by Kenton Shepard and Nick Gromicko, CMI®
Although homes have been constructed using straw bales (also written "strawbales") for several hundred years in Europe, and since the late 1800s in the U.S. (originating in Nebraska), the modern straw bale home is a relatively new type of home with its own unique construction techniques.
Rising energy costs have encouraged the construction of homes designed to minimize air leakage. Home systems and components must be combined in ways that work well to keep the home safe and comfortable. Tighter building envelopes affect the way moisture produced by activities such as bathing and cooking accumulates inside homes and in wall assemblies. The addition of all the moisture-emitting appliances and conveniences that we now enjoy also makes the straw bale home a wetter home -- one that, at times, has some problems breathing. When a home is built using methods that minimize air leakage, ventilation must be increased to avoid the accumulation of excess moisture in the home's interior and in wall assemblies, and straw bale homes are no exception.
Additional Knowledge Required
Building science is the study of how buildings are affected by moisture, temperature changes and air movement. Because modern straw bale homes employ fairly new building methods, it has taken a while to find out what works and what doesn't, especially since what works changes with the design, materials used and local climate.
Inspecting straw bale homes requires basic knowledge of commonly used straw bale construction techniques, an understanding of building science, some additional research in the building methods that are appropriate for the local climate, and some special instruments.
Types of Straw Bale Homes
There are two types of straw bale homes:
- Load-bearing straw bale walls -- including the plaster interior and exterior wall coverings -- support the roof and lateral loads (such as wind).
- Non-load bearing straw bale walls typically consist of a post-and-beam structure, which supports the roof and lateral loads. This structure is in-filled with straw bales that provide only insulation.
The exterior and interior wall coverings are plastered with cement stucco, earthen plaster, lime, or some combination of these. Plaster is usually applied directly to the straw.
CONCERNS WITH STRAW BALE HOMES
Moisture intrusion is the #1 concern with straw bale homes, just as it is with conventional homes. Because straw bales can provide food for decay fungus, widespread, long-term fungal activity can destroy a straw bale home.
In addition to decay of the straw, decay fungus is a concern because mold fungus releases spores. A high concentration of mold spores in indoor air can cause health problems in infants and the elderly, and especially in people with compromised immune systems, lung disease and allergies.
A number of design methods are used to keep moisture out of the wall cavity and allows it to escape once it gets in there.
- The foundation should include a stem wall, the top of which should extend a minimum of 8 inches above the exterior grade to minimize splashback onto the exterior wall coverings.
- The stem wall should extend a minimum of 6 inches above the interior floor. This will prevent soaking of the bottom of the lower course of straw bales if the floor should flood.
Pictured above: two exterior walls, but a deck just outside the wall to the left could route water under the wall.
To avoid turning the bottom course of bales into a giant sponge, the first course is raised above the floor.
- Roof overhangs should be extensive to protect the walls underneath from the elements.
- Flat roofs should be avoided because they're more likely to leak.
- Windows should be installed so that they extend out past the exterior surface of the plaster. If windows are inset from the exterior wall face, they should have high-quality pan flashing installed on the window sills, and the sills should slope. The pan flashing should extend beneath the window and not butt to the exterior face of the window.
Windows should be installed so that they extend past the exterior surface of the plaster.
In the photo above, the windows will be installed in the 2x4 frames, which will also act as a stop for the plaster.
In the photo below, you can see flashing details using Grace Ice & Water Shield® and expanded metal lath.
When windows are held to the outside, the insides of window openings can be formed with expanded metal lath, then plastered with gypsum plaster. The gypsum is then plastered over with subsequent coats of the same plaster used on the rest of the walls.
- Water supply pipes should not be run through straw bales. Pipes should be installed in the floor or in horizontal channels designed into the stem walls for that purpose. This includes supply pipes for exterior faucets.
- Moisture Barriers and Retarders
- Interior and exterior plasters should be applied directly to the straw bales. No plastic, polymer or other barriers or retarders should be installed in wall assemblies, as this may trap moisture in the wall cavity.
This PVC pipe is part of a moisture detection system devised by Laura Bartels.
It's inserted into a hole drilled into the bale. The wires on the right end allow
moisture levels to be read by a moisture meter.
