Natural building
techniques
A natural
building involves a range of building systems and materials that
place major emphasis on sustainability. Ways of achieving sustainability
through natural building focus on durability and the use of minimally
processed, plentiful or renewable resource, as well as those that, while
recycled or salvaged, produce healthy living environments and maintain
indoor air quality. Natural building tends to rely on human labor, more than
technology. As Michael G. Smith observes, it depends on "local ecology,
geology and climate; on the character of the particular building site, and on
the needs and personalities of the builders and users.
The basis of natural building is the need to lessen the
environmental impact of buildings and other supporting systems, without
sacrificing comfort, health or aesthetics. To be more sustainable, natural
building uses primarily abundantly available, renewable, reused or recycled
materials. The use of rapidly renewable materials is increasingly a focus. In
addition to relying on natural building materials, the emphasis on the
architectural design is heightened. The orientation of a building, the
utilization of local climate and site conditions, the emphasis on natural
ventilation through design, fundamentally lessen operational costs and positively
impact the environmental. Building compactly and minimizing the ecological
footprint is common, as are on-site handling of energy acquisition,
on-site water capture, alternate sewage treatment and water reuse.
Porch of a modern timber framed home
Materials
The materials common to many types of natural building are
clay and sand. When mixed with water and, usually, straw or another fiber, the
mixture may form cob or adobe (clay blocks). Other
materials commonly used in natural building are: earth, wood, straw,
rice-hulls, bamboo and rock. A wide variety of reused or
recycled materials are common in natural building, including urbanite
(salvaged chunks of used concrete), tires, tire bales, discarded bottles and
other recycled glass.
Several other materials are increasingly avoided by many
practitioners of this building approach, due to their major negative
environmental or health impacts. These include unsustainably harvested wood,
toxic wood-preservatives, Portland cement -based mixes, paints and other coatings
that off-gas volatile organic compound (VOCs), and some plastics,
particularly polyvinyl chloride (PVC or "vinyl") and those
containing harmful plasticizes or hormone-mimicking formulations.
Techniques
Many traditional methods, techniques, and materials, are now
experiencing a resurgence of popularity, however the relative popularity of
these techniques differs around the World.
Adobe
One of the oldest building methods, adobe is simply clay and
sand mixed with water. Often, chopped straw or other fibers are added for
strength. The mixture is then allowed to dry in the desired shape. Usually
adobe is shaped into bricks that can be stacked to form walls.
Various claims are made about the optimal proportions of
clay and sand (or larger aggregate). Some say that the best adobe soil contains
15% - 30% clay to bind the material together. Others say equal proportions of
clay and sand are best to prevent cracking or fragmenting of the bricks.
Sometimes adobe is stabilized with a small amount of cement or asphalt emulsion
to provide better weatherproofing. The blocks can either be poured into molds
and dried, or pressed into blocks. Adobe colored with clay and polished with
natural oil makes an attractive and resilient floor.
To protect the walls and reduce maintenance, adobe buildings
usually have large overhanging eaves and sizeable foundations. Adobe can be
plastered over with cob or lime-based mixes for both appearance and protection.
Adobe has good thermal mass, meaning that it is slow to transmit heat or cold.
It is not a good insulator, however, so insulation can be added (preferably on
the outside), or a double wall built with airspace or insulation in between.
The traditional thick, un-insulated adobe has proven to perform best in regions
without harsh winters or where daily sun is predictably available during those
cold periods.
Cob
A small cob building with a living roof
The term cob is used to describe a
monolithic building system based on a mixture of clay, sand, straw and earth.
The construction uses no forms, bricks or wooden framework; it is built from
the ground up. Various forms of "mud" building have been used in many
parts of the world for centuries, under a variety of names, and date from at
least 10,000 years ago. Cob building began use in England prior to the 13th
century, and fell out of favor after World War I, although it is seeing
resurgence today. Cob is one of the simplest and least expensive building
techniques available, though it is typically very labor-intensive. Cob's other
great advantage is versatility; It can easily be shaped into any form. While
cob building was falling out of favor in England by the late 19th century,
thousands of cob structures have endured to the present. It is estimated that from one third to one
half of the world's population lives in earthen dwellings today. Although
typically associated with "low-rise" structures, in Yemen and
other Middle-Eastern countries, it has, for centuries, been used in
"apartment" buildings of eight stories and more.
Cob-like mixes are also used as plaster or filler in several
methods of natural building, such as adobe, earth bags, timber frames,
cordwood, and straw bales. Earth is thus a primary ingredient of natural
building.
Cordwood
A section of a cordwood home.
Cordwood construction is a term used for a
natural building method in which "cordwood" or short lengths pieces
of debarked tree are laid up crosswise with masonry or cob mixtures to
build a wall. The cordwood, thus, becomes infill for the walls, usually between
posts in a timber frame structure. Cordwood masonry can be combined
with other methods to produce attractive combinations. Cordwood masonry
construction provides a relatively high thermal mass, which helps to
minimize fluctuations in temperature.
Earth bag
Earth is the most typical fill material used in bag-wall
construction techniques. This building method utilizes stacked polypropylene or
natural-fiber (burlap) bags filled with earth or other mixes, with or without a
stabilizer such as Portland cement, to form footings, foundations, walls and
even vaulted or domed roofs. In recent years, building with earth bags has
become one of the increasingly practiced techniques in natural building. It
facilitates self-contained, often free-form rammed-earth structures. Its
growing popularity relates to its use of an abundant and readily available
often site-available material (earth) in a potentially inexpensive building
technique that is flexible, and easy to learn and use. However, because earth
is a poor insulator, in more extreme climates other filler variations are now
being explored, substituting pumice, rice-hulls or another material with better
insulating value for all or part of the earth.
Rammed earth
Rammed earth is a wall system made of compacted earth, or another
material that is compacted. It is extremely strong and durable. Quality
rammed earth walls are dense, solid, and stone-like with great environmental
benefits and superior low maintenance characteristics. As an option depending
on climate or seismic concerns rigid insulation can be placed inside the wall
as well as steel reinforcement. Rammed earth has been used for around 10,000
years in all types of buildings from low rise to high-rise and from small huts
to palaces.
Rammed earth walls are formed in place by pounding damp
sub-soil (containing sand, clay and sometimes gravel) into movable, reusable
forms with manual or machine-powered tampers. In traditional rammed earth, a
mixture of around 70% aggregate (gravel, sand) and 30% clay is optimal. Pigmentation
may be added if the mix to achieve the desired color. Around 5-10 inches
of mixed damp sub-soil are placed inside the forms and pounded to total
compaction and the process is repeated until the desired height is achieved.
What is left after the forms are removed is a wall that is structural and can
last over 1000 years.
Stucco
Stucco or render is also often employed. Particularly lime
render is popular with natural builders. In some
instances, concrete is added to the mix; however due to the relatively
high co² emissions of concrete, it is often avoided.
Stone
Dry Limestone Bridge across Hubb Creek, Wellington, Ontario,
Canada.
It was built by members of 'Dry Stone Walling Across Canada' using 38
tons of stone.
Stone has been used as a building material for thousands of
years. It has long been recognized as a material of great durability. The
pyramids in Giza, burial chambers in the UK and temples in Malta were all built
from stone over 4000 years ago and are still standing. The earliest form of stone
construction is known as dry stone, or dry stacking. These are freestanding
structures such as field walls, bridges and buildings that use irregularly
shaped stones carefully selected and placed so that they fit closely together
without slipping. Structures are typically wider at the base and taper in as
height increases. They do not require any special tools, only the skill of the
craftsman in choosing and placing the stones.
Traditional stone masonry evolved from dry stone stacking.
Stone blocks are laid in rows of even (courses) or uneven (un-coursed) height,
and fixed in place with lime mortar pasted between the stones. Traditional
stone masonry is rarely used today because stone is expensive to quarry, cut
and transport, and the building process is labor and skill-intensive.
Stone is a highly durable, low maintenance building material
with high thermal mass. It is versatile, available in many shapes, sizes,
colors and textures, and can be used for floors, walls, arches and roofs. Stone
blends well with the natural landscape, and can easily be recycled for other
building purposes.
Straw bale
Straw bale construction in Santa Cruz, CA
Although grasses and straw have been in
use in a range of ways in building since pre-history around the world, their
incorporation in machine-manufactured modular bales seems to date back to
the early 20th century in the Midwestern United States, particularly the
sand-hills of Nebraska, where grass was plentiful and other building
materials were not. Straw bale building typically consists of stacking a series
of rows of bales (often in running-bond) on a raised footing
or foundation, with a moisture barrier between. Bale walls are often tied
together with pins of bamboo, re-bar, or wood (internal to the bales or on their
faces), or with surface wire meshes, and then stuccoed or plastered, either
with cementaceous mixes, lime-based formulations or earth/clay
renders. Bale buildings can either have a structural frame of other materials,
with bales between (simply serving as insulation and stucco substrate),
referred to as "infill”, or the bales may actually provide the support for
openings and roof, referred to as "load bearing" or
"Nebraska-style", or a combination of framing and load-bearing
may be employed, referred to a "hybrid" straw bale.
Typically, bales created on farms with mobile machinery have
been used ("field-bales"), but recently higher-density
"re-compressed" bales (or "straw-blocks") are increasing the
loads that may be supported; where field bales might support around 600 pounds
per linear foot of wall, the high density bales bear up to
4,000 lb./lin.ft. and more. And the basic bale-building method is now
increasingly being extended to bound modules of other often-recycled materials,
including tire-bales, as well as those of cardboard, paper, plastics and used
carpeting, and to bag-contained "bales" of wood-chips, rice-hulls,
etc.
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