homestead AT lists.ibiblio.org
Subject: Homestead mailing list
List archive
[Homestead] Earthen house construction---advantages, problems to avoid
- From: tvoivozhd <tvoivozd AT infionline.net>
- To: homestead AT lists.ibiblio.org
- Subject: [Homestead] Earthen house construction---advantages, problems to avoid
- Date: Tue, 01 Feb 2005 21:05:52 -0500
| Back to Contents Page | Home Energy Index | About Home
Energy |
| Home Energy Home Page | Back Issues of Home Energy |
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.
Home Energy can be reached at: contact AT homeenergy.org
Home Energy magazine -- Please read our Copyright Notice
- [Homestead] Earthen house construction---advantages, problems to avoid, tvoivozhd, 02/01/2005
Archive powered by MHonArc 2.6.24.