward Mazria, Rodale Press about 1982, out of print but in many libraries and at amazon.com Absorptance depends on the color of the wall presented to the sun and varies incredibly from light to dark colors. Source: New Mexico Energy Conservation Code Applications Manual available from the State of New Mexico Construction Industries Division, 725 St Michaels Drive, Santa Fe, 87501
Thermal Conductivity, BTU/SQ FT/DELTA T/HR also known as or related to thermal diffusivity, rho, has never been determined as far as I know. It is the most important number and its value, whatever it is, is what makes adobe the planet's best passive solar storage medium. Rho is less than concrete, stone, brick, and concrete block so it does not loose its heat as fast as those materials which have nearly the same specific heats. Its rho-value is higher than wood or paper which are so low that they are very slow to transfer their stored heat even though they also have nearly the same specific heat. The paper I refer to is in the form of books or other tightly packed forms of paper. Mazria hints at these relative conductivity in a table of appropriate wall thicknesses for Trombe Walls. A person might use Mazria's table and extrapolate for adobe if the numbers can be found for the other building materials.Ed Mazria, himself, is a well-known architect practicing in Santa Fe. He might respond to a well structured question requiring a short answer.
Sometimes, Thermal Conductivity or its reciprocal, Thermal Resistance is quoted from determinations made by standard laboratory calorimeter tests at steady state. As soon as steady state is mentioned, the results should be thrown out. The planet is not steady state. The sun comes up and goes down, the air cools off and warms up. The NM Energy Code Applns Manual hints at these planetary truths in the several tables that give Effective U-Values for adobe walls depending upon:Climatic Zone in NM Orientation of the wall (n,s,e,w) Color of wall (light, medium, dark)
The State of NM got its numbers from computer simulations run in the mid-1970's. Along with that, four adobe buildings and buildings of several other types of materials were built at the Tesuque Pueblo Solar Thermal Project at the same time. Real data points measured on the buildings confirmed the millions of bits of information produced by the computer simulations to convince the researchers at the University of New Mexico to publish the tables of Effective U-Values in the Manual.
Q: I am trying to find the R-value on 9" thick adobe brick. I live 12 miles from Oklahoma in Texas, 75 miles north east of Dallas. I do a/c for a living and need to figure a heat load on two houses.
A: That's a mighty tough question. Adobe has about 140 Effective U-Values according to the NM Energy Conservation Code Applications Manual. I am looking at Zone 9 which is Clovis and Roswell which is as close to the Dallas area as we are going to get minus some of the humidity. For a Medium Colored Wall the U-Values are North: 0.227; East: 0.198; South: 0.160: West: 0.204 For dark colored walls the values go down 10 to 30%. For light colored walls the values go up 8 to 15%. The ASHRAE Steady State U-Value is 0.263. Those numbers are for 10-inch thick adobe walls.
Q: I would like to build a home with some adobe walls. One of the walls I would like to be adobe would be on the south side of the home but would also between the living space and an attached greenhouse. I still need to explore the attached greenhouse option, but would you recommend adobe in this application? I am concerned about the higher humidity levels. Are there any changes you would suggest? i.e. a vapor barrier between the stucco and adobe?
A: This is a perfect use of adobe. It stores the excess heat of the greenhouse and conducts it to the interior of the home. The elevated humidity will not hurt the adobe one bit. Adobes achieve their maximum strength at 60% humidity. They also have a great ability to soak up moisture and return it as the atmosphere inside dries out. We are starting to see some papers to this effect at our yearly Adobe Conferences here in El Rito. May 20-22 in 2005. The Germans have long been aware of adobe's moisture modulating capabilities and Gernot Minke has a chapter in his Handbook of Earthen Construction.
If the adobe wall is between the living space and the greenhouse, you can save money by omitting stucco. Exposed adobes, a mud wash or mud plaster will work fine. Try to find a light color to avoid having to paint the wall. Paints are less breathable than is desirable in this situation.
