Building with Straw
The rebirth of interest in traditional construction techniques, in part a
search for new sustainable construction approaches, means that many people
are looking at "alternative" construction materials. Cobb,
rammed earth and straw bale are just a few of the approaches being
revived. Each has its aesthetic and technical features. However, it is
important to remember that each also has its technical and practical
Straw bale construction is an option that has attracted wide attention.
Straw is a natural, affordable and annually renewable building material
that has been used to shelter people around the world for thousands of
years. Straw bale is simply one of many options for more sustainable
construction. Its use depends upon accessibility to surplus bales.
Building with bales is a North American approach that emerged with the
invention of the horse-drawn baling machine in the 1880s.
Perhaps the highest profile straw bale homes are a series of 100-year-old
pioneer houses in Nebraska, a dry area.
The prime design consideration must
be to protect the straw from exterior wetting. When designs are
inappropriate, straw bale walls can fail, even in dry, low
precipitation climates. Successful designs incorporated
combinations of the following strategies:
the use of verandahs, oversize gables and overhangs for direct control of precipitation
and overhangs with a combination of other factors including:
- protection afforded by topography
- minimal wind driven precipitation
- infrequent exterior wetting
accompanied by prolonged drying
- cement-based parging used on
exterior and interior
- bales elevated above grade to
produced borderline or unacceptable moisture readings included:
minimal or no
no capillary break
between foundation parging and above grade stucco
to extreme exterior wetting without drainage
It is important to recognize that there are significant differences
between straw bale and standard frame construction. A major difference is
the lack of a vapour barrier on the interior and the absence of exterior
sheathing in straw bale. The parging and stucco are applied directly to
the straw bale wall. As a result, moisture build-up is possible.
High and sustained levels of moisture can have negative effects in at
least three ways. First, prolonged wetness could cause structural damage.
Second, mould growth associated with cellulose-based materials can cause
serious health problems. Third, high levels of moisture reduce straw¹s
Do we know enough to build stuccoed straw bale houses anywhere? We think
not. Straw bale should not be used where there are concerns with stuccoed
wood frame buildings. That includes a lot of coastal areas.
Perhaps straw may rot no worse than wood when equally wet and warm but a
stuccoed straw bale is not as moisture-load tolerant. The drainability and
breathability of most wood frame walls is generally much better.
Whether straw, cobb, rammed earth, or wood frame, we have to design
building envelopes with the underlying thought that water will penetrate
the exterior cladding, so the system should be able to accommodate
incidental moisture. Water will get in, so it must be able to get out
without damaging the structure.
Compared to standard frame construction, straw bale walls generally
incorporate higher permeance (more "breathable") interior and
exterior protective layers. Although breathability of a wall can refer
either to air movement through the wall or moisture movement, it commonly
refers to moisture diffusion and not air movement. Much theoretical
discussion about straw bale houses centres on the healthfulness of this
breathability but there is not much information available on the subject.
Most of the straw bale houses built to date are in warmer arid climates
where moisture concerns are easier to deal with because of dry conditions.
To satisfy regulatory authorities, technical research on straw bale
construction has concentrated mainly on the structural capacity and
thermal resistance properties of straw bales.
The high vapour permeability properties of a straw bale wall means that it
has the capacity to absorb moisture and diffuse this moisture to either
the exterior or interior of a structure. However, this capacity must not
be used as an excuse for inappropriate designs and applications. Remember
that vapour diffusion through interior finishes, good vapour retarder or
not, does not move large quantities of moisture into the wall. If there is
wall moisture from indoors, it can be tracked back to air leaks moving air
through the wall and driven by sustained pressure differentials.
Although straw bale houses do not use a traditional vapour barrier, they
must still be airtight. A straw bale house, when properly plastered
outside with cement-based stucco and inside with cement stucco or wet
plaster, will not offer many air leakage points, unlike drywall which
often has gaps under the trim details. In straw bale construction, it is
the plaster finish that is the air barrier. But as in conventional frame
construction, airtight detailing must be followed to achieve air
Air tightness tests of several Nova Scotia straw bale houses proved that
air sealing techniques can be applied to any building material. The
tightest house was built by owner/builders who had paid special attention
to air sealing in such areas as the wall/ceiling junction, the light and
plumbing penetrations into the attic space and the spaces between the
window units and the rough openings. However, it registered 3.13 air
changes per hour (the R-2000 limit is 1.5 air changes).
