Building With Straw Bales and Adobes
By Karin Deneke
“This looks like the inside of a barn,” a friend of mine exclaimed when she visited my unfinished straw bale house years ago. And she was correct. All she could see when she stepped inside were straw bales, the timber frame and the roofed trusses overhead. The interior floor space was wide open, void of framing for the individual rooms.
“That is where the adobe walls will come in later,” I explained.
An unconventional house requires a much different approach—from planning to finish—than that for stick built or log construction. It just happened that my hybrid house is located in a mountain community where log cabins are extremely popular. It created quite a stir among some of our residents, folks that may have never had a chance to watch the construction of a straw bale/timber frame/adobe hybrid house. And I am sure a few folks were extremely skeptical this would work.
Planning for such a home starts with the selection of a suitable lot. This 13,000-acre gated subdivision is heavily timbered. The elevation varies between 8,400 to 10,300 feet. Winters are long, with night-time sub-zero temperatures, and snowfall averages of up to 250 inches per season. Such conditions call for a well insulated home and a home with maximum solar exposure.
Finding a lot with southern orientation for my passive solar house was a prerequisite. Passive solar in a nutshell means taking advantage of the sun. The lot I selected is at an elevation of 9,000 feet. It has under-ground grid connection—a plus for me because I did not have to invest in a costly solar electric system to provide power for my house.
In a cold, or moderate climate—whether you build a conventional or an unconventional home or live on or off the electric grid—you should always make sure the longest wall of your house is south-facing. By that I mean, the home’s south flank should be sited no more than 20 degrees from the true south or the overhangs won’t work properly to block the sun’s heat energy during the summer months. The sun travels high in the summer sky—thus the overhang shades the south facing windows. During the winter months, the path of the sun is much lower, allowing full sunshine to enter the house.
In this community, having a view of the surrounding mountain peaks ranks high on the priority list. A south-facing lot is secondary. However a million dollar view does not pay the heating bills.
The support system for my straw bale/ hybrid house is the timber frame, anchored into the concrete slab. As recommended by my contractor Charles Reel, we had poured a Frost Protected Shallow Foundation, a foundation pioneered by Frank Lloyd Wright. It uses insulation vertically, and horizontally around the exterior perimeter, and actually requires less concrete than a traditional slab with its footings and stem walls. We reinforced the foot-print points where the vertical 8-by-8 timbers would be placed—and also reinforced the areas where we would set the adobe walls—with extra concrete.
A timber frame can add quite a bit of expense to your building costs, unless you cut your own lumber and craft your own frame. In my case, the frame was ordered from a timber framer who erected it with the help of my contractor and crew. A crane was needed to lift the heavy timbers.
When it comes to the overall cost for a straw bale house, don’t assume it is low. If you want to build it correctly by hiring a professional, someone trained in alternative building methods, it can easily exceed the dollars spent on a stick-built house due to its hand labor.
A load-bearing house is a cheaper option. You stack your bale walls to the desired height and add a bond beam around the perimeter to accommodate the roof. You could also consider a shed roof, which is the least expensive option. But first consult your county planning and zoning department to learn if it’s building code permits load-bearing construction.
Once your plans are approved, don’t get into a hurry to have your straw bales and adobe blocks delivered. A common mistake folks make, is storing these materials on their building lot, subject to deterioration by rain and snow. First get your slab under roof, then store your adobes and straw bales underneath.
In the agricultural valley below me, fields of wheat, oats and barley are harvested each season. The straw bales from all three of these small grain varieties can be used for building a straw bale house as long as they are tight, heavy construction type, two or three string bales from the latest harvest. I would not recommend old bales. We were fortunate to have a farmer in the valley who produced just what we needed.
Locating the adobes for our interior walls was not as easy. Some folks in the Southwest still make their own adobe blocks on-site from the clay-containing soil on their land. This was not an option for me. The clay content of my forest soil was minimal. In addition, time constraints would not have allowed the lengthy process.
