Straw Bale Home Basics
by Kenton Shepard Edited by Laura Bartels
Certified Master Inspector GreenWeaver Inc.
Boulder, CO Straw Bale Consultant
NACHI member Carbondale, CO
www.peaktoprairie.com www.greenweaverinc.com
peaktoprairie@msn.com laura@greenweaverinc.com
Although homes have been built in the US using straw bales for over a hundred
years, methods for their construction have evolved to meet the changing needs
of their inhabitants.
Building Science is the study of how buildings are affected by moisture,
temperature changes and air movement.
Rising energy costs have encouraged the construction of homes designed to
minimize air leakage. Tighter building envelopes affect the way moisture
produced by activities such as bathing and cooking accumulates inside homes
and in wall assemblies. Straw bale homes are no exception.
Home system types must be combined in ways that work well to keep the home
safe and comfortable. When a home is built using methods that minimize air
leakage, ventilation must be increased to avoid accumulating excess moisture in
the home interior and in wall assemblies.
Pre-purchase or pre-sale inspection of straw bale homes requires basic
knowledge of commonly-used straw bale construction techniques, some special
instruments, a working knowledge of Building Science and some research into
what methods are appropriate for various climates.
There are two types of straw bale homes:
1. Load-bearing straw bale walls (including the plaster interior and exterior
wall coverings) support the roof and lateral (such as wind) loads.
2. Non-load bearing straw bale walls typically consist of a post and beam
structure which supports the roof and lateral loads. This structure is
infilled with straw bales which provide insulation.
Exterior and interior wall coverings are plastered using cement/ stucco, earthen
plaster, lime, or some combination of these. Plaster should be applied directly to
the straw.
CONCERNS WITH STRAW BALE HOMES
Moisture Intrusion of the Straw Bales
Moisture intrusion is the number one concern with straw bale homes, just as it is
with conventional homes. Because straw bales can provide food for decay
fungus, wide-spread, long term fungal activity can destroy a straw bale home.
In addition to decay of the straw, decay fungus are a concern because mold
fungus release spores. High concentrations of mold spores in indoor air can
cause health problems in infants or the elderly, people with compromised
immune systems, lung disease or allergies.
Preventing Moisture Intrusion
A number of design methods are used to keep moisture out of the wall cavity or
allow it to escape once it gets in there&
1. Foundation
a. The foundation should extend above the exterior grade far enough
to keep the bottom of the outside wall plaster a minimum of 12
inches above grade to minimize damage from splashback.
b. The bottom of the lower course of bales should be a minimum of 3
inches above the interior finish floor. This will help prevent soaking
the bales if the floor should flood.
c. The lowest course of bales should rest on a waterproof material
which will provide a capillary break (such as plastic sheeting) to
prevent bales from wicking up moisture from below.
2. Exterior
a. Roof overhangs should be extensive to protect walls from weather.
b. Flat roofs should be avoided because they re likely to leak.
c. Windows should be installed so that they extend past the exterior
surface of the plaster. Whether windows are installed to the
exterior or inset from the exterior wall face, high-quality pan
flashing should be installed under the windows. Any sills should
slope and extend past the exterior face of the plaster.
3. Interior
a. Shower stalls should be placed against interior walls.
4. Plumbing
a. Plumbing pipes should not be routed through straw bales. Pipes
should be installed in the floor or in waterproof channels designed
with a clear drainage path out of the straw bale wall. This includes
supply pipes for exterior faucets.
If pipes must be routed through straw bales they should be
contained within watertight (preferably seamless) conduit which is
sloped to the exterior.
5. Moisture barriers or retarders
Wall plasters should be applied directly to the straw bales. No
plastic, polymer or other barriers or retarders should be installed in
wall assemblies. In addition to trapping moisture in the wall cavity,
installing a barrier will limit the shear strength of the wall assembly,
which relies on good adherence of the plaster to the straw.
PLASTER BASICS
Straw bale homes are typically plastered with one of four different types of
plaster&
1. Stucco-cement is a cementicious material. Although in the past it has been
the material of choice, difficulty in repairing cracks and low permeability to
water vapor are causing it to be seen by many as inferior to earthen or
lime plasters. Low permeability may result in moisture becoming trapped
in the wall cavity where it can encourage the growth of decay (mold)
fungus.
2. Gypsum plaster has been used for many years and before drywall become
the interior wall covering of choice, interior walls of most conventional
homes were covered with gypsum plaster. Because it is relatively soft and
water soluble its use is limited to interior applications.
3. Earthen plasters are composed of various combinations of clay-based
earth, lime, sand and chopped straw. Other additives such as mica,
various fibers and pigments for color are often added.
Because clay in earthen plasters is Hygroscopic (water absorbing),
earthen walls can help to temper interior humidity.
These plasters act as a barrier to water in its liquid form, but will allow
water vapor to pass through so that moisture is not trapped within the
walls.