Straw bale homes are typically plastered with one of four different types of plaster:
Above left: sprayed slip coat Above right: hand-applied slip coat
- Cement stucco is a cementitious material. Historically, it has been the material of choice, but difficulty in repairing cracks and its low permeability to water vapor have made it less popular in recent years. Its low permeability may result in moisture becoming trapped within the wall cavity where it can encourage the growth of decay or mold fungus.
- Gypsum plaster has been used for many years. Before drywall became the interior wall covering of choice, the 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.
- Earthen plasters are composed of various combinations of clay-based earth, lime, sand and chopped straw. These plasters act as a barrier to water in its liquid form, but they will allow water vapor to pass through so that moisture is not trapped within the walls. Such walls are also very easy to repair. Cracks and changes are easily repaired and blended. Because clay in earthen plasters is hygroscopic, meaning that the moisture content changes with changes in relative humidity, earthen walls can help to temper the interior humidity. Other additives typically include mica, various fibers and pigments for color.
- Lime plasters are made from limestone that has been heated and powdered. When mixed with water and allowed to dry, it hardens, providing durability and acting as a barrier to water in its liquid form while remaining permeable to water vapor.
- Plaster is applied in three coats:
- The first or scratch coat is used to build out low spots in the wall and build up the thickness of the wall surface.
- The second or brown coat adds thickness and further flattens the surface.
- The third or finish coat provides color and texture, and produces the visual surface.
- Allowing each coat to dry completely will help prevent cracks from being transmitted from the underlying coats to newly applied coats.
- 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 the 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.
Cracking is a natural process with the earthen and cementitious plasters that are typically used to cover the interior and exterior walls in straw bale homes. They cure in a manner similar to concrete; they shrink as they dry, and they 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 freeze-thaw cycles. 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 in order 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:
- in parts of the wall where it has been applied thickly to fill in low spots;
- if the plaster has been mixed with too much water; and/or
- if the plaster has been mixed with an inadequate amount of binder, such as 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:
- cracks in underlying coats were not allowed to cure completely before subsequent coats were applied; and/or
- cracks in underlying coats were not filled completely when subsequent coats were applied.
When the wall surface is covered with extensive, multi-directional vein-like cracks (resembling a spider web), the reason is probably improper curing due to inadequate hydration (moisture) 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. Improper curing can cause a failure of the bond between coats.
Inadequate hydration can be caused by:
- exposure to sun and wind, which causes rapid evaporation;
- inadequate wetting of the straw before applying the plaster;
- inadequate wetting of the dry plaster before the additional coats were applied; and/or
- inadequate wetting of the plaster coats during the curing process.
If the cracks are wide and close together, and the plaster detaches when it's tapped or scraped, the bond to the underlying material has failed, and the entire coat in the problem area 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.
The most common type of cracking in clay, lime and cementitious plaster wall coverings of straw bale homes is caused by shrinkage of the plaster as it dries.
Cracks typically run diagonally between the longest diagonal distance in a wall -- for example, from the lower left to top right -- and usually don't extend all the way to the corners.
Very thin cracks can be left alone, but cracks that can trap moisture should be repaired. Cracks in cement stucco may have to be widened first in order to be repaired properly.
Plaster applied over different substrates, such as straw and wood, may crack at the transition for two different reasons:
- differential curing caused by dissimilar materials absorbing moisture from wet plaster at different rates; or
- differential thermal expansion and contraction rates of dissimilar materials.
Cracking will occur where different substrates meet.
One method for preventing cracks resulting from different substrate materials is to staple
expanded metal lath over wood framing members after the second earth coat is applied.
The lath is then plastered with a coat of gypsum plaster. Once the gypsum has dried, the next
plaster coat of lime or earth is applied uniformly over the entire wall.
Movement of the home structure may result from:
- foundation movement, which is caused by:
- expansive soil;
- inadequate soil compaction;
- excessive moisture in the soil;
- inadequate foundation design; and/or
- seismic activity.
- structural movement, caused by
- wind loads;
- inadequate structure design; and/or
- 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 they may appear in other areas, depending on the nature of the problem.