Air Leakage Areas
Most air leakage points found in
the four Nova Scotia straw bale houses examined are the same
as those found in conventionally built houses. They are not inherent to the process
of straw bale construction, except that there seems to be a higher
ratio of owner/builders to professional builders. (All four of the
houses tested were owner-built). Major air leakage areas
electrical outlets on exterior walls where interior plaster
was cracked or pulled away from the electrical box.
and/or 'truth windows' without glass in front of the exposed
at gaps/cracks at
the top of wall to ceiling/rafter connection
at window frames
at the connection between the frame and the rough buck or
straw where no silicone or latex caulking was used to seal the
opening at the buck/stucco connection
at the connection
between door bucks to door and/or window frames
penetration through exterior wall and/or through slab
an attic hatch
HRV ductwork in
attic (one house)
ceiling penetrating to attic
Moisture issues in mainstream housing range from the common use of wet or
unseasoned local lumber to foundation leakage, poor site drainage, drywall
nail pops, leaks, floor squeaks, truss uplift and condensation problems.
Poor installation of air and vapour barriers leads to deterioration of
wall, ceiling and floor materials and finishes, excessive moisture in
crawl spaces due to 'rising damp' and high humidity related to poor
ventilation of living spaces. Straw builders have to deal with all of
these issues plus ones that are specific to straw: liquid moisture
penetration, potential freeze/thaw spalling of exterior stucco finishes
resulting in water damage.
Generally, the moisture in a wall is outdoor moisture penetration.
Entrapment of exterior moisture guarantees trouble when the straw is
wetted, because straw dries slowly. The straw could stay too wet into the
warm weather and rot in a few years. Mould growth can occur at
temperatures as low as -5ºC, and at humidities as low as 62%, although
the optimum temperature ranges from +20ºC to +28ºC, and at relative
humidity (RH) levels of more than 95%.
The exterior of straw bale walls is faced with stucco, an absorbent
material, which is intimately connected to thousands of tightly packed,
poorly draining wicks -- straws -- embedded in it and leading inwards.
When the construction is located in a region with frequent wind driven
rains and weak drying conditions, there will be problems.
Because of the increasing interest in straw bale construction, the Canada
Mortgage & Housing Corporation (CMHC) has supported the monitoring of
several straw bale houses in various parts of Canada. Results show that
straw bale houses can be designed and built to function properly but that
moisture is also a concern.
Taking measurements in straw bales is not easy. Part of the CMHC studies
involved assessing tools to make the measurements. Measuring the relative
humidity was one technique used. Another was inserting a block of wood
with wood moisture pines. The assumption was that the wood would establish
moisture equilibrium with the surrounding straw, so the readings would
reflect the moisture content.
Although both measuring techniques have their limitations, they do provide
a reasonable indication of moisture content.
The straw bale houses examined were in a variety of climate zones:
Southeast British Columbia, in the semi-arid region of the Okanagan
Valley; in Alberta, on the west coast of Washington; as well as in Quebec
and Nova Scotia. All buildings displayed seasonal and random fluctuations
in moisture readings. Although daily variances in RH are not an indication
of straw degradation, prolonged high RH values (over 85%) generally
indicate a problem.
Walls seem to have two peak 'wet' times: one in late winter and one in
late summer/early fall. This matches the pattern of moisture collecting in
a house during the heating season, dispersing over the spring/summer when
windows and doors are open and fewer moisture-producing activities are
confined to the interior of the house. The summer peak, which is higher
than the winter peak, is indicative of the higher absolute moisture levels
experienced in the summer months.
The results suggest straw bale walls do not have any unique propensity for
moisture retention but some walls examined, especially north walls, had a
moisture content readings of 14-17% during the summer months. These levels
suggest borderline to unacceptable conditions. Walls with a southern
exposure were generally much drier than walls with other orientations and
could handle significantly more exterior wetting.
