In the late 19th century American Great Plains pioneers were inspired by circumstance--plentiful grasslands and wheat fields, very few trees--and the recently invented baling machine to build comfortable, energy efficient homes that are still used today, over one-hundred years later. Renewed interest in straw bale building in the 1980s continues to this day, and has carried on that pioneer spirit of ingenuity driven by necessity. Modern-day builders have improved on these original straw bale buildings, creating energy efficient, affordable, long-lasting structures from this simple, natural building material. Today’s straw bale homes are built to last, and offer comfortable, healthy, quiet, inspiring dwellings.
How is a straw bale home different from a home built with conventional materials?
Straw bale and conventional homes are far more similar than they are different. Both have roofs and interior stud-frame walls. They both have bedrooms, bathrooms and kitchens, windows and doors, sinks, tubs, and toilets. The only significant difference is the walls, and how the walls connect to the foundation and support the roof. Since a straw bale wall assembly is at least three to four times thicker than stud frame or SIPS (Structural Insulated Panels) walls, and twice as thick as ICF (Insulated Concrete Form) walls, the footing is usually wider. For a slab-on-grade foundation, that often means more concrete. Instead of a single sill plate, straw bales sit on a double sill plate—two sills spaced the width of the bales and running parallel to the building perimeter. Bale walls must have a plaster on both interior and exterior surfaces. The finished surfaces can be exterior siding or interior drywall with plaster underneath, but the straw bale surface must be sealed with a plaster to retard fire, exclude pests, and achieve the expected insulation and thermal performance. In a conventional structure services like electrical and plumbing are usually installed after the wall framing is completed, but in a straw bale wall these are normally installed as the bale walls are being raised. Other than a few sequencing issues, the straw bale and conventional systems are remarkably similar to build, but they have very different thermal performance qualities.
What is the insulation value of a straw bale wall?
Testing regimes and test walls vary, but research shows that the R-value of a thick (18” or 24”) straw bale wall is between R-26 and R-34. The California Energy Commission assumes an R-30 value, but there's much more to the story! When looking at performance it’s important to look at the entire straw bale wall assembly—many inches of densely packed insulating straw, sheathed on both sides by an inch of an earth or lime/cement plaster. In addition to resisting heat flow, the bale and plaster density functions to store energy, like a battery, which moderates interior temperature fluctuations. For example, the temperature inside a straw bale building might change only six or eight degrees on a day when the outside temperature climbs from 60 degrees to 100, then drops back down to 60 degrees. The day to night “diurnal” temperature swing in a straw bale wall corresponds to, but lags behind daytime-nighttime temperatures, making for a stable, comfortable interior. During hot summer periods most straw bale home owners keep their buildings cooler by opening windows in the evening and closing them in the morning. This unique combination of insulating walls with distributed thermal mass accounts for why plastered straw bale walls perform beyond their rated values.
Where do the posts go in a timber frame or post-and-beam wall?
That depends on where the engineered load paths are for both gravity (roof) and live (wind and seismic) loads. In a post-and-beam or timber frame structure, the straw bales are usually treated as in-fill, or sometimes as a curtain wall. Placing the timbers or posts and beams in the exterior plane of the plastered bale wall is most common, but structural elements can be placed on the interior plane of the wall, or set either inside or outside the wall altogether.
Where’s the best place for window and door placement in the wall?
Windows and doors placed in the interior wall plane create terrific depth and shadowing when viewed from the outside. That’s the classic “southwestern” look of thick wall construction. However, it’s more difficult to flash this placement against moisture intrusion—deeply set windows have sills where water might sit and soak into the bales below them, so this area must be very carefully detailed (sloped and flashed). Windows placed in the exterior wall plane create large interior window stools that serve as window seats, counters, and plant stands, and are easier to flash against water intrusion. With thoughtful planning, windows and doors can be mounted to structural posts, reducing the use of wood and facilitating an applied plaster surface with fewer dissimilar material interruptions, which reduces the chance of developing vertical plaster cracks.
