Mass and Insulation with Adobe
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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: I’m 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.

R-factor is the reciprocal of U-Value. ( R = 1/U ) Because adobe is a storage device rather than a resistance device it generally performs well beyond predictions based on steady state numbers. The NM numbers are based on the Tesuque Thermal Study Project of the 1970's which used full-sized adobe test cells and computer numbers to mimic adobe's dynamic performance. I do know that there was an entire school district built in the Dallas area with adobe in the 1930's.

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?

A: Adobe walls don't have any specific R-values. In the State of New Mexico it is recognized that adobe walls will perform as if they have different R-values depending on the thickness of the wall, the climatic zone, the cardinal direction that the wall faces and the color of the wall. The walls can range from 2.5R to 22R. In your case a wall on the north side or below grade would act like the 2.5R. The preferred insulation to add to the exterior of a wall is 2-inch polystyrene. Dow Styrofoam Utilityfix xps has a rated R-value of 10 and a crush resistance of 15psi. It is a closed cell foam and will not loose its R-value to moisture uptake if it is below grade.

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
polystyrene, two-inches thick gives R 10 or 11 but, coupled with the Capacity/Conductivity of the adobe, an unexpected boost in performance will occur as if there are additional phantom R's

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
point of diminishing returns. Or have the wall foamer foam the roof too. I favor TC&I in Santa Fe. They have developed the best coating for the top of the roof insulation - gravel.

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.

As for the insulation, it's true that all the foamers drive big, diesel, dually, crew cab trucks that cost as much as a baby Freightliner. Being the simple guy that I am, I just bought the Freightliner. I make sure that no prospective clients see it until the contract is signed. One client was sure he had paid for my Dodge Dakota in 1992 so I learned my lesson.

My choice on my own home was to install the blue, rigid Dow foam myself and even do the three coat stucco myself. That saves a lot on the insulation but there are hairline cracks at the joints in the insulation. One solution is to tape the joints with some now forgotten waterproof tape and follow it with two layers of Jumbotex building paper before the wire and stucco.

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.

An adobe house performs differently than a lightweight structure. We were in a student's home last week that she is renovating. She has installed radiant floor heating, new doors and windows but no roof insulation yet. The original dirt is still in place on the roof. Her living room and kitchen felt warm to us. We found that the floor surfaces were averaging 74 degrees. The walls averaged 66 and the ceiling was about 68. In standard lightweight construction with conventional heating we would have expected to have found the floor to be around 68 degrees, the walls around 72 degrees and the ceiling around 80 degrees. It is not uncommon to find a temperature difference of fifteen degrees between the floor and ceiling due to heat stratification. This is why ceiling fans have become popular in the winter. With your walls any surface temperature below 80 degrees would feel cold to the touch as the adobe would draw heat out of your hand rapidly. This thermodynamics is tricky stuff and not for sissies.

This time of year is best for heat problems diagnosis. As the spring warms up and summer arrives the heat starts moving the other direction. See if you can verify the wall and ceiling insulation.

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

Q: My husband and are retirees and are currently negotiating to purchase an adobe home in Green Valley, AZ, south of Tucson. We are from California and know nothing about adobe construction. The house was built in 1969 by Fairfield (a big Green Valley builder), the "Hermosa" model, and all exterior walls are adobe brick which we are told are 8" thick (but not sure). It doesn't have any other insulation on the walls, inside or out. It is a 1378 sq. ft. house, with standard 8 foot walls, and a tile roof. We asked the realtors for information re: insulation "R" values for this house, and no one can give us a clue. We are on a limited retirement income and don't want to fall into a "propane black hole" in the winter, like I read in one of the posted Q & A's on your website. The house has a forced air gas heater and electric "refrigeration" type cooling. Can you please enlighten us as to what we might expect to encounter to heat and cool this house year around?

A: That's a tough one. On our adobe discussion group right now, we are hearing about some wonderfully cool adobe houses in Texas and Arizona from our roving Butopia2003. Most of them have very well insulated roofs or thick dirt roofs. And most of them have thicker walls than your house of interest. I don't know about Green Valley, but much of the area around Phoenix is
covered by the codes of several counties most all of which require 16-inch thick walls.

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.
Or ask for utility records, or get a trial month. If you can survive August, you can survive anything. If it stays cool then, it will most certainly stay warm in January.

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?

A: I would love to find a place with extreme moisture here in the Southwest. Adobe and other types of earthen structures are found in hot arid climates because the material is near at hand but also because the material is simply the best at moderating heat. There are many ways in which the adobe building can be built to enhance its abilities to fend off the unwanted extremes of the climate. There are also ways in which the building can be designed to enhance the climate's ability to create thermal discomfort. The key, then, is to make sure the house is designed by someone who understands how to make it work with the climate. In extreme moisture, wood rots, steel rusts and microbes eat most everything else so adobe is no more delicate than any other material.

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.

It won't be as strong as concrete hollow blocks since concrete is around 3000psi and adobe runs around 300psi. We do not see much hollow adobe production in the USA but several German companies do it routinely. You will just have to do some experimentation but do it with the knowledge that it is possible.

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.