Q: Do you know of any instances where this type of setup was used as a significant portion of heating a residence? I was looking to use the thermal mass and attached sunspace as a means of heating the conditioned space and also tempering the thermal conditions in the greenhouse. I have heard rumors that this can be effective in both these goals.
A: (Kelly) Yes, Quentin suggests that this is a good strategy, and I agree. I know of one home where an adobe Trombe wall has been used quite effectively in heating the home during the winter in New Mexico.
Q: I live in western North Dakota which typical sees 12-15" of moisture(6-8" as rain) a year. Winter temperatures can be arctic (-10 to -30 F for a week at a time) I am most curious about insulating properties. What exactly are the R-factors for an eight inch, ten inch and twelve inch adobe walls? How many R-factors does outer insulation (polypropylene?) add to a wall? I ask these questions because I wonder if adobe would work as a basement wall if adequate moisture barriers were installed? (I am thinking a trench filled with bentonite.) However is the effort, lower cost and mainly the R-factors, worth the effort?
I don't recommend that you use adobes below grade even with the Styrofoam. If there is one little pinhole, moisture will get through and cause trouble for the adobes. I would stick with concrete or pumice or cinder blocks for below grade construction basement walls.
Strange things will happen if you add insulation to the adobe. The wall will perform better than the sum of the two R-values. I call it phantom R's. It's because the adobe is not an insulator but a capacitor and conductor that mimics insulation in certain situations. When you couple capacity with resistance you get more resistance than expected in a non-steady state situation. Electrical engineers understand this as capacitive reactance or a band pass filter.
Down in the basement in Western North Dakota, you are as close as anyone will get to steady state on this planet. Adobe will be at its least wonderful state there. If you can have an adobe wall where some sunlight hits it, it will start to perform as a low grade, unglazed Trombe wall. That's adobe at its best level of performance.
Q: My wife and I recently bought a rambling adobe house outside Ranchos de Taos. The house is not that old, but when it seems as though relatively little attention to insulation was paid when it was built. The roof is a straightforward viga/truss/brai structure, and it does have about 4" or so of blown in insulation. The walls are 20" of adobe - but with no insulation of any kind.
Here's the problem - the place is a propane black hole in the winter, with prodigious quantities of the stuff being burned for seemingly little/no internal temperature rise. (Plus, the house was conventionally designed in the Northern NM vernacular, and it would be difficult to attempt to retrofit passive solar features without messing up the "feel".) An architect we know in Taos (Karlis Viceps) has suggested that we should put a bunch of active solar heating on the roof in an attempt to warm the mass up in the daytime. One of the local solar contractors reckons that is pointless until the house is better insulated.
So, here's the question - would it be better to: a) insulate the walls (styrofoam panels, and then re-stucco; or spray foam on), b) add extra blown-in insulation to the roof, or c) add active solar hot air heating? The driver is less cost, per se, but more about cost effectiveness and comfort.
A: I'm gonna have to shoot that Carlos Biceps! Talk about blowing the NNM vernacular, not to mention the heat cart before the insulation horse. I agree that the house needs to be insulated better. It will cost more, but if you can afford it, spray on polyurethane on the North East and West walls then restucco. Two inches works well. Nail-on
If there is space in the roof cavity add all the blown in cellulose or loose fiberglass or fiberglass bats possible. Current standards require roof insulation to be R-38. 40 or 45 would be nice, beyond that is beyond the
Just promise me no solar warts on the roof. I would think the same amount of tempered glass could be used in a solarium, greenhouse, sun gallery, glassed in south porch, or to fill giant holes blasted into the south wall for direct gain. There are plenty of stately vernacularly aligned solar adobes in Taos. If hot air collectors cannot be avoided, give some thought to ground mounting them in front of the house or behind it beyond the winter shade line.
