Natural Building Colloquium


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Straw-Bale Dome Report

[Editor's note: This article is included in the interest of presenting the cutting-edge research of some natural builders. There are still no definitive results on any of the work presented here, and readers are cautioned to keep this in mind if they want to try any of these ideas.]

Brief History

Construction of the experimental straw-bale dome at the Black Range Lodge in Kingston, New Mexico, began during The Natural Building Colloquium held there in October, 1995. This project was initiated and spearheaded by straw-bale pioneer Matts Myhrman. The dome sits upon a coarse pumice foundation. Wood wedges were temporarily placed at each course of straw bales to tilt the bales inward and begin to form the dome shape. Cob was shaped between the bales at these horizontal joints, with the wood wedges then removed. Upper layers used a straw/clay mixture to replace the cob.

The south-facing entry to the dome was constructed out of a "wishbone" shaped juniper tree and dried yucca stalks, and a four-foot wide square skylight capped the dome for interior lighting. A clay/sand plaster with minimal straw content was applied in successive thin layers to the exterior surface. The interior was finished with earth plaster containing a higher degree of short straw and then coated with a fine light colored aliz (clay paint).

Dome Damage Report

While the unfinished dome was at times protected by a tarp, it was hit with quite a bit of rain and snow over the next 18 months. By the spring of 1997, the dome showed signs of damage both on the exterior and interior surfaces; therefore, it was assumed that some bale damage had occurred as well. The exterior plaster was very fragile and peeled off in layers and chunks. Examining the removed layers revealed very little straw, aggregate or other bonding or interlacing agent. While each layer seemed bonded to the next, there was little bonding between the plaster and the underlying structure. With every rainfall the plaster continued to erode.

High moisture content (20% and above) in bales can cause decomposition to begin. Depending on the length of time bales are subject to these moisture levels, results can be minor or catastrophic. In the case of the dome, much of the water that did penetrate the exterior plaster passed through the bales, was reabsorbed by the earth plaster and/or evaporated out of the bale. In a few specific locations, the combination of indentations in the structure allowing water to pool, infiltration through weakened earth plaster, and dense cob wedges which served to funnel water to the interior allowed some areas of some of the bales to decompose significantly. Analyzing the decomposed straw showed earthworms and other organisms had taken up residency. In areas not water-damaged, the straw is bright and sound.

The interior finish of earth plaster and aliz is damaged as well. Dark water streaks are the telltale signs. Moisture tests performed by David Eisenberg in June 1997 showed moisture levels hovering around 20% in several locations; some were much higher, and the majority were at safe levels.

Recommendations for Repair

Recommendations were discussed amongst a number of natural building practitioners during the 1997 Natural Building Colloquium. An extensive recommendation was outlined by Cedar Rose of CRG Design as shown below. An experimental waterproof coating was developed by Lance Durand and Keely Meagan.

1. Demolish existing plaster and remove any bulges or indentations which could catch water. Slightly wet down the dome to liquify any adhering clay particles and rub them into the straw bales.

2. Create an "adhesion coat" of clay slip, medium length straw and wheat paste and apply to dome, working it into the moistened clay remnants on the surface. Let dry.

3. Apply a base coat of 10 parts clay soil, 2 parts sand, "medium" amount straw and 1/2 gallon flour paste to moistened adhesion coat. Let dry.

4. Apply shaping coat of the same mixture above in #3, but with a higher proportion of sand (4 parts) and straw. Use to fill any significant depressions or deformities.

5. Apply to moistened base coat a finish coat of 6 parts clay soil, 2 parts sand, 2 parts sifted horse manure, 2 parts fine straw, 1/8 cup canola oil (for durability) and 1/2 gallon of wheat paste. This should be a smooth, easily spreadable mixture. Leave to dry until it is "set up." Burnish with a plastic disk, filling cracks with more mud mixture, and moistening surface using a water bottle. Let dry.

6. Apply experimental waterproofing base coat of 16 cups raw linseed oil, 50 cups bentonite clay, 3-4 cups local clay soil (for color), 2-3 cups of red art clay, using stiff paint brushes. Let dry for a few days before the next coat. Apply waterproof finish coat of 14 cups raw linseed oil, 20 cups chalk (whiting or ground limestone), and 12+/- cups of red art clay and let cure completely.