Cracks or changes can be more easily repaired or blended when using
earthen or lime plasters.
4. Lime plasters have been used around the world for centuries. They are
made from limestone which has been heated and powdered. When mixed
with sand, water and fiber and allowed to cure it hardens, providing
durability and acting as a barrier to water in its liquid form, while
remaining permeable to water vapor.
Lime plasters are easily repaired and have anti-microbial properties.
Application Procedures
Plaster is often applied in three coats&
1. The scratch coat is worked well into the straw to provide a good bond
between the plaster and straw. It is used to build out low spots in the wall
and build up the thickness of the wall surface.
Sometimes, a thinner layer of diluted plaster called a slip coat will be
applied first in an effort to get better penetration into the straw.
2. The brown coat applied next adds thickness and further flattens the wall
surface.
3. The thinner finish coat provides durability, color, texture, and is what you
see when you look at the wall.
Allow each coat to dry&
Allowing each coat to dry completely will help prevent cracks from being
transmitted from underlying coats to newly applied coats.
Mist before applying mud&
The straw or plaster substrate should be misted with water before a fresh coat is
applied. This will help prevent a dry substrate from sucking the water out of
newly applied plaster. If the new coat loses too much water to thirsty substrates,
it may not bond or cure properly and the result can be an easily eroded, abraded
or detached layer.
New coats will need occasional misting for a couple of days after they are applied
and all coats should be protected from the sun and wind as they cure.
CRACK DIAGNOSTICS
Cracking is a natural process with the earthen and cementicious plasters typically
used to cover interior and exterior walls in straw bale homes. They shrink as they
dry - they can crack as they shrink.
Cracking plaster is the #1 maintenance issue with straw bale homes.
Cracks through the finish coat should be repaired to prevent increasing damage
from the freeze/thaw cycle. Hairline cracks are not a problem, but should be
monitored and repaired if they widen.
Cracks through multiple coats may allow moisture intrusion and are a defect
requiring immediate repair to prevent moisture intrusion of the straw-filled wall.
Sagging or Weight Cracks
If the heavy, wet plaster has not bonded well to the straw or mesh to which it
has been applied, gravity will begin to pull it toward the floor&
1. in parts of the wall where it has been applied thickly to fill in low spots.
2. if the plaster has been mixed with too much water.
3. if the plaster has been mixed with inadequate amounts of fiber (chopped
straw).
This kind of cracking is common in the first coat and to a lesser extent, in the
second.
These cracks will often extend across a section of thick plaster and take the
shape of a frowning or smiling mouth.
If you see this kind of cracking in a finish coat it may mean that&
1. cracks in underlying coats were not allowed to cure completely before
subsequent coats were applied.
2. cracks in underlying coats were not filled completely when subsequent
coats were applied.
Inadequate Plaster Mix
While excessive amounts of binder in the plaster can cause cracking, too little
binder can cause the plaster to be weak or crumbly. Especially with earthen
plasters, additives are often used to modify or augment the qualities of the
clay/sand mix.
Improper Curing
When the wall surface is covered with extensive, spidery, multi-directional vein-
like cracks, the reason is probably improper curing due to inadequate hydration
(moistening) during the drying process.
Dry plaster will suck the moisture out of any wet plaster applied to it before the
wet plaster has a chance to hydrate completely, causing the bond between coats
to fail.
Inadequate hydration can be caused by&
1. exposure to sun and wind causing rapid evaporation.
2. inadequate wetting of the straw before applying plaster
3. inadequate wetting of dry plaster before additional coats are applied.
4. inadequate wetting of plaster coats as they are curing.
If cracks are wide and close together and plaster detaches when it is tapped or
scraped, the bond to the underlying material has failed and the entire coat in
problem areas needs to be removed and a new coat properly applied.
If cracks are widely spaced and narrow, it may be possible to simply patch them.
Shrinkage Cracks
The most common type of cracking in the clay, lime and cementicious plaster
wall coverings of straw bale homes is caused by the shrinkage of the plaster as it
dries.
Cracks often run diagonally between the longest diagonal distance in a wall (for
example lower left to top right) and may not extend all the way to the corners.
Some shrinkage cracks also emanate from the corners of doorway and window
openings.
Very thin cracks can be left alone, but cracks which can admit moisture to the
wall cavity should be repaired.
Cracks in stucco/cement may have to be widened to be repaired properly.
Different substrates
Plaster applied over different substrates, such as straw and wood, may crack
where different materials meet for two different reasons&
1. differential curing caused by dissimilar materials absorbing moisture from
wet plaster at different rates.
2. differential thermal expansion and contraction rates of dissimilar materials.