Oregon building code once required that straw bale walls have “pinning.” What is pinning?
In addition to the vertical post uprights or timbers that support the roof, straw bale walls benefit from pinning to handle out-of-plane loads. When the straw building revival was just getting started, and until only ten or twelve years ago, most builders pounded rebar through the bale courses to tie the wall together. Today many builders use external pinning, which is also in the 2015 IRC Code Appendix S Strawbale Construction that Oregon recently adopted. Wood or bamboo poles are inserted into kerfs in both sides of the straw wall with ends inserted into holes pocketed along the edges of both top and bottom plates. The poles are then tied tightly together with baling twine. This effectively and inexpensively stiffens the wall, and can be used with either lime stucco or earth plaster regimes. Similarly, metal or plastic mesh lath or netting on both sides of the bale wall can be sewn through with baling twine--this system is used when the straw bale walls play a structural role
Where do shear walls go in a straw bale wall assembly?
Short answer—wherever the engineer puts them! There are several ways to design for shear in a straw bale wall—and all can be effective. Plywood shear walls, tension-only x-bracing, moment frames, and timber frames have all been successfully used in straw bale structures. The engineer can also use the wall’s plaster skins as a shear wall, and this may require 2 x 2 welded wire mesh or 17 gauge lath stapled in a prescribed way to top and bottom plates, a cement/lime plaster regime, and every effort made to keep the mesh more-or-less in the center of the plaster. Prefabricated steel brace frames like Hardy Frames are also popular because they integrate easily into a straw bale wall and can be detailed for lime or earth plasters.
Do straw walls need to be plastered/stuccoed?
Yes. This ensures thermal performance, excludes pests, and makes the wall much more fire resistant. Several materials can be used to cover a straw bale wall, but all of them must be vapor permeable. Lime, earth, and gypsum plasters allow water vapor to disperse through the materials. Earth (clay) and gypsum plasters have the highest vapor permeance—permitting the most water vapor to pass through. Lime plasters have the second highest vapor permeance. Cement plasters with some lime added are suitable, but avoid straight cement plasters, or acrylic plasters and additives unless you’re confident they are at least as vapor permeable as a lime-cement mix. Even 20% lime in a lime-cement mix increases vapor permeance considerably—a necessary condition for long-lasting straw bale walls. Interior plaster can be the same as the exterior—and might need to be if the plaster skin is part of the structural design. Many builders use a three-coat earth plaster on the interior of a building. Clay soils found in and around the Rogue and Applegate Valleys are quite good for the scratch and brown coat plasters when mixed with sand and chopped straw. Many have a chocolate brown color—which may be too dark for an interior wall finish surface. A finish clay plaster color coat can be ordered from suppliers or fabricated from clay, sand, and pigments acquired from local pottery suppliers. Be very cautious about painting a plastered straw bale wall surface with latex paint—a much less permeable material that reduces or stops the movement of water vapor, leading to all kinds of problems! Shop around for paints with a high permeability rating--but be aware that multiple paint layers reduces vapor permeability. Finally, there are appropriate situations where straw bale buildings use exterior siding attached to furring strips, and interior straw bale walls are sheet rocked over furring strips, but the bales themselves still have a plaster coat to exclude pests, reduce fire risk, and achieve desired thermal performance.
Straw bale buildings evoke the tradition of a “barn raising.” How does that work?
Two phases of building with straw bales—the bale raising and the plastering—are sometimes done with the help of friends and family. Many people who eventually build straw bale homes first got involved by helping at a neighbor’s or friend’s “bale raising” or “plaster party.” Sometimes this works well, and other times it doesn't — a lot depends on the site, the owners, the building design, who’s managing the effort, and the skill and ability of the gathered community. It’s really important to be safe, organized and prepared so the job is accomplished, everyone feels their time was well spent, and nothing has to be done over by a professional crew.
Can passive solar features be combined into a straw bale building?