7. Seal skylight using waterproof mud mixture and metal flashing.

Reparative Work Performed

During the Natural Building Colloquium in June 1997, an experimental composite waterproofing material was applied over the cob and juniper branch/yucca stalk entry. Damage to the earth plaster at the entry was minimal probably due to the steeper slope of the entry, so the surface plaster was not stripped as in other areas.

Two different application techniques were performed on the right and left sides of the entry. The right side used the procedure outlined in #6 above, while on the left side a mix of similar proportions was used to coat cloth which was then draped on the structure. An old sheet was used on the upper half and a piece of wide mesh burlap below. Fine "hairs" from the burlap stuck through the material and it was felt these might allow water to infiltrate through to the surface below. This was not the case, however, as after the material was dried in the sun for a few weeks, the hairs simply brushed off without allowing any moisture penetration.

On both sides of the entry the material cured to a firm putty consistency. A very thin "skin" developed on the surface. On the left side with cloth below, the skin was easily disturbed, while on the right side the skin was less easy to disturb. Although minor disturbances of the skin healed quickly, deep gouges or tears would probably require a new coating of waterproofing.

The only major cracking occurred at the depressed joint between the cob and the large exposed juniper at the very front of the entry. More layers of waterproofing may eliminate this problem. This material sheds water very well; water beads up and washes to the ground. [Editor's note: observation of this coat after a year reveals much cracking and peeling of the final coat, but the underlying coat doesn't seem to be as badly affected. In addition, a good deal of bleaching has occurred, lightening the original color substantially.]

Several different tactics, depending on the location and density of the plaster, were used to demolish the faulty earth plaster. Garden hoes were used to dislodge and scrape off the larger chunks of plaster. Raking or sweeping with a broom loosened a large portion of the plaster remnants from the straw bales.

After the earth plaster was removed and the straw bales and cob exposed, the bales, and in some cases the cob, needed to be reshaped. Protrusions and other irregularities were removed and rounded to a smooth dome shape. A small hand saw was used to reshape the bales, and a hammer worked well to reduce cob bulges.

During the demolition and reshaping process, damaged bales were uncovered. Two locations were significant and large portions of individual bales had to be removed. The location with the worst bale damage and decomposition was just under the corner of the skylight. The second location that proved almost as bad was the exact opposite corner under the skylight. Both of these areas had been originally indented (like a step), possibly for foot placement when installing or working on the skylight. Decomposed and damaged straw was pulled out, and in both locations the interior plaster was punched through in order to allow for complete removal of damaged straw and for aeration of the remainder of the bale. When the remaining portions of the bales had dried considerably, fresh straw was stuffed tightly into the crevice and cobbed into place.

After some experimentation, it was decided that the new earth plaster would be a thick (3"-6" depending on location) one coat system, with a high content of medium length straw in a mix of approximately 7 parts clay soil to 1 part horse manure and enough water to create a sticky yet moist texture. Batches of the new earth plaster were mixed in a shallow trough and agitated by foot.

Much of the dome was uncovered to the straw bales without any remnants of the original earth plaster. In these situations, the earth plaster was worked into the bales until there was a full integration of the new plaster and existing straw bale. In areas where the original earth plaster had good integration with the structure, the old plaster was moistened and reconstituted and the new earth plaster worked into it so as to bind the old and new into one single element.

Areas of the dome that needed to be built out or rounded into a smooth shape were first filled in with a slightly modified earth plaster. The shaping plaster had less water content and more straw content. Some areas required several applications that were allowed to partially dry. Successive layers were worked into each other until the appropriate shape was achieved. At this point the dome was ready for the composite waterproofing material to be applied.

Further Recommendations

Although the new earth plaster should resist rapid deterioration and eliminate or significantly reduce water penetration, it is still a relatively porous and water soluble material and will eventually succumb to erosion. Based on the performance of the experimental portion of waterproofing at the entry, it is recommended that at least the top half to two thirds of the dome be protected by applying the composite waterproofing material.

It is also strongly recommended that the skylight area either be configured with a drip edge that distributes the water runoff evenly and not just at the corners of the skylight; or a small, simple gutter and downspout system be created to direct the skylight runoff away from the dome structure into the adjacent landscaping.

[Note: as of Spring 2000, this experimental straw-bale dome shows no signs of catastrophic failure.]

After fifteen years in construction management, Elizabeth Lassuy obtained a Masters of Science in Sustainable Systems from Slippery Rock University, PA.


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