C and Q: I should note, in Karlis' defense, that he was familiar with the house (he designed the retrofit rainwater collection system). It's a somewhat unusual house layout and I agree with him that it would be very hard to do a retrofit of passive solar features. (We discussed that as the first option.) It's also true that, because of the way the house lies on the land that the solar collectors would be totally invisible from most angles. So, solar warts might be in our future - after we've improved the insulation... One last question on the wall insulation - the local spray on foam guy in Taos drives a brand new shiny full-size pickup. Like you, I am extremely suspicious about that, on the basis that folks like me are paying for it! On the other hand, I have a certain amount of construction experience and could install the blue styrofoam panels, along with the self-furring mesh, myself. (I would get someone else to do the three coat plaster, regardless.) So, here's the question - given those parameters, would you go with spray on foam or the styrofoam blocks?
A: Sorry, I over-reacted on Karlis as I certainly know him to be a careful, thoughtful designer. Of course, he is a little too open minded on certain non-adobe types of construction.
Q: I bought an adobe construction home in Pueblo Co last summer. This is my first winter here and I am freezing in this home. The walls are 14" thick adobe with 2" of foam board over that, then stucco. I expected to be living in a cozy warm home, instead the cold goes right threw me. My propane bill is running $100 per week with me keeping the temp at 72 during the day and 66 during bed time. I just don't understand what the problem is. Can you help?
A: Something is very wrong. Pueblo has a relatively mild climate and no house there should be using so much propane. It could be that there is little insulation in the roof or there may be a source of air leaking into the house. Perhaps there is a heating flue or kitchen range vent or bathroom vent that is loosing heat upward. Too many windows on the north? French doors are the most notorious for infiltration. (Letting cold air in.) There could be a problem with the heater. There are some installations of radiant floor heating which had no insulation under the floor. We have always known that to be a problem but the plumbers were slow to catch on. Uninsulated radiant floors use twice the fuel as insulated floors. Once adobe walls cool down, they are hard to warm up and they suck the heat right out of you. But they should work the other way, be toasty warm, and radiate back at you. We will keep thinking. We might have to send a deputation team up to visit to see if we can determine the problem. An electric blanket would help until help arrives.
R: I have checked for air leaks etc. and not found the place too drafty. Most windows are on the south and west. I don't think I've ever been able to pump enough heat into the place to heat these walls up. The walls are COLD. The furnace takes hours to raise the heat in the house just a few degrees. My rooms are large and I feel a damp coldness everywhere. I'm thinking of having a fireplace insert installed just so I can get a lot of heat in here for several days in order to heat the walls. What do you think?
A: (Penny, if you have been putting $100 worth of heat into your home each week, the adobe walls should be very warm by now. Rather than, or in addition to, spending the money on an insert, you might consider having an energy audit done on the house. There should be one or two certified home inspectors in Pueblo who can do this. A blower door test will determine the airtightness of your home and pinpoint the area of loss if there is one. A house that does not feel drafty can still have major infiltration problems. An infrared camera will find conduction losses through the walls or roof. Clever use of a non-contact thermometer such as a Raytech can find floor problems.
Q: I have some land in Southern Colorado and I am starting to make choices necessary to start building a 1200 Sq. Ft. house on it. Your web page is very, very helpful. Right now I am looking at the possibilities of using either Shotcrete Sandwich Panels or adobe. It is very cold in the mountains of Southern Colorado so I need heavy insulation want materials that hold head and radiate it out. But I also have to look at cost. I'm trying to figure out how to cost compare a house with Shortcrete Sandwich Panels and an adobe house build with two rows of brick and insulation between them with both structures having the same R Factor. Can you tell me where I can start to get information to calculate this?
A: Take a look at Vishu Magee's Taos Construction Costs Handbook: archetype-design.com
Adobe is always my choice because besides having the thermal capacity needed, it also has a slower rate of heat transfer than concrete, stone or brick. Therefore for a given thickness of wall, the heat is returned to the living space over a longer period of time
Adobe itself is not an insulator. It is a capacitor and stores heat nicely. On big thick walls, heat or coolth does not get to the interior before the next phase of the sun cycle starts and the heat, or cool starts to move back out. With an 8-inch wall, heat or coolth might get to the interior before the sun comes up or goes down and reverses the cycle. It would work badly.
