This article was originally published in the May/June 1999 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online May/June 1999
Earth Building Takes New Shapes
by Christina B. Farnsworth
Christina B. Farnsworth is a freelance writer and one of only three life members of the National Association of Real Estate Editors.
Earth as a building material can be great for both builders and homeowners--it offers low material costs and low transportation costs, especially for earth dug directly from the site, and high thermal mass, which can mean huge savings in energy.
Earth building does pose some problems. Earthen walls don't span open spaces or window and door openings very well, so they tend to crack near windows and doors that have inadequate metal or wood lintels. If the roof fails, moisture that seeps in can quickly erode the walls. Also, most earthen materials are unsuitable for homes of more than two stories because in order to carry the load of the upper walls, the lower walls would need to be thicker than is typically practical to build. Furthermore, labor costs for a two-story adobe home would be very high indeed.
Turn Off Your Air Conditioner
In hot, dry climates, the high thermal mass of earth-built homes can render them substantially more energy-efficient than stick-built homes, thanks to the flywheel effect of walls with high thermal mass (see Mass Walls Mean Thermal Comfort). A recent study by Oak Ridge National Laboratory (ORNL) found that the thermal mass benefit of high-mass walls is a function of the wall's composition and the climatic conditions where it is built. For example, in a hot, dry climate such as that in Phoenix, the energy demand of a one-story house with high-mass walls is equivalent to the energy demand of a similar wood-frame house made with R-25 light-weight walls. (More information on the ORNL research will appear in the next issue of Home Energy.)
When I don't use supplemental heating or cooling, the indoor temperature of my adobe house in Tucson, Arizona, seldom rises above 85°F in the summer or falls below 61°F in the winter--this in a climate with roughly 22 freezing nights a year and frequent summer temperatures above 105°F. Tucson has 2,000 annual heating degree days and 3,000 annual cooling degree days.
I only run the air conditioning during July, August, and part of September--the Tucson monsoon season. Otherwise, on most mornings, when the desert air temperature is coolest, I simply close all the windows and doors. Because it takes as long as 14 hours for the outdoor heat to pass all of the way through the adobe block and heat the indoor air, night has fallen before it gets hot indoors.
The humidity of the rainy season changes the equation and forces me to turn the air conditioning on. Humidity lessens the differential between daytime highs and lows, so drawing in night air no longer serves the same purpose. After mid-June, the days are their longest, meaning that the sun is heating the block longer and there are fewer night hours and higher night temperatures. That means the adobe block hasn't enough time to give up the heat gained during the day before it is daylight again. Nights also don't get as cool (somewhere in the 70s). However, running the air conditioning at night cools the house and serves the same effect as opening the windows. I usually turn off the air conditioning in the morning and let the house coast with its doors and windows closed until nightfall.
Thermal intertia is also at work in the winter, when fewer daylight hours mean the sun simply doesn't shine long enough to warm the adobe block. This means the house is cold for a much longer portion of the day than it is warm. I have often been surprised to go outside and find it warmer outdoors than inside. Forced air heat, which merely blows warm air around, isn't as effective in a high-mass house as radiant heat, which physically radiates warmth to people. Radiant floor heating warms the feet, the person, and eventually the adobe. It make take longer to get the walls warm, but once they are warm, they stay that way.
Thoughtful and selective insulation can efficiently modulate the thermal properties of earthen homes. Although an external thermal barrier would eliminate the earthen walls' role as solar heat collectors, that same insulation placed within the walls acts as a heat sink for the home's interior volumes. Adding insulation within north and east walls would be useful in a very cold climate, while insulating within a west wall would be advantageous in a hot climate.
Water is the enemy of all earthen construction. Water wicking up from the ground erodes the bases of earthen walls, causing them to crumble and fall away. Water from leaky roofs gets trapped in an earthen wall and oozes mud-laden water onto the ceiling; if the problem is not fixed, it will cause the plaster to pop off.
The degenerative effects of water can cause some earthen walls to simply melt, although it may take a long time. Vertical surfaces exposed to as much as 25 inches of rain per year will erode approximately 1 inch in 20 years. Horizontal surfaces like the top of a wall, on the other hand, can erode as much as 2 to 3 inches in a single year. Therfore, earthen homes must be carefully stuccoed or sealed. Most earthen walls are stuccoed, but if there is a deep roof overhang, clear wall sealer may be sufficient. How well an earth-built home stands up to water depends on how well it is protected by roofing and stucco or sealing, but also on how it was constructed and whether it contains stabilizers.
New Technologies Come to Market
Thermal comfort isn't the only draw of thick earthen walls. They also have an aesthetic appeal that is driving strong customer demand. Luxury homes lead this market; in New Mexico, half of all new homes selling for more than $300,000 are some form of earthen construction. Most of these are adobe, but a growing number of them are built using new earthen technologies.
The most significant structural innovation in earthen construction was the introduction in the 1930s of concrete foundations to protect the structure from water. Concrete overhead bond beams to increase structural integrity were another important change. More recently, various additives for strengthening the bricks and increasing water resistance have been incorporated into adobe mixtures. New construction methods and a revival of older building methods are also being used.