Are there advantages to straw bale
The most controversial issue is
affordability. Proponents suggest there is a cost advantage, but there
is no real cost advantage in building with straw - apart from the fact
that you can use unskilled labour for a great deal of the work. Costs
can range from $50 to $75 per square foot, and depending on finishing
details can run over $95 per square foot.
the bales go up quickly, the plastering process is slow and
available, the raw materials may be relatively cheap to buy.
Straw bales are
sound proof, reducing noise from outside, and make a building
feel very solid inside.
Plastered straw bale
construction has excellent fire resistance. Studies have
shown that plastered straw bale walls provide a two-hour rating.
It is aesthetically
pleasing because you can round off or soften edges, and make
things like wide casements.
Dry straw walls can
make for a healthy home because of the high insulation values.
The natural materials do not
off-gas synthetic products.
Two Alberta houses had sustained higher moisture readings and observed
indicators that would be considered borderline acceptable. High moisture
readings were accompanied by straw samples that were wet and decomposing.
In both cases, the problems were the result of two or more design flaws.
The third questionable case was a straw bale house on the west coast of
Oregon. The sustained high levels of moisture in the north wall were the
result of high atmospheric humidity levels and not from external wetting.
In one house, areas of damp, slowly rotting straw were found under poorly
built windowsills, even under a three-foot roof overhang (but which was
two stories above). Some water may also have seeped down from condensation
on the indoor glass surface.
In another house, problem areas were one foot above the slab-on-grade. The
straw was found to be damp, dark and musty, slowly rotting in a
ten-year-old wall. The source of moisture was "rising camp"
which is found moisture wicking through the concrete slab-on-grade that
did not have a polyethylene or other effective ground cover. The amount of
water rising was greater than the amount of water that could dry, so the
straw was rotting from the slab upward.
Houses in Pompeii had slate damp-proof courses near the base of all
masonry walls. Are we super-slow learners?
straw bale construction only for rural applications?
image of straw bale houses with their thick walls being suited only to
rural or large suburban properties is being challenged by Julia Bourke, a
professor of architecture at McGill University. She recently completed her
own straw bale house on an infill lot in downtown Montreal.
The house is a 2x4 timber frame structure with bales packed between the
struts as insulating infill. The straw was covered with a 4 to 5 cm stucco
coat inside and out.
Prior to installation, a straw bale must be tested to ensure that its
moisture content does not exceed 11 or 12% and must be kept dry during
Building code issues remain problematic, as not all building officials
will be comfortable with straw bale construction and may require
engineering sign-off. There is also the issue of bale size. Straw bales
are large, so a wall can be up to 2 feet thick, which can result in less
usable floor space on a small lot.
ISOBORD Engineered Strawboard
can be used in more ways than as a structural or insulation
product in walls. Several engineered straw board products are
manufactured around the world. In Canada, Isobord is an engineered
board that has been available since 1998. It is made in Manitoba
from finely chopped residue wheat straw and non-formaldehyde
isocyanate resins, so formaldehyde emissions are not an issue.
Before Isobord manufacturing started in Elie, Manitoba, farmers
burned off residue straw in their fields, creating air pollution
problems in the Red River Valley area. Now, the Isobord
Enterprises plant uses up to 200,000 tons of the straw each year
and has helped alleviate this pollution problem.
Isobord's smooth surface makes it ideal for laminates, foils and
hardwood veneers. It is a premium panel with superior properties
of elasticity, internal bond, density and strength with no harmful
emissions. These qualities mean that Isobord is suited for normal
or sensitive applications in both residential and commercial
settings. It can be used in many of the same applications as
medium density fiberboard (MDF), such as countertops, shelving
cabinetry and as a component in furniture and flooring. The
company also manufactures StorageBoard™ and
Isobord is lighter in weight than MDF and the 1.2 by 2.4 m (4x8
ft) Isobord panels come in a variety of thicknesses. Isobord is
not considered a building product like oriented strand board or
Plans are underway for the startup of several additional Isobord
manufacturing sites in North America over the next few years.
Sales Office 503/242-7345
with permission from
SOLPLAN REVIEW January 2001
Richard Kadulski, Editor-Publisher
Box 86627, North Vancouver, BC V7L 4L2