[Homestead] Earthen house construction---advantages, problems to avoid

tvoivozhd tvoivozd at infionline.net
Tue Feb 1 21:05:52 EST 2005


 

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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.
Embodied Costs--The Real Price We Pay
Embodied cost is the complete life cycle cost of

gathering, manufacturing, transporting, assembling, and

even recycling building materials (see "Reducing the

Embodied Energy of Buildings," HE Jan/Feb '95, p. 19). As

long as the materials are gathered locally, earthen

structures of all kinds are real winners in terms of

embodied costs. In 1976, the Energy Research Group,

University of Illinois at Chicago, and architects Richard

G. Stein and Associates did a complete study of building

materials and the embodied energy that each represented.

They found that earth's construction methods gave it

definite advantages in terms of resource consumption.

Because adobe is primarily soil, the energy costs of

gathering and manufacturing are very low, and if it is

made from soil on site, the energy cost of transport is

also negligible.
Water is the enemy of earthen construction. This water

damage was caused by water leaking around old exterior

flashing. It has leaked earth into the interoir plaster,

causing stains and making the plaster peel off.
Print Resources
Easton, David The Rammed Earth House. White River

Junction, Vermont: Chelsea Green Publishing, 1996.

McHenry, Paul Graham, Adobe and Rammed Earth Buildings.

Tucson: University of Arizona Press, 1984.
Internet Resources
* Rammed Earth Works
http://www.rammedearthworks.com

* Cast Earth
http://www.castearth.com

* New Mexico Energy Conservation and Management Division
http://intertech.albuquerque.nm.us/ecmd/html/
Publications/Adobe/archives/Adobe_TOC.htm

* U.S. Department of Energy
http://www.eren.doe.gov/EE/strawhouse/

* The Earth Building Foundation
www.earthbuilding.com
To manufacture pressed adobe block, workers dump earth

into a hydraulic adobe press. A large screen at the top

sifts out rocks.
The dirt drops into the pressing machine and is

hydraulically compressed by almost half. The machine can

produce up to 900 blocks per hour.
The blocks drop from the machine onto a conveyor, ready

for a quick cleaning and immediate stacking on pallets.
A Cast Earth home under construction, with some walls in

the curing phase and forms set up to pour new walls.
This Cast Earth home features a passive solar design,

propane and wood heat, and an evaporative air conditioner.

The walls took just two days to pour and cure. Note the

wall's stair pattern, an indication of the building

process.
The walls of this Cast Earth house and garage show the

sandpainting effect of the building process, as well as an

extensive roof overhang that protects the walls from water

damage.
It takes a long time for even the hottest summer sun to

warm up a heavy earthen structure. That's one reason that

earth has been a popular building material in hot climates

for many centuries and is still common in the sizzling

Southwest where I live. Earth is one of many alternative

materials that can be used in place of residential stick

building. Due to its high thermal mass, in climates that

demand lots of cooling energy and have cool nighttime

temperatures, nothing works like earth.
Pros and Cons of Earth Building
Earth is a sustainable material, meaning that it has

little impact on the environment and that building with it

expends little or none of the earth's finite resources,

such as fossil fuels. It also has low embodied costs--the

costs to individuals and society together of creating,

storing, distributing, using, and disposing of a given

material (see "Embodied Costs--The Real Price We Pay").

And earthen construction has passed the test of time--

until recently, earth-building technology changed very

little from the mud brick homes that were first built

during the Neolithic period, back in 7100 bc.

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--Earth's Enemy

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

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

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.
 
 
This article is adapted from the author's book Alternative

Building Systems to be published by Fisher Books in

October 1999.

 

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