Energy-Efficient Straw-bale Home in the Colorado Rockies

Energy-Efficient Straw-bale Home in the Colorado Rockies

Carbondale, CO

Oct 11 2010 By | 4 comments

General Specs and Team

Location: Carbondale, CO
Bedrooms: 1
Bathrooms: 2.5
Living Space : 2465 sqf

Completed: 2004

Offices: 2 (potential bedrooms)
Greenhouse: 440 sq. ft.
Garage: 535 sq. ft.

Builder: Wolfe Brand Construction Inc.
Architect/designer: Graybeal Architects LLC
Mechanical Engineer: Resource Engineering Group
Structural Engineer: White Horse Consultants Ltd.
Lighting designer: David Nelson and Associates

Construction

Foundation: Insulating concrete forms (ICFs) with insulated concrete slab on grade
Walls: House main level walls – wood post and beam with straw-bale infill walls with lime plaster finish (R-37); upper level walls – OVE 2x6 FSC-certified framing with non-formaldehyde batt insulation and ¾-in. rigid insulation on exterior (R-27); garage – structural insulated panels (SIPs) with reclaimed Douglas-fir siding
Roof: Main level roof – garden roof over 9 inches of rigid insulation on 3x6 structural wood decking supported by glulam beams (unvented); upper level, garage, and greenhouse roofs – SIPs with high-temperature waterproofing and corrugated metal roofing (unvented)
Windows: South glazing – double-pane low-e 178, 79% visible light, SHGC = 0.65, SC = 0.76, U-factor = 0.37; north, east, and west – double-pane low-e 171 SHGC = 0.41, SC = 0.41, U-factor = 0.37; greenhouse glazing – triple honeycomb polycarbonate, U-factor 0.37, SC = 0.83
Garage: Separate structure attached by double lock enclosed bridge
Additional features: Main-level central thermal mass structural wall is 2-ft.-wide cast-earth wall.

Energy

Heating equipment: Primary passive solar with timer to open all zone valves and circulate radiant floor fluid during maximum solar gain, distributing heat gain throughout house. Radiant floor heat in concrete slabs with NTI Trinity 92.7 AFUE gas boiler (50,000 Btu).
Mechanical ventilation: HRV for makeup bathroom exhaust and introduction of fresh air into the house.
Air-conditioning equipment: None. Thermal mass of cast-earth wall keeps main level at 72º F to 76º F during 90º F summer days.
Water heating equipment: Two 4x10 solar hot water panels (closed loop), 50-gal. storage tank. Excess hot water pre-heats main gas-fired water tank.
Yearly energy use: Projected, 34k Btu per sq. ft. per year (not including offset of PV system); actual, 3k Btu per sq. ft. per year. Annually, 660 CCF of natural gas and 1045 kW of electricity. PV system generated 358 kW of electricity feedback into the grid.

Renewable Energy Use
Photovoltaic power: 4.8 kW
PV cost: Approximately $43,000 before rebates; approximately $23,000 after local and state rebates and tax credits
Solar water: Two 4x10 panels; domestic hot water in closed-loop system
Cost: Approximately $6,000

Water Efficiency

  • Toto and Kohler low-flush toilets, low-flow shower and faucet heads
  • Pressurized, efficient septic system
  • Composting toilets, graywater system, and rainwater catchment prohibited by local and state codes

Indoor Air Quality

  • Natural ventilation
  • HRV exhausting from bathrooms
  • Outdoor air source for gas appliances and for kitchen exhaust hood makeup air
  • Non-formaldehyde plantation-grown cherry cabinetry

Green Materials and Resource Efficiency

  • Reclaimed Douglas fir trim.
  • Countertops of regional stone, concrete, And Richlite
  • Cooking exhaust ventilation has outside air source.
  • Clay plaster fireplace accent wall.
  • Solar shading, daylighting, and exterior light shelf.
  • Passive solar heating.
  • A water feature in the great room's cast-earth wall provides humidity while also cleaning the air.
  • Non-formaldehyde batt insulation from an in-state company used in upper-level walls.
  • Green (living) roof (R-39.5).
  • Thermal mass for storage and temperature tempering.

Green Building Product Sources
Cast-earth wall: Earth Homes Inc.
Windows: Loewen with Cardinal glazing
Cabinetry: Hallmark, Salt Lake City, Utah
Green roof system: American HydroTech
Greenhouse glazing: Marcolux
ICFs: Eco-Block
SIPs: AFM R-Control

Certification

AIA Colorado West Chapter: Award of honor for outstanding achievement in design excellence, 2004
Colorado Renewable Energy Society: Exemplary Building Award, 2005
Homes Across America: Showcase home, 2004

An architect designs a home for himself, his wife, and their vegetables

By Doug Graybeal

Our house is located outside of Carbondale, Colo., at an altitude of 7,000 feet. The architecture is both responsive to the climate — a dry mountain environment — and reflective of the local mountain style. I’m an architect, and I designed the house to be as energy efficient and environmentally friendly as possible within a reasonable budget. Of course, my wife and I, both self-employed professionals, also wanted a wonderful space to live and work in. The use of natural materials has made it a warm, cozy and comfortable home.

Choosing the components
We selected building forms and materials that blend with the natural surroundings and local architectural vernacular. In the process, we reviewed each building component’s environmental impact, from material extraction through manufacturing and installation to future recycling, including delivery distances.