Initially my contractor and I were prepared to order adobe blocks from a yard in northern New Mexico, nearly 140 miles away. We learned transportation charges can add up to 50 percent to the cost of these heavy earth blocks. We also required adobe dirt—to make the mortar that glues the blocks together. Fortunately we discovered a small ad in a local advertiser/shopper’s guide that helped us tremendously. It read: “Traditional toxic-free clay-mud adobes. Adobe houses are warm in winter and cool in summer. Help save our forests—build with adobes.”
It turned out this adobe-making operation was located along the Conejos River here in south central Colorado near a small community by the same name, and 90 miles from my building site. The owner, a man by the name of Demetrio Valdez, made his adobes the old-fashioned way, by sun-drying the mud bricks outdoors in wooden molds.
Unlike Valdez’ natural adobes, the yard in New Mexico produced traditional and stabilized blocks. The latter contain an asphalt emulsion additive. We preferred the natural adobes because our intentions were to build a healthy house, using natural materials only, thus avoiding out-gassing of the finished home, so common in conventional construction, a condition that can pollute the interior of a home up to seven years. We stored the adobes under roof, just like we had done with the straw bales earlier. Natural adobes are somewhat fragile and must not be dropped, and should be carefully stored on edge.
As soon as my house was enclosed completely, with windows and doors in place, we started laying the interior adobe walls. The adobes were set on a course of half height concrete blocks, mortared to the slab. The blocks matched the width of the adobes and served as a necessary safeguard against accidental spills or flooding. Contact with standing water could damage the adobe walls structurally, thus all the earthen walls rest on these blocks.
My exposure to adobe construction prior to building this house was minimal. I knew building with sun-cured earth goes back thousands of years, and is a wide spread practice all over the world. During my visits to the sunny and dry Southwest, I had admired the adobe architecture in the settlements of Taos and Santa Fe. The R-value of my straw bale exterior walls of around R-50, by far exceed that of 4/6 adobe exterior walls of around R-3. Yet, when you couple the insulation value of the exterior straw bale walls with the interior adobe walls’ heat-retaining mass, you have a winner. The adobes add thermal mass to a house—storing heat in winter and providing cooling properties in summer. Combine that with blown-in cellulose insulation in the attic and you have a super-energy-efficient house.
We had wrapped the straw bale walls inside and out in plastic netting. Our stucco sub contractor would have preferred expended metal lath. But living inside a metal cage, was not our idea of a healthy house. The stucco coats—the scratch, gray and color coat, contained lime to allow the walls to breeze. Later on we finished the exterior walls with a three-foot high rock layer, applied over the stucco coating. It not only enhances the visual appeal of my home but also protects the lower part of the stucco walls from moisture such as rain and snow.
To achieve a professional finish look for the interior straw bale walls, my contractor first coated them with a clay slip, and then finished with a natural plaster mix to which an earth-tone pigment had been added. Prior to that, he had spent hours shaving off bumpy spots protruding from the bales with an electric chain saw, to assure later on the finished walls had an even appearance.
It took a while for the mortar between the adobe blocks to dry. In the end, I was not happy with the appearance of the adobe walls—they made my rooms look dark. I preferred a lighter more airy look. It took some experimentation to come up with just the right coating mix, to achieve the desired change. A mixture of wheat paste, Kaolin clay, fine, and medium-fine sand, and water, did the trick. Instead of applying it with a trowel, I used a paint brush. The results justified all the work that went into this project, from brushing down the adobe walls to removing loose debris to hauling buckets of the coating mix up and down ladders.
The exterior foot print of my single-story, three-bedroom, 2.5-bath house is a bit over 2,000 square feet. The interior measures 1,630 square feet. The ceilings are nine feet high. It is the most energy efficient home I have ever owned. And that says a lot, considering my sub-alpine location in the Rocky Mountains where snow often is covering the ground from the end of November, through late April or early May.