Straw Bale House
Originally posted as "Post and Beam Frame, Multi-story, Hillside, Straw Bale Home" in Experiments in Sustainable Urban Living / February 02, 2005
Our family is gradually refining plans for an ecologically low impact home to be constructed in Portland, Oregon. The site is a hillside below SW Broadway Drive about 5 minutes south of downtown Portland, Oregon. The slope is approximately 50%, southwest exposure, and size is 2/3 acre. The upper portion of the site is blessed with full sun, even at the winter solstice, on cloudless days. Most of the site is covered with small trees, a few of which are native species. There is ample cover, food, and space to raise young, providing an ideal habitat for wildlife. We have provided fresh water and nesting boxes to encourage this. We are gradually replacing nuisance species like English ivy and blackberries with native plants that require minimal care and irrigation once established.
Access
Currently, we access a small (12'x24') cottage on the site via a tree and fern lined walkway surfaced with recycled wood chips, a permeable surface that prevents runoff. Besides the pathways, the only other developed areas of the site are the three small cisterns. Downtown is a short bicycle ride away. The area is also served by frequent bus service about three blocks away.
Our plans continue to evolve. We plan to construct the straw bale structure as an addition to the cottage, linked via a three-foot wide bridge. Here is a three dimensional rendering of the addition (which doesn't show the link to the cottage):
House levels:
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Top: attic and utility area for solar water heater and furnace for heating air, hot water heater, and front load washer and indoor clothes line
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Master: bedroom, bath, study area
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Main: kitchen, dining, living, pantry, half-bath, and recycling center
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Ground: entry, bath, and children's realm consisting of two bedrooms and common shared area.
Rough site plan for the property
three dimensional rendering of the addition
House Dimensions and Floor Plans
The house will consist of three levels plus an attic and utility space. Each level is approximately 24 feet by 24 feet (576 square feet) living (or attic) space with a three (six on the east side) foot wide balcony surrounding each level. Except on the roof, where it will be surfaced with a waterproof finish, the wrap-around balcony will be covered, providing wide eves to keep the walls dry. We plan to construct walls using 16-inch thick two-string straw bales. All of the south side will be glazed for solar collection using a sunroom. Including the wrap-around balcony, the outer dimension of the structure will be approximately 36 by 39 feet, enclosing a roof footprint area of 1404 square feet. Building up rather than out preserves the vast majority of the site in an undeveloped state. Because the addition is positioned close to the existing cottage in an area with non-native plum saplings and blackberries, no large or native trees will need to be removed for construction.
In the following floor plans, doors and windows are shown as rectangles spanning the (magenta) straw bale walls. Actual wall width is about 18 inches (16" for bale, one inch each side for rebar and stucco/plaster). The bridge to the existing cottage is not shown in these plans. The red rim surrounding the perimeter is a handrail.
Structural system
The structure will consist of a steel post and beam frame resting on concrete piers attached by pins drilled into the underlying bedrock. Neither the inner partitions nor the straw bale outer walls will be load bearing. Inner walls will consist of 1/2-inch sheet rock over sheet metal or wood studs using standard construction technique. We plan to use steel and wood trusses and solid wood joists for horizontal structural members. The following elevation plