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Bale construction differs qualitatively
from wood-frame construction in how moisture collects and travels
within the wall, and should therefore be treated differently.
The use of sheet moisture barriers, although common practice in
wood construction, runs counter to successful historical building
practice and generally accepted current practice, is unsupported
by either empirical or test data, and is not required by straw-bale
building codes.
Background
The Uniform Building Code requires the use of sheet moisture barriers
underneath stucco to protect wood-frame structures from external
moisture. Wood-frame structures tend to concentrate moisture entering
a wall into localized areas. Plywood significantly blocks the
movement of air and vapor through a wall, and the stud wall matrix
of dense structure (wood) alternated with sparse material (air
or fiberglass), promotes to moisture concentration at certain
locations. Moisture concentrations can lead quickly to the growth
of dry rot.
Empirical and test data for bale walls, on the other hand, strongly indicate
that adequate moisture protection exists without the use of sheet membranes.
Consequently the use of sheet moisture barriers is not required in the "Guidelines for Straw-Bale Construction"
(California Health & Safety Code Section 18944.303(l)
and is expressly prohibited in some codes.
Moisture Mechanics
The mechanics of moisture within buildings is complex and dependent upon several
variables such as seasonal climate, building usage, heating and cooling systems
and wall construction. Moisture trapped within the building envelope (walls,
floor or roofs) can cause rot in either wood or straw. The composition of wood
and straw are quite similar, both consisting largely of cellulose plus inorganic
materials. At about 18% moisture content fungi which which are present in
wood and straw as spores, become active in breaking down cellulose, creating
what we know as dry rot. Below 18% r.h. the active fungi die off, but leave
behind millions of spores. Good building design does not allow enough moisture
to accumulate in the building envelope to reach a level where fungi will grow
and multiply.
Moisture can enter and exist in walls in two forms: liquid water,
such as might come from a leaky window; and water vapor, which
is water suspended in air. The mechanics of each form is different:
liquid water is moved by primarily by gravity and sometimes by
wind pressure; water vapor is moves with the air in response to
air currents within a building or its shell. These air currents
are often driven by pressure differentials created by temperature
differences between the interior and the outside, or pressures
created mechanically by heating and cooling equipment.
Water vapor, except in extremely humid environments, is generally
not a threat to wood or straw unless it condenses and becomes
liquid water. Warm air is capable of holding more water vapor
than cold air. When warm, moist air encounters cold air or cold
surfaces, the warm air cools and the water vapor it carried condenses,
creating rain or dew. The point within a wall at which condensation
can occur, called the dew point, varies with the difference in
temperature between the inside and the outside and with the moisture
content of the air. This point is not always inside the wall,
but in hot or cold climates the dew point, and the potential for
rain inside the walls, commonly falls inside the wall. Vapor entering
the wall will condense at the dew point and change to liquid.
This liquid may accumulate and create the conditions for dry rot
to occur.
The water entering a wall, then, can change from vapor to liquid;
and liquid inside a wall can also evaporate, that is, change from
liquid to vapor. We have used the term "moisture" to
include both liquid and airborne water when referring to conditions
which could include both.
top
Principles of Moisture in Bale
and Wood-frame
Although their organic composition is similar, bale and wood walls
differ significantly in structure, creating major differences
in how moisture enters, leaves and travels within the building
shell.
Bale walls are made up of millions of individual straws which
tend to absorb and disperse relatively large quantities of moisture
uniformly. Imagine, for instance, turning a hose on a straw bale.
Considerable water will be absorbed before runoff will occur,
and after a short while the water absorbed will be fairly evenly
distributed throughout the volume of the bale. Now imagine turning
a hose on a stack of studs and plywood--the water would begin
to run off almost at once, and any water which would remain would
collect in cracks and crevices.
This tendency of bales to absorb and disperse moisture, works
to protect the interior of the wall from high concentrations which
would support fungal growth. Moisture entering a bale wall can
be `stored' over the very great surface area of the straw itself;
and then be transpired back through the stucco when conditions
permit.
Whereas plywood and other wood elements block moisture migration,
straw bales transpire moisture relatively efficiently. The uniform
permeability and density of a bale wall makes it able to transfer
and transpire moisture more equally over its entire volume and
surface. Imagine, for instance, building a plywood box the size
of a bale, and dumping in a gallon of water before sealing the
top. Next to it, place a straw bale and pour a quart of water
into the bale. After a month, what will be the condition of each?
Problems with (and studies of) non-breathable stucco finishes
have shown that the ability of a wall to transpire moisture is
vitally important in protecting the wall from dry rot. The health
and longevity of older bale structures, built without moisture
barriers and often with poor exterior finishes, may be attributable
to the ability of bales to naturally breathe off moisture and
maintain safe humidity levels.