The framing lumber is FSC-certified, and the garage siding is Douglas fir reclaimed from warehouses in the state. Straw-bale walls with lime plaster exterior and interior finishes are on the north, east, and west sides of the main level. We planted a vegated roof for the main roof using native grasses and flowers (and someday wild strawberries). Other roofs and the upper level siding are corrugated metal made with recycled content, for low maintenance. The fascias are borax-treated engineered wood.

The main structure is elongated along the east-west axes and faces true south. We constructed it on an abandoned road running through the site and worked around the existing vegetation. Our driveway follows the alignment of the abandoned road; what vegetation that was removed was ground up and used as mulch. The property is rural in character, yet is located less than ten minutes from a major highway and commercial center.

Energy performance
Even though we maximized the south glazing on the house to take advantage of the panoramic views of the Elk Mountain range, overhangs and light shelves prevent overheating and also reflect natural light into the depths of living spaces. Small windows on adjacent exterior walls offer additional balanced daylight and cross ventilation.

Natural ventilation to cool the building is provided by a low awning window in the great room and by casement windows in other living spaces, with smaller high awning windows on the north side of the house. During the summer the windows are opened in the evening to cool the structure, and remain partially closed during the day.

During the design process, computer modeling showed a 3º to 5ºF temperature swing in the house year-round without the use of conventional heating or cooling systems. The calculated temperature swing has proven to be correct. The temperature in the house during the cold winter construction, when outdoor temperatures were below zero, ranged from a low of 35ºF to average daytime highs of 50ºF plus. Minimal supplemental heat was used during two extremely cold weeks of construction in January. The house maintains 72ºF to 76ºF interior temperatures during 90ºF degree summer days.

Our home is heated by passive solar energy. When needed, backup heat comes from a 92.7% efficient gas-fired Trinity compact boiler. Concrete floors provide thermal mass and incorporate radiant heat tubes for the backup conventional heating system (as required by mortgage companies). A pump circulates fluid through sun-exposed floors to the rest of the floor slabs to distribute solar heat gain.

A photovoltaic array (4.8 kW) on the upper pitched and garage roofs provide almost 100% of our annual electrical needs, and the grid is used as a backup battery. The utility meter runs backward during daytime energy gains. An HRV system provides fresh air while simultaneously exhausting stale air from the bathrooms. Two solar hot water collectors supply domestic hot water needs.

Green inside
Waking up with a shower in the master bath is rejuvenating. The tiles in the shower are made of recycled glass in wonderful colors with a matte finish. Another shower features slate tile from Vermont. The doors in the bathrooms have sandblasted glass upper panels to share natural light between the spaces.

When we can't rely solely on natural light inside the house, we use energy-efficient compact fluorescent lighting. It provides up-lighting in the main living spaces; in work and reading areas, task lighting does the job. The mix of natural and thoughtfully designed electric lighting is an attractive counterpoint to the recycled black steel used as accents throughout the house.

The drive for energy efficiency continues with efficient appliances, including a Sun Frost refrigerator and a Whirlpool Duet front-loading washer and dryer. We do not use a dishwasher.

Indoor air quality is just as important to us as energy efficiency, so we chose to use no-VOC paints. To achieve darker colors in some cases, we used low-VOC paints. Interior wood finishes and stains are water based, including the sealer used on the stained concrete floors. A water-based sealer covers the plantation-grown cherry cabinets as well. All of the cabinetry is formaldehyde-free, as is the wheatboard used for closet, display, and office shelving.

Earth friendly takes on new meaning
A 2-ft.-thick cast-earth wall divides the main living spaces in the house from those that see secondary use. Accents and art nooks pebble the wall, which has the appearance of a sliced river bank. The wall provides additional thermal mass for heating and cool tempering of the structure. Data sensors in the wall show it stays a consistent 72ºF, reducing the temperature swing in the house. The wall keeps us warm in the winter, and definitely keeps us cool in the summer. Additional sensors inside the house and the attached greenhouse, at the house’s exterior wall, in the concrete floors, and in the greenhouse rock bed collect temperature and relative humidity data.

The attached passive-solar greenhouse provides organic fruits and vegetables year-round. We're getting a lot of use out of the greenhouse sink made from an old galvanized bucket. The thermal mass of the greenhouse includes 15 10-ft.-tall Kalwall water tubes, along with an inline fan that pulls hot air from the top of the structure and circulates it through rock bed storage under the soil in the planter beds. Working together, they keep the greenhouse comfortable for plant growth all year long.

Lessons Learned

A majority of the project goals were achieved, and our house is exceeding its performance goals. The only thing I would rethink is the use of the water storage tubes in the greenhouse because they tend to collect condensation, which drains off onto the slab below. Even though I have provided drainage for this, the tubes sweat enough to actually raise the humidity in the greenhouse. They also sit too close to the north wall and would benefit from more air circulation around them. It might have been better to use a solid exposed concrete wall, insulated on the exterior, as thermal storage for the space.

I would also reconsider the interior finish and use a more durable metal or cement board on the walls; I used a water-resistant gypsum wallboard, but there are now paperless gypsum wallboard products on the market that would hold up better under such moist conditions.


Doug Graybeal is an architect in Carbondale, Colo.

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Image Credits:

  1. Doug Graybeal