Structural movement
Movement of the home structure may result from&
1. foundation movement caused by
a. expansive soil
b. inadequate soil compaction
c. excessive moisture in the soil
d. inadequate foundation design
e. seismic activity
2. structure movement caused by
a. wind loads
b. inadequate structure design
c. Poor construction practices
These cracks also often appear at the corners of doors and windows or at the
upper and lower corners of the structure, but may appear in other areas,
depending on the nature of the problem.
You may be able to apply some of the guidelines for diagnosing poured concrete
foundation problems to determining the cause of the cracks you see in plaster
walls. This is where having a copy of the original plans and/or photographs of
the construction process may help.
PAINTS and SEALERS
Paints that form a membrane which is impermeable to moisture vapor should be
avoided in order to prevent sealing moisture into the walls. Other paints such as
lime paint and silicate paint are an appropriate final application.
Typically, non-toxic pigments are added to the final finish plaster or can be
applied with washes using a brush or roller.
Sealers such as siloxane can be used to reduce moisture intrusion, improve
durability while maintaining good vapor permeability to allow moisture in walls to
escape.
The ADVANTAGES to BUILDING with STRAW BALES
High thermal insulation value
Straw bale wall assemblies provide an R value somewhere between 30 and 36
according to authorities such as the California Energy Commission and Oak Ridge
National Laboratories. Actual R values will very depending on how well voids
within walls are filled. A typical 2x6 exterior wall assembly with fiberglass
insulation is approximately R-22.
High sound insulation value
In conventionally-framed buildings, framing members act as sound bridges,
transmitting sound through walls. Because straw bales are non-rigid, they
dampen sound rather than transmitting it, making for an a wall with highly
effective acoustic insulating characteristics. This is one area in which loosely
compressed straw bales are superior to tightly compressed bales.
The Ability to Store Water
Both the interior and exterior plaster and the straw bales are capable of
absorbing and later releasing large amounts of water while remaining below the
levels at which decay fungus are active. This means that it takes more water to
bring these walls to the point at which mold will start to grow than wall
assemblies that have less water storage capacity. Because of the high
permeability of the straw bale/plaster wall assembly, it is able to efficiently
release this water through evaporation (and to a lesser extent, diffusion).
Fire Resistance
The majority of straw bale homes that burn are lost during construction due to
careless subs igniting loose straw. Once straw bales are sandwiched between
plaster, wall assemblies are extremely fire resistant. Because the straw inside
walls is compacted there is little oxygen available for combustion.
Properly constructed, plastered straw bale walls can withstand temperatures of
1800 degrees F. for up to 2 hours with little or no damage. Fire resistance varies
with straw bale density, the effectiveness with which interior wall voids have
been filled and the type and thickness of the plaster.
Walls plastered with earth and cement/lime have now passed 1 and 2 hour fire
rating tests respectively during full-scale ASTM e-119 tests.
Natural building material
Straw is the stems of cereal grains that have had the seed heads removed. It
contains no toxic glues or resins, it s relatively inexpensive and often locally
available. It s comparatively benign to work with, no trees are cut to provide it,
and very little energy is required to cut, bale and deliver it (embodied energy)
compared to materials used in conventional wall systems.
The DISADVANTAGES to BUILDING with STRAW BALES
Most problems relating to building straw bale homes can be traced to the fact
that there is much less experience with them and that methods involving
innovative techniques and ideas about what works best for a given climate are
still changing.
Lack of Qualified Professionals
Designers, building officials, general contractors, sub-contractors and home
inspectors are often ignorant of good building practices unique to straw bale
homes. This can lead to difficulties in getting plans approved, problems being
built into homes which are sometimes difficult to identify and/or correct both
before and after the home is complete and a lack of qualified pre-sale and pre-
purchase home inspectors.
Problems obtaining financing
The main concern of mortgage lenders is resale value. If they should have to
foreclose on the property, they want to feel confident that they can recover the
cost of the loan by selling the home. Areas with a larger percentage of straw
bale homes will be more likely to have financing available. This condition may
also improve with time.
A couple of companies offering construction, mortgage or green system loans for
systems such as photovoltaic and hot water systems in Boulder, Colorado are:
Colonial Savings
2500 30TH STREET #300
BOULDER, CO 80301
(303) 443-4427
Heritage Bank
3002 Bluff St
Boulder, CO 80301
(303) 413-0700
Problems obtaining insurance
There are companies who will write fire insurance policies on homes containing
straw bales, such as Farmer s Insurance, but at this time (Sept. 2006), many
insurance companies will not. is. Difficulty in finding coverage will depend to
some extent on the area in which the home is located. Availability will probably
change with time.
For more information :
The Last Straw http://www.thelaststraw.org
Quarterly magazine, comprehensive, covering straw bale and natural building.
Colorado Straw Bale Association http://www.coloradostrawbale.org/
extensive green links http://www.dcat.net/resources/links.php#straw
Ecological Building Network http://ecobuildnetwork.org
Non-profit organization developing educational programs for and promoting
green/sustainable building.
Straw bale testing results are available through this site.


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