Today, several types of earth construction compete with traditional adobe: semistabilized and fully stabilized adobe, pressed adobe block, rammed earth, Cast Earth, and Pneumatically Impacted Stabilized Earth (PISE). See PISE--Thermal Comfort Made Practical for more detail on this last process.Adobe The word adobe refers variously to earthen blocks or bricks, to the mixtures used to form them, to a type of mud plaster, and to the entire building. New Mexico is the largest domestic producer and user of adobe. The state is home to more than 59,000 adobe dwellings, one-third of all adobe homes in the United States.
Most older, traditional adobe homes were built with earth excavated from the site. The resulting hole became the basement, so these homes have basements only as large as the volume of earth needed to build the walls. Then, as now, typical adobe bricks measured 4 inches x 10 inches x 14 inches and weighed approximately 30 lb.
Earth for modern adobe homes is typically quarried from commercial sites where the soil content is known, and the bricks are made at adobe brick factories. Adobe walls are typically 10 inches or 14 inches thick. Walls taller than 6 to 8 feet high are thicker at the bottom than at the top, to better support the load.
The kinds of adobe available today include traditional, semistabilized, and stabilized adobe. (Stabilizers are additives that make the adobe stronger and more water resistant.) Machine-pressed adobe block, often simply called pressed block, is another type of adobe that is gaining popularity in the Southwest. Such variations were developed in an effort to ward off the destructive effects of water. More unusual variations of traditional adobe include New Mexican terrones (cut-sod brick) and quemados (burnt adobe), but these are not used very often. According to New Mexico State records, semistabilized adobe is the most common variant currently in use today; before 1970, most homes were the traditional untreated adobe.
Traditional adobe bricks are found mostly in older homes. These untreated bricks are made out of soil and straw. The sandy alkaline silt and clay soils of the Southwest are much prized for adobe. The straw adds strength and prevents cracking.
In constructing these bricks, the adobe makers moisten the mixture of straw and soil to make a thick goop, then slap the goop into a mold, often a simple, four-sided wooden mold. The adobe may stay in the mold anywhere from two or three minutes to three or four hours to several days, depending on weather conditions and on how square the bricks are meant to be. (Bricks that have set for only a few minutes tend to slump or bulge out at the edges and are not always square.) Then they knock the brick free of the mold and let it dry out and cure in the sun for up to 30 days.
If the walls are dry and water is kept from wicking up into the building from the earth, an adobe building can easily last 100 or more years. Maintenance is important, though, since moisture can get in through cracks. My own house is 62 years old and is in fine shape. Over the years there has been minimal cracking in the plaster that covers the inside walls and the stucco that covers the outside walls. The house does have one advantage in that it sits on a concrete foundation.
Semistabilized adobe brick was developed in New Mexico. Previously, throughout the Southwest, sap from agave or prickly pear, straw, and manure were among the additives used to stabilize adobe. Today, stabilized adobe is made of plain earth mixed with a stabilizer that classifies the brick as water resistant.
Liquid asphalt emulsion stabilizer, 3%-5 % by weight, is the most popular additive because it is easy and inexpensive to use (it is a byproduct of the road building industry). Portland cement, 5%-10% by weight, is also an excellent additive, one that can be added to the dry earth and mixed in the same way as concrete. The Portland cement augments the structural integrity of the brick, so that the finished product is less crumbly than traditional adobe.
Fully Stabilized Adobe
Fully stabilized adobe contains enough asphalt emulsion or Portland cement to limit the brick's seven-day water absorption to less than 4% of its dry weight. This amounts to about twice as much stabilizer as is used in semistabilized adobe--6 to 12% by weight of the dry mixture. Because they are so well protected, these bricks will last longer under exposure to the elements (although they should also be sealed).
In 1994, 79% of the adobe bricks manufactured in New Mexico were semistabilized; 21% were the traditional untreated adobes. Fully stabilized bricks accounted for roughly 1% of the state's adobe production and were available only on special order. Fully stabilized adobe is more expensive than the other types and looks much more like concrete than like adobe.
Pressed Adobe Block
Pressed adobe block is the latest improvement to adobe. There are two kinds of pressed adobe block: natural and semistabilized. Semistabilized adobe pressed block contains 5% by weight Portland cement. Both types of pressed adobe block are tougher than and less crumbly than other types of adobe.
The molds used for pressed adobe block are almost twice as deep as the molds used for other types of adobe. Rather than simply curing the molded blocks in the sun, the manufacturer uses a hydraulic press to set the mud under extreme pressure. When the press goes to work, 4,000 lb per square inch (psi) of pressure quickly compresses the blocks to the traditional 4-inch thickness. Rather than the traditional 30 lb, each pressed adobe block weighs a hefty 40 lb. As for tensile strength (modulus of rupture), it takes 100 psi of pressure to break pressed adobe block--double the uniform building code requirement of 50 psi.