Moisture within a stud wall is commonly led along a few passageways
within the wall: air channels, eddies and backwaters within cavities
direct vapor laden air; and solid wood elements working with gravity
concentrate the movement of water into certain limited areas within
the wall. This channeling leads to collections of water and localized
areas of high humidity and potential rot, making it extremely
important to minimize the amount of moisture entering a wood wall.
Water does not travel so easily within a bale wall. Although breathable,
bales are dense and difficult to penetrate--like standing under
a dense tree canopy in the rain, water doesn't penetrate easily.
Because stucco wicks liquid readily, the combination of the plaster
coating and the density of the bales forms a breathable membrane
which accumulates and transpires moisture in response to exterior
humidity and climactic conditions.
top
Use of Sheet Moisture Barriers
The almost universal practice among straw-bale builders, whether
in California, Arizona, Washington or Nova Scotia, is to avoid
the use of sheet moisture barriers or impermeable stuccoes over
the bales. Experience with straw-bale structures, in a variety
of climates, indicates that these barriers are not necessary and
may even be detrimental.
The introduction of a sheet moisture barrier, even a `breathable' product, inhibits
the natural transpiration of the bales and may even create a surface which would
concentrate moisture within the wall. Although products such as Tyvek transmit
vapor, they block liquid moisture. Consequently vapor traveling from the building
interior condensed inside the bale wall would be unable to leave the wall except
as vapor and could collect at the membrane and cause rot.
The straw/stucco membrane, which allows both vapor migration
and transpiration of liquid, can allow such moisture to wick out to the exterior
more readily.
Centuries of experience has shown that water will get inside building
walls, either as condensed vapor or as water entering from the
exterior. Because a bale wall can accommodate accumulations of
moisture which a wood-frame wall cannot, common practice has been
to maintain a breathable wall as the best safeguard in preventing
harmful moisture accumulation in bale walls.
top
Structural Advantages of a
Direct Plaster/Bale Bond
The natural mechanical bond between plaster and bare straw bales
is impressive in its strength and tenacity. Because the stucco
finish on a building can contribute significantly to the strength
of the building, incorporation of the stucco into the entire wall
system can improve the structural performance of the building.
The decision to use a sheet membrane over bales effects not only
the permeability of the bale wall, but the strength and integrity
of the plaster coating as well.
Individual straws protrude from the bales into the plaster and
form a natural reinforcement for the plaster; and the uneven surface
plane of the bales creates a multitude of large and small mechanical
keys into the bales. The strong bond between bales and plaster
makes lathing unnecessary for adhesion of plaster, although lathing
may be used as reinforcement for the plaster. If used, lathing
may be attached as needed to prevent `bellying', but need not
have extensive ties through the bales for adherence to the wall.
When plaster is applied directly to bales, a network of thickened
plaster is created at the bale joints, creating a `waffle truss'
which significantly strengthens the stucco layer. In addition,
a light stress-skin panel is created by the bales when cement
stucco is used both inside and out. And because the straw behind
the plaster has some flexibility, movement of the bales over time
can be accommodated without transferring stresses to the plaster
layer as stress cracks. These structural advantages are lost,
of course, when a sheet barrier is applied over the bales.
top
Recommendations
Good bale building design would concentrate on preventing gross
leakages of water into the wall and on providing a means for accumulated
moisture to exit the wall easily. This often involves the use
of breathable exterior finishes, as well as exterior design details
which promote, or at least don't block, air-flow around the building.
Care should be taken in flashing horizontal bale surfaces, such
as at window sills, and any wood at the outside of a bale wall
should be wrapped in paper as with wood-frame construction. A
splash guard consisting of a one- or two-course membrane at the
base of a wall is commonly used to protect against rain-splash.
Good design would also optimize the structural advantages of bale
assemblies. Even in post-and-beam structures, good design would
use the bale walls as an important redundant assembly in the event
of failure in the primary structure. top
Data and Information
There are no historical precedents of bales being used with moisture
barriers, and consequently there is no data on how the two perform
together. Most historical data for unwrapped bale walls demonstrates
the importance of maximum breathability of bale walls:
Buildings constructed early in this century which survive today
in and around Arthur, Nebraska, show no evidence of rot or deteroiation
within the walls. These buildings typically were constructed with
1-2' roof overhangs and were originally plastered with earthen
materials and later replastered with cement-based plaster. (See
"Historic Straw bale Structures, unpublished paper by David
Eisenberg).
A mansion in Huntsville, Alabama has successfully endured Southern
humidity since 1938; a 1978 building near Rockport, Washington
receives up to 75 inches of rain a year; and an unplastered building
near Tonasket, Washington, with no foundation and unplastered
walls shows no apparent deterioration of the bales since 1984.
Of the hundreds of bale buildings standing in the Southwest, none
have used a paper moisture barrier. Recent bale structures in
northern New York (humid winters) and Nova Scotia (cold humid
winters) have been monitored and demonstrate good performance
in these difficult climates.
Prepared by John Swearingen
Skillful Means
PO Box 207
Junction City, CA 96048