I don't know if there is anyway to figure it out short of finding a smart heating cooling engineer who actually has an idea how adobe performs. It can't be all bad or the miners in Death Valley or the Mormons by Safford, Arizona would not have used adobe. But again, they had the thicker walls.
Q: I'm a student at university in Tanzania, East Africa and am searching for a PhD thesis topic. My interest is the use of clay/mud/ soil to achieve thermal performance in walls. Its almost hot throughout the year here. Could you please advise?
A: Give me a little time to think this over. Adobe works well where there are differences in temperature throughout the 24-hour daily cycle. With it's thermal flywheel effect, it can capture the desirable temperature at the cool or hot part of the day and save it for the hot or cool part of the day. If it's always hot, we have to think this through very carefully.
Q: My walls are two courses thick (10 by 14 by 4) with two inches of airspace in between, making a 22 inch wall. I read the previous questions and answers, some of which touched on the use of the air space, but the other folks all talked about putting some kind of insulation in that airspace. It seems to me that simple dead air, with no insulating substance, would be just as effective as any insulation. Do you have any thoughts on that?
A: Dead airspace is around an R-value of 1. Fill the two-inch space with rigid polystyrene insulation and you will have R-11. Several lightweight loose insulation materials can be poured in later and will give similar R's. If you have gone to all the trouble to build double walls with an airspace, fill it. It would not hurt to bridge across the two walls with JDurowall to tie them together about every four courses.
Q: I am considering employing a builder within the next year or more to build an adobe home for me in the southwest. I am concerned with extreme heat and extreme moisture. How will the structure respond to both heat and moisture?
Q: I am from Philippines. They have said that adobe isn't a good insulator. Is it possible to make adobe hollow blocks and then use rice hulls as mortar in green building? How can I make it strong enough just like reinforced concrete so that it could pass the specifications?
A: Adobe is not an insulator at at all. It is a capacitor and stores heat wonderfully. No heat can be stored in insulation. So adobe does wonderful things when it is understood and for that reason much of the world's population lives in earthen homes quite comfortably while nearby homes of new, manufactured, technological sources leave the occupants panting for air conditioning or screaming for heat. All those engineers who grin and leer at adobe are just plain dead wrong. They can't fool 3 billion people.
You can certainly add rice hulls. There are a couple of papers in our AdobeUSA 2007 proceedings from Argentineans regarding rice hulls. The bricks are not hollow but have a large percentage of hulls. It is in essence what some call "cob". We call it monolithic adobe or "daub." Adobe without hotrodding is the original green building material. It is as autochthonous as anything can get.
Q: I want an adobe or comparable earth home. I am a long time crafter, always thinking of alternative ways to do things! Can I build a curvy (pine wood) facade and other walls, then add adobe brick (while wet) mixture one stage at a time, meaning wait till dry then go along say a day or two later and add another, till the wall height is reached, considering doing it in summer, hot dry? What benefit could I have from using a series of wood forms and how thick do you envision I could go? Also in one wall, a rustic fireplace with stone inside and outside?
A: That would be a good system for building. Systems with two walls and adobe mud filled in between are found in various parts of the world. It is sometimes called cribbing as in corn crib. It is also one permutation of wattle and daub. Doing it with just one wall will work as you propose by waiting for each layer to dry. The truth is you can even do it with no wood wall. Hand formed stiff mud layer by layer is the world's most widespread building system over history. It is called cob in some areas, monolithic adobe in others, and perhaps the root word for adobe was dub which became a'tob or daub as in wattle and daub. The problem with building the curvy pine wood wall is thermal. An inch of thickness of pine is enough to isolate the interior of the building from the powerful ability of adobe to store heat. The result is a wall with thermal properties more like a pine paneled room.
Also, it just might turn out that a brick and mortar wall is easier to build with curves than is a pine wood wall.