You may be able to apply some of the guidelines for diagnosing problems associated with poured-concrete foundations to determining the cause of the cracks you see in plaster walls. This is where having a copy of the original plans and/or photos of the construction process can be useful.
Earthen plaster has an advantage over cementitious finishes, such as cement stucco, because the cracks are easier to repair. Even so, some types of cracks may require some imagination to repair adequately.
Paints that form a membrane impermeable to moisture vapor should be avoided in order to prevent sealing moisture into the walls. Non-toxic pigments can be added to the final finish plaster, or they can be applied with washes using a brush or roller.
Sealers, such as siloxane, can be used to reduce moisture intrusion and improve durability, while maintaining adequate vapor permeability that allows moisture in the walls to escape.
ADVANTAGES OF BUILDING WITH STRAW BALES
The advantages include their:
- High Thermal-Insulation Value
Straw bale wall assemblies provide an R value somewhere between R-30 and R-36, according to authorities such as the California Energy Commission and Oak Ridge National Laboratories. Actual R values vary, depending on how well voids within walls are filled. A typical 2x6 exterior wall assembly with fiberglass insulation is rated at approximately R-22.
- High Sound-Insulation Value
In conventionally framed buildings, the framing members act as sound bridges, transmitting sound through walls. Because straw bales are non-rigid, they dampen sound rather than transmit it, making for a wall with highly effective acoustical insulating characteristics. This is one reason why loosely compressed straw bales are superior to tightly compressed bales.
Both the interior and exterior plaster and straw bales are capable of absorbing and later releasing large amounts of water while remaining below the level 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 a lesser capacity for water storage. 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.
In order for the walls to perform well thermally and to improve their fire resistance, it's important that the walls be tightly packed with straw during construction. If there are spaces left over after stacking the bales, the flakes must be tied into mini-bales which can then be used to fill the space.
These steel rods (above) are actually big sewing needles used to push baling twine through the bale. After it's been pushed through, it's tied, creating a smaller bale, which is used to fill in the small space pictured below.
The majority of straw bale homes that burn are lost during construction due to careless subcontractors igniting loose straw. Once straw bales are sandwiched between plaster, the wall assemblies are extremely fire-resistant. Because the straw inside the walls is compacted, there is little oxygen available for combustion.
Because combustion requires oxygen, packing all small spaces tightly
with straw is crucial to keeping oxygen in the wall assembly to a minimum.
Extensive testing (including full-scale ASTM E-119) has been performed on these walls. Properly constructed, they can withstand temperatures of 1,800° F for up to two hours and sustain little or no damage. Fire resistance varies with the straw bales' density, how well the interior wall voids have been filled, and the type and thickness of the plaster used for the wall covering.
- Properties as a Natural Building Material
Straw is comprised of the stems of cereal grains that have had their seed heads removed. It contains no toxic glues or resins, it's relatively inexpensive, and it's 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, compared to materials used in conventional wall systems.
DISADVANTAGES OF BUILDING WITH STRAW BALES
Most problems related to building straw bale homes can be traced to the fact that few contractors and inspectors have experience with them, and the methods involving their innovative building techniques and the ideas about what works best for a given climate are all still evolving.
- Lack of Qualified Professionals
Designers, building officials, general contractors, sub-contractors and home inspectors are often ignorant of the best practices unique to the construction and inspection of straw bale homes.
This can lead to:
- difficulties in getting plans approved;
- problems being accidentally built into straw bale homes;
- difficulties in identifying and/or correcting such accidental problems, both before and after the home is completed; and/or
- inadequate inspection at either the pre-purchase or pre-sale stage.
- Problems Obtaining Financing
The main concern of mortgage lenders is a home's resale value. If a lender should have to foreclose on a straw bale home, its confidence in the likelihood that it can recover the cost of the loan by selling the home is essential. Financing is more favorable in areas that have a reasonably large percentage of straw bale homes.
- Problems Obtaining Insurance
There are companies that will write fire insurance policies on homes containing straw bales, but many insurance companies will not. The ability to find coverage depends, to some extent, on the area where the home is located.
In summary, straw bale homes are viable, advantageous designs if they’re designed, constructed, maintained and inspected properly.
For More Information
All photos are courtesy of Kenton Shepard.