Adobe as Mass?

Question: 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.

Answer: 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 diffusivity which results in its releasing heat more slowly that the other materials which all have higher diffusivity.

Question: 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?

Answer: 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.

Question: I’m a 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.

Answer: 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 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 lose 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 this 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 Applications 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.

Question: I am trying to find the R-value of 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.

Answer: 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.

Question: 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 be 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?

Answer: 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.

Question: 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.

Answer: (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.