According to Keith Guffey of Tucson's Pascua Yaqui Adobe Company, pressed adobe block has been proven by the manufacturers to be stronger than other types of adobe. Other types of adobe, Guffey says, are strength rated at 400-600 psi, meaning that they can handle roof loads of that weight. Pressed adobe block is strength rated at 1,000-2,000 psi.Rammed Earth Rammed earth was used in the Great Wall of China. Puddled mud ruins, such as those at Casa Grande, an Indian ruin in southern Arizona, are at least 400 years old. Rammed-earth walls range from 12 inches thick to a hefty 18, 24, or even 36 inches thick. They can be as much as 22 feet high.
As well as increasing thermal mass, the thicker walls are often considered more aesthetic. The thickness of the wall depends partially on its height; like adobe walls, rammed-earth walls are thicker at the bottom than at the top. Wall height is a factor of load bearing; it is determined by the Uniform Building Code and local engineering standards.
Technically, rammed earth is a mixture of slightly damp, sifted earth (often from the site itself) and a small amount of cement (roughly 3% by weight, depending on soil composition). The best soils for rammed earth contain roughly 30 percent clay and 70 percent sand, but other soils such as caliche (a calcium-rich soil layer formed through water leaching) may also be suitable.
This mixture is tamped under pressure into wooden or metal wall forms. To begin the process, the wooden or metal form is filled 6 to 8 inches deep with moistened earth. Hand or hydraulic tampers pack the earth, compacting and reducing the volume by 25%. Once the forms are tamped full, which can take many hours for the entire house, the builder moves them up the wall to construct the next layers. It often takes three or more weeks to complete rammed earth walls.
Topping the finished wall is a poured-in-place beam of steel-reinforced concrete made with the same form that is used for the walls. Experts say that rammed-earth walls continue to harden, or cure, during the first year after construction and will last at least 100 years. Finished walls may be stuccoed, plastered, painted, or left natural and sealed. Like adobe, rammed earth is far from cheap. Because it is labor intensive, most rammed earth homes have been custom homes.Cast Earth Though Cast Earth is the new kid on the block, it is potentially an affordable method of earthen construction. Its advantages are enormous: embodied acquisition and transport costs on par with site-dug adobe, much lower labor costs than adobe or rammed earth, and a structural plasticity comparable to concrete. For example, Michael Frerking of Living Systems Architecture & Construction in Prescott, Arizona, says it used to take him three or more weeks to build the walls of a rammed earth house. That same house built in Cast Earth takes just a day and a half.
Harris Lowenhaupt of Phoenix, Arizona, is the inventor of the patent-pending Cast Earth wall system. Lowenhaupt's formula of earth, 10% to 15% calcined gypsum, and other additives creates a hard, cementatious wall that sets up in three to eight hours, depending on the exact composition of the formula. Although the gypsum imparts better rain resistance than adobe, the completed wall can be sealed with clear sealer. Like other stabilized earthen systems, Cast Earth needs good weater protection, such as an overhang, or some kind of finish, such as paint, stucco, or sealing.
Gypsum is a crystal of calcium sulfate and water. Heating drives off much of the water, yielding calcined gypsum, or Plaster of Paris. According to Lowenhaupt, this common and inexpensive industrial mineral adds properties similar to those of cement (rain resistance and added strength), but its strength is not affected by fine particles of soil.
When the gypsum is mixed with water and earth, then dries, it creates a lattice crystaline structure. This unique structure allows Cast Earth to set rapidly, gives it sufficient strength to support itself while wet, and allows it to dry to a much higher strength without cracking and shrinking.
Cast Earth develops a final compression strength of 600-700 psi, comfortably competitive with adobe and rammed earth. Lowenhaupt points out that the actual compressive load at the base of an 8-ft wall is only about 10 psi.
The crystalline lattice structure of the calcined gypsum gives Cast Earth a high tensile strength. According to Lowenhaupt, it consistently tests at about 300 psi, two to three times the tensile strength of adobe.
Cast Earth is usually constructed using concrete mixers. The contractor sets up wall forms, typically metal forms similar to those used for concrete. First a small mixer pumps the concrete stemwall into the forms. Then the Cast Earth dry ingredients and water are mixed to slurry in the concrete mixer. The mix is then pumped into the walls three feet deep or so at a time. By the time the forms have been partially filled around the perimeter of the house, the Cast Earth has set up enough to pour the next layer. The lines seen inthe finished wall are where two pourings overlap. Colorants can be added to the wet mixture or painted on the walls.
Lowenhaupt's wall-building process uses lightweight aluminum forms, but Cast Earth can be poured into forms of any size and shape. This makes it easy to create walls with radius curves, serpentine curves, and unusual angles. Foam insulation board can be placed in the center of the forms before the earthen slurry is poured into them.
The costs of Cast Earth construction are job and site specific; materials, labor, water, site accessibility, and the size of the project all have considerable influence on total costs.Consider the Merits Builders committed to sustainability and effective energy utilization need to consider the merits of earthen construction. Building codes and lenders, for the most part, no longer discriminate against these ancient construction materials. Current technological developments are bringing them into parity with other commerical materials while maintaining--and even in some cases, improving--the well-known thermal comfort qualities of high-mass earthen construction.
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