You can certainly make forms that are 4- to 6-inches high and build your wall in lifts as the drying time permits. One of my students, Dave C, left some of those type of forms hanging on the wall of our shop should you wish to see them. Curved forms could be built with several thin strips bent around a form and then glued together. This system saves human energy since the adobe is only placed once. However, the process stretches over more time as you wait for each lift to dry.
New Mexico now requires construction permits throughout the state. Remote sites have been known to escape detection. For permitted construction it will be hard to build a stone fireplace without a liner of fire brick that meets several requirements. It is hard to prove to the officials that stones do not burn. For an energy efficient home it is a disadvantage to have the fireplace pierce through the building envelope and present a stone exterior surface. Stone transmits heat much faster than adobe or insulated frame construction. It would be a cold spot in the room when a fire was not blazing.
Q: I live in southern Arizona and am told my home is of adobe brick. The brick is 15"x7". The interior of the exterior walls has 2x2's nailed to the walls with 1.75 insulation batting. I am having to rip the walls out due to termite damage. What would be the best method to put in a vapor and/or moisture barrier between the new walls and the exposed, uncoated interior adobe. I have been told everything from do nothing, to seal the wall, to install tar paper on the inside of the new frame. Code requires R13 insulation, which apparently requires new 2x4 walls. Cost has become a real factor. What would you suggest?
The best thing would indeed be to do nothing. The thermal power of adobes are lost once there is insulation applied to the inside of the wall. What is left is the thermal power of insulated frame construction which is nearly nothing. A vapor barrier should go on the warm side of a wall assembly which normally is on the inside surface of a wall or just under the sheetrock so tarpaper, but better yet, a real vapor barrier would be best located on the inside of the new frame. Just in case an inspector is advocating tarpaper, then he is to be ignored since dedicated vapor barriers have long since replaced tarpaper.
Q: I have a 2 story adobe house in Silver City, NM that I am trying to get insulated and re-stuccoed. I am having a very difficult time finding someone with skills to either put on 2" insulation board or spray on insulation. Do you know where I can find a list of contractors in my area that might have these skills. So far the only person I found locally has given me an estimate of $34000 to insulate and stucco. The area of the house is approximately 240 sq. yds that need insulation and 185 sq. yds. that are insulated frame and just need matching stucco.
A: I am out of touch with the contracting scene in Silver City. The yellow pages might be your best bet and you might have to look at Las Cruces or even El Paso. Start looking for an insulation company and see if they have their own stucco crew or one they work with frequently. Foam and stucco are fairly routine from Santa Fe North. It might be more rare around Silver City.
Q: I am working with a team on a greenhouse. We are a group of Engineers and architects. The house is located in Jordan, near the Dead Sea. My question is what materials we should use for insulation, and is it recommended to have airspace insulation?
A: Most often we insulate the exterior of an adobe wall with nailed on rigid polystyrene insulation or sprayed on polyurethane foam. We would rather use a more planet friendly insulation but have not yet found one. Architects in Portugal have used cork which is wonderful but not easily available or affordable in sufficient quantity in the USA.
Q: I have a recently constructed adobe with 2" of sprayed foam insulation on the outside of the outer walls. My issue is that the bedrooms are on the north side of the house and this winter, our first, have been cool to cold. I would like to add 2"-4" over the outside wall and replaster. Can the insulation (rigid foam board) be applied directly to the current plastered wall and then be re-plastered with no problem?
A: That will probably work Jim. If you are in a dry climate then you do not have to worry about moisture migration through the assembly and possible entrapment in the plaster between the two applications of foam. Another consideration is the weight of the foam and plaster which will be hung on the existing plaster. It might be worth considering long screws that will go from the stucco netting al the way through and two or more inches into the adobe.
Spend some time looking for other possible causes for your home being cold. There might be some heat loss through the ceiling/roof system or even the floor. Windows can loose heat through conduction and air infiltration. The new, cheap infrared thermometers give quick point and shoot temperature readings of surfaces. Sometimes they uncover a surprise cold spot. A little smoke can help show an air infiltration source.
