R- and U-values, however, do not tell the Adobeas a building full story of what constitutes thermally ef- material ficient walls (Smith, 1982).Both of theseval- byGeorge S. Austin,New Mexico Bureau of Minesand Mineral Resources, Socorro, NM 87801 ues reflect the rate at which heat passes through the wall only after it has achieved a Introduction Pressedadobe brick is manufactured from steady-statecondition. Steadystate is when Adobes are dried mud or unburned bricks traditional or stabilized adobe materials that heat passes uninterruptedly from one side that have been used for thousandsof years are pressed into a dense brick with a hy- of the wall to the other at a constant rate. in the construction of dwellings and bther draulically operated machine or a hand- The heat capacity of a wall is not considered structures. Even today the majority of the operated press. The advantage of pressed in thesecalculations but is determined by the people in the world use mud-brick construc- adobe bricks is that generally they have a length of time that passes before a steady tion. The term adobe generally is used to higher compressive strength and cure more state of heat flow is achieved. In practice, describe various building materials made of rapidly than traditional ones. However, external temperature changesconstantly earth and the techniques for using these ma- without a stabilizer, the pressedadobe bricks during the day, so a true steady-state con- terials. Most often it refers to sun-dried brick, distintegraterapidly when wet. ciition is rarely achieved. currently the most widely used in the United Burntailobe, or quemado,is a traditionalsun- The net result of the thermal properties of States, but puddled earth material, mud- dried adobebrick that has undergonemod- adobebricks is the preservationof coolernight plasteredlogs or branches,cut soil horizons, ification by low temperature firing. Com- temperaturesinto the next day and of warmer and even rammed-earthconstruction also can bustible materials,usually wood, kerosene, afternoon temperatures into the following be identified as adobe. Generally, any struc- or old tires, are burned in a stacked-brick evening. Thus, there is a "flywheel effect" fure that has been made with soil or mud as kiln, which is built to allow air circulation that moderates temperatures within adobe a primary building materialis consideredto within. Combustibles are fed through small buildings. be adobe. doors at the end of the kiln, and smoke es- Burch et aI. (1982\, in a seminar held on capesthrough holes in the top. It takes from thermal-masseffects in buildings, observed Types of adobe two to four days of firing to produce 300 to that the most significant reductions in en- 500quemados. ergy during heating or cooling were found smith (1982)divided adobe bricks into six in the summer cooling season.During these types: 1) traditional, 2) semi-stabilized, 3) Thermal properties times the buildings "floated" during a por- stabilized, 4) terr6n (cut sod), 5) pressed Building materials are evaluated for ther- tion of the day (i.e., no heating or cooling adobe,and 6) burnt adobe(quemados). Each mal performancebased on conductivitymea- load was used during a part of the day, and type of brick is made somewhat differently. surementsknown as R- and U-values. The outdoor temperatures both rose above and Traditionaladobe brick is made with pooriy R-value indicates the ability of a wall to in- fell below the indoor temperature).The tests sorted soil composedof a more or leis uni- sulate effectively-the higher the resistance they conducted were performed at Gaithers- form mixture of sand, silt, and clay. Straw is to the conductive transfer of heat, the higher burg, Maryland, and involved uninsulated usuallyadded to the bricks to prevent crack- the R-value and the better the insulator. The masonry,insulated masonry, log, and wood ing when they are cured. Traditional adobes R-value is calculated by dividing the thick- frame (both insulated and noninsulated) have been used for centuriesin the south- ness of the wall by the wall's tfiermal con- construction. west United States.The majority of these ductivity, which is the amount of heat per Testsconducted in New Meico with adobe structureswere and are built on iield stone squarefoot per hour flowing from the hotter confirm the general results of Burch et al. or river-rock foundations to prevent under- to the cooler side of the wall. The U-value, (Gustinis and Robertson, 1983).However, cutting of the walls at ground level.This un- or conductance,is representedby the recip- these latter tests did not result in large sav- dercutting during weathering is the most rocal of the R-value and reflects the rate at ings in power during the summer because common cause of structural failure. Mud which heat is conductedthrough a material. the principal cooling devices in New Mexico mortar is used between bricks, and plaster