HomeLink Magazine Summer 2017: Straw Bale Homes 

Natural Building with Straw Bales

By Morgan Cate, Adaptive Habitats

Photos: Daniel O’Conner Photography

 

Natural building is the practice of constructing with natural local materials and designing with sustainability objectives. One such method of natural building is straw bale construction, using bales of straw as structural elements, building insulation, or both.

The Geiser Residence

This straw bale home, located in the mountainous foothills outside of Lyons, CO. was designed by Boulder-based Architect Robert Ross AIA, Engineered by Ian Smith, PE, and built by Morgan Cate with Adaptive Habitats.

The owner and architect were inspired by the surrounding beauty to create a high performance structure that fits perfectly into the surrounding scenery. The architect chose the use of natural stone, heavy timber, and southwest design elements to provide the Colorado sense of place. The thick straw bale walls covered in earth-tone natural plaster complement the colors of the surrounding landscape, while the deep window wells and organic contours of the massive walls reflect the surrounding mountainous terrain and provide a sense of both serenity and security.

Design coordination is critical for successful natural building as the entire building needs to function as a system. Bale walls provide significant insulation, while thermal mass properties of floor and walls integrate with the in-floor heat to regulate interior comfort. The passive solar design and roof system are important components of the building that require close attention in the design phase. The home takes advantage of solar energy with a grid-tied 10kW photovoltaic system and a domestic hot water solar thermal panel to preheat domestic hot water. The solar technology along with back-up off-grid energy generation provides energy security and lower utility costs.

Straw bale wall—positive attributes

         In addition to the obvious thermal insulation benefits, straw bale walls provide beautiful results, which can only really be appreciated standing in a finished room. The room is extremely quiet and smells fresh and clean. The bale wall provides 30 – 35 R-value insulation, and the thick walls give a sturdy architectural element. The straw is a renewable resource available everywhere; bale walls also have the same fire rating as heavy timber and the material is relatively inexpensive.

         The purpose of straw bale construction is to minimize energy demand with insulation and thermal mass. To optimize the value in bale walls the house sits on a 4-inch heated concrete slab. When the mass of the concrete and bale walls reach comfort levels, they require much less energy to maintain the comfort zone. In summer months, open windows with stacked air convection keep the space naturally cool resulting in no need for air conditioning.

         The use of straw bales for exterior walls is primarily for insulation; the architectural element is a bonus. Many infill straw bale wall systems are composed of an advanced framing technique using posts spaced six to eight feet apart spanned by a structural box beam for a continuous roof system support. The roof assembly includes tapered foam white EPDM (ethylene propylene diene monomer) roof membrane built on engineered I-joists. The straw is then in-filled for walls, while closed-cell foam and blown cellulose insulation are integrated into the rafter cavities and dropped ceilings.

         Straw bale substrate systems require metal flashing for windows, doors and any potential drainage plain and material break. It is a good idea to use a water-resistant membrane across exterior structural members such as posts and box beams. The use of stucco lath is also a good idea for any flat surface that is not straw. On exterior straw surfaces, chicken-wire is stapled to the bale walls for stucco to adhere. The exterior stucco system is a scratch coat and base coat of 1 to 1 ratio Portland cement/ lime stucco with a color integrated final coat. Vertical expansion joints with sealant are recommended every 20 feet of wall length to prevent excessive cracking of wall stucco. In this building the exterior stucco system was applied using compressed air, similar to the shot-crete system used in pool construction. Bale walls need to acclimate to surrounding atmospheric conditions, meaning the walls need to breathe, so total encapsulation of the wall is not an option. The lime/ cement stucco is porous allowing the acclimation process.

         Interior walls are traditionally framed with half-inch sheet rock finished with a base coat and finish coat of plaster to match exterior wall color and texture. Non-priority space is painted drywall. Here, all finishes were non-VOC to maintain excellent indoor air quality. A continuous running bath fans provide the air exchange required by code.

Straw Bale challenges

         Straw bale construction is a labor-intensive endeavor. The advanced post and beam framing is not traditional, so the learning curve needs to be accounted for by the framing team. The plaster systems are a multi-step process requiring lots of handwork. When building straw bale walls, interior finish requirements need to be considered, such as blocking for shelving, base and wood trim or furnishings. Each project is unique, so a builder cannot anticipate average market pricing from sub-contractors.

         Straw bale construction systems may not be suited for all locations. In dry climates, such as in the west and central mountains, bale walls work well with little risk of decay, as the humidity is relatively low and the moisture can be controlled. Wet climates such as the East Coast and Pacific Northwest may not be best suited for a straw bale wall assembly.

A Tranquil Space

The owner wanted to create a tranquil quiet space where he can disconnect while being connected. The office/ residence location deep in the mountains provides an inspiring refuge to both work and live. Custom features represented in this straw bale structure provide modern luxuries while embracing an old, though slightly improved, building technology that has been around for generations. Straw bale construction has been utilized for over a hundred years—people in rural areas build with what they have locally.

Project shell specifications:

  • Advanced framing post and beam with straw-bale infill
  • 1-1/2” lime/cement plaster exterior skin, 1-1/2”  base coat lime stucco with American clay finish plaster
  • 10kW solar electric with back up generator
  • Solar thermal domestic hot water
  • 4” radiant heat flooring with multi-zone
  • Stack convection ventilation
  • White reflective flat roof membrane
  • EPA compliant wood fireplace
  • Performance thermal window glazing
  • Water efficient landscaping
  • Low flow water fixtures
  • LED lighting and Energy Star appliances


Interior finish Specifications:

  • Beetle kill flooring and millwork
  • Milled stone counter tops
  • Locally crafted tile and Saltillo tile flooring
  • Zero-VOC paints and sealers
  • Local quarry flag stone
  • American clay wall plaster
  • Energy Star Appliances
  • Natural day-lighting (Solar tubes)
  • Walk in beer cooler and custom local cabinetry