I live in Arroyo Hondo, just north of Taos so it is pretty dry. Would it be better to go with spray foam insulation rather than rigid foam board?
The spray foam will cost more but it has an R-value of around 7 or 7.5 per inch. Rigid polystyrene is around R-5.5. The spray is preferred if the cost is not prohibitive since it is polyurethane which is the basic ingredient in Gorilla glue and other glues. The spray will stick to almost any substrate including existing stucco. The foam is closed cell and is a very good moisture and moisture vapor barrier. There are no seams and most applicators do a good job going right up to window and door frames so that the house is very air tight. The foam is self supporting but might need help to support the stucco and you would still need to attach the stucco netting mechanically through the insulation, the stucco and the first insulation into the adobe. I think. Your spray insulation application contractor might have another opinion. I would trust that opinion over mine.
Q: I have an adobe home built in 1968 with adobes 14" think laid for extra thickness. It is so cold in winter and I have been thinking about the spray foam insulation on the exterior. I live in the cold north of Taos County and I have been advised to use 4 inches. Recently I attended an event and spoke with some construction/contractor people here in NM. I am concerned about moisture and a vapor barrier. I had always heard to put the insulation on the outside. Now this contractor is advising 3 layers of regular stucco on the outside, and placing a vapor barrier on the inside walls, and then the insulation over that, on the interior, and finally the cosmetic plaster over that. What do you think?
A: Totally wrong.
Q: I live in Queretaro state (central plateau of Mexico) at about 6500' elevation. 18-20" of rain per year. Summer temps run 60º nights/75º-80º days. Winters run 33º nights/65º days. I would like rooms that are 12' wide with 24" adobe walls (14' on centers).
A: (Kelly) I lived in Bernal, Queretero for awhile, so I know that region pretty well. While a thick adobe-walled home will certainly be more comfortable than the common brick or cement block, especially in the spring, summer, and autumn, the winter time might be a bit chilly. You might consider leaving a cavity in the middle of those 24" adobe walls and fill it with pumice to isolate the inside from the outside. This will definitely help with the thermal properties of the home.
Q: I would like to make use of the thermal mass properties of Adobe. I have a freestanding fireplace against a wall in my lounge. Behind that wall is a bedroom which I would like to keep reasonably warm during the night. I was thinking of replacing the current timber wall with an Adobe wall, so that the heat, which is emitted from the fireplace can heat up the Adobe wall in the evening and then later when I go to bed I am hoping that the wall would release the stored heat during the course of the night. Do you think this will work well and, if so, what would an ideal wall thickness be so that the warmth will not be lost too quickly? (Note: The wall is not structural)
A: A ten-inch wall will give a heat lag of about six- to eight-hours. A fourteen-inch wall will add another two hours. This will work well if there is a way for the freestanding fireplace to put its heat into the wall. Direct contact is the best method.
Q: I have the idea to build a compressed earth block (CEB) or compressed stabilized earth block (CSEB) adobe style house in Australia. I like the idea of using natural materials in the house's construction that won't off gas VOCs in/around the house. I appreciate that the CEB / adobe style house has great thermal mass but understand that it needs a layer of insulation too. I thought about building a double wall with insulation or just air in between the two layers. I though to make the inside wall a the typically thick CEB/adobe block and the outer wall a thinner smaller block similar to the size of a regular brick (to save on labour). Now, in terms of insulation, I know that what is often talked about with adobe is the hard polystyrene panels tacked on the outside and then stuccoed. I don't like this idea because the polystyrene panels are not a natural material and it also seems to go against the idea that the house would breathe if made from CEBs. My question is, in this double cavity wall I have described above would it be suitable/acceptable to insert wool insulation batts instead? I think this would provide the insulation needed but also be 1) natural and 2) breathable.
A: (Kelly) Yes, I think that you could fill that void with wool, and this would effectively insulate the structure and keep it breathable. Another natural material for that is sawdust mixed with some lime. With either of these you would want to assure that they stay dry and don't settle too much.