Absolutely! Here in Southern Oregon, designing the structure to take full advantage of passive solar heating and cooling can reduce winter heating loads by up to 50% and eliminate summer cooling loads completely! With the appropriate roof overhang and window sizing and placement, the winter sun can boost interior temperatures up to ten degrees on a sunny winter day, and prevent unwanted heat in the summer time. Opening windows placed at different heights in the building can speed summer cooling as well. Combined with the super insulation and thermal mass virtues of a straw bale wall assembly, passive solar design permits the use of smaller sized mechanical heating systems, and may eliminate the need for summer air conditioning altogether!
How much does it cost to build a straw bale house?
This is a really important question to understand, but hearing a figure like $35 or $75 or $110 or $225 per square foot isn’t useful if you don’t know the structure’s size, features, or finishes.
Assuming these figures are just for the structure—not including land, road, permits, and utilities— for them to have meaning you need to know what they cover. Do these prices represent passive solar structures built to Net Zero standards, or are they drafty and energy inefficient (Yes, there are drafty, energy inefficient straw bale structures.)? Do they have basements, crawlspaces, or are they slab-on-grade? Are they built into hillsides or on flat ground? Do they have earth floors? Are they multi-story? Are they designed for wheelchair access, or to easily handle an addition one day? Are the interior finishes simple or highly refined and decorative? Did the owners find lots of recycled materials, or contribute sweat equity by handling some of the bale raising, base plastering, or other finish work themselves or with friends and family? Are the structures relatively small by American standards—less than 1000 square feet? Or relatively large—over 2000 square feet? Size matters, because the total price of smaller homes is usually much lower than larger homes, but smaller homes usually have a higher square foot cost because relatively high cost building features like heating systems, kitchens and baths, etc. are distributed over fewer square feet.
Location matters, too. Here in Southern Oregon it’s possible to build a very comfortable, simple, energy efficient straw bale structure for around $150 per square foot (note that terms like “very comfortable” and “simple” and “energy efficient” conjures different meanings for different people). That same building would cost far less in regions where seismic issues don’t influence the design, or where there are no county or municipal building codes or inspections. It would cost far more in large urban areas.
If your goal is to build a structure for the lowest possible square foot price—and you don’t give much thought to that structure’s design (e.g., its initial and on-going environmental impact, longevity, resilience, etc.), then building the largest possible structure with the lowest cost materials to the absolute minimum acceptable standards will get you there. You’ll have a house with a low cost per square foot…and you might not want to live in it.
A more effective way to get a handle on this question is to visit straw bale homes in your area, note the size, features and finishes, and ask about energy performance and building costs. Most straw bale home owners are eager to share what they know. Be sure to ask if they’d do anything differently—because most of us would make some changes if we could! Then you can make more informed decisions about what you want in a home, and better estimate what you’ll spend to get it.
Do straw bale structures cost more than conventional buildings?
Yes and No. If you’re comparing a net-zero straw bale building with a net-zero green building made with more conventional materials, they probably cost the same. If you compare either net-zero building to a conventional building that isn’t particularly energy or resource efficient, then they both probably cost more. Straw bale buildings in particular have heavier walls, and require a larger roof to shelter the same size interior space of a conventional or green building. The heavier walls are usually supported by a larger concrete footing, which costs more. On the other hand, straw bales and earth plasters are extremely inexpensive. Even though straw bales define the wall system, in most cases bales comprise less than 1% of a building’s cost. And earth plasters are often dirt cheap if found locally, or free acquired from the building site. This low cost is offset by somewhat higher labor as preparing earth plasters and plastering is more labor intensive than conventional wall sheathing. But this higher cost in turn enables the much simpler and lower cost heating systems since thick plasters are a distributed thermal mass that helps maintain a stable interior temperature. Thick walls and earth plasters invite creativity in the form niches, sculpted wall forms and arches, built-in shelves, and window seats, and because there’s a rich variety of plaster finishes ranging from simple to highly refined, straw bale homes with “jaw dropping” plaster finishes and details will cost more.