or cement stucco covers the walls to prevent erosion of the adobe by water. Annual ap- plications of a wall covering are often used. Semistabilizedbricks area new classof adobe production developed by large-scaleadobe producers.A smallamount of stabilizingma- terial such as portland cement or asphaltic or bituminous emulsion is added to the adobe in order to obtain a partially water-resistant brick. The brick is made in essentiallythe same way as a traditional adobe bricli, but with, for example, 2-3% (by weight) as- phaltic emulsion added to the mix. Stabilizedadobe bricks contain enough sta- bilizer to limit water absorption of thebricks after seven days of immersion in water to less than 2.5Voby u'eight. A fully stabilized, commercial adobe brick contains between 5 and l2Vo asphaltic emulsion. Tetdn is Spanish for a brick made from cut sod or turf. In parts of the southwest U.S. terr6nes are still used widely; they usually measure7 x 7 x l4inches or4 x 7 x l+ inches.An ordinary garden spadewith a flat- tened blade cut down to measure7 x 8 inches commonly is used to cut the sod. Terr6nes FIGLJREl-South-facing home in Socorro, New Mexico, with exterior, lightweight, conventional in- are used in the same manner as traditional sulation covering adobe walls. Winter heating is provided by windows on upper level and fireplace adobebricks. on southeast (right) comer of home. New Mexico Geology November 1984 69 are evaporative coolers. These coolers use as a framework or structural element and Energy usage only a fraction of the energy needed to op- appearsto be the best combination. McHenry (1983)described a study done eratethe refrigeratedcoolers used in moister Adobe walls do have a tendency to crack by the energy researchgroup at the Univer- climates. with time. However, repair work is quite easy sity of Illinois and the architectural firm of The thermal efficiency of heating adobe becausethe samemud materialsand plaster RichardG. Stein and Associates,New York, buildings in the winter is not significantly that were used in the original wall can be that compares the energy costs of manufac- better than for conventional wood frame used for repairs. The resulting repaired turing nearly all common building materials. houseswith adequateinsulation because the structure is as strong as the original if the The total energy investment must include indoor temperatureis commonly held above repairs are made carefully, provided that the mining, shipping, processing, storing, han- both the day and night outsidetemperature. original crackingwas not a result of under- dling, shipping to the point of use, and in- pulls AIso, the thermalmass the interior heat cutting. Adobe structuresat Indian pueblos stalling the materials.Some of the common into the wall, causing greater use of gener- in New Mexico that received proper annual itemsreported in this study include common ated heat. As can be seen from many excel- carehave stood for centuries. brick, with an investedenergy rating of.13,570 lent examplesin the southwest U.S., past Btu per brick; concreteblock, with 29,0L8Btu adobebuilders responded to this problemby Building codesand tests per block; and portland cement, lime, and increasingthe thickness of walls. The ma- paving brick, all with large Btu values. Al- jority of adobeproducers today manufacture Building codes govern the construction of new buildings. The New Mexicobuilding code though traditional Southwestadobe brick has 10 x 4 x 14-inchbricks that weigh 30 lbs not been evaluated,McHenry indicatesthat each. Many older buildings contain adobe requiresthat adobe bricks have a compres- sive strength averaging 300 lbs per square the value would be approximately 2,500 Btu bricks with dimensions up to 6 x 12 x 24 per brick. inchesthat weigh up to 100lbs each.In some inch. The importanceof this test for a heavy materialsuch as adobebrick is apparent when casesthese bricks were used to constructwalls Utilization up to 2 ft thick. Even with the typical R-value the greatamount of weight a typicalwall unit of 2 for adobe, thick walls can preserve mod- must bear is considered.In addition to the Smith (1982)chiefly discussedadobe pro- erateindoor temperaturesin all but the cold- weight of the roof, each layer of brick in a ducersin New Mexico, but commercialop- est parts of the year. load-bearingwall must support the column erationsalso exist in the southwestU.S. from (1943) Burchet al. (1,982)also reported that a wall of bricks above it and it depends upon the Texasto California. Hubbell extends mass (such as that of adobe) has a larger compressivestrength of the underlying brick the area suitable for adobe construction effectwhen placedinside conventionalwall layersfor support. Another test required for northward into Oregon, Idaho, Wyoming, insulation as opposed to outside the same all types of adobe brick is the modulus of and even Montana (Fig. 2). Long and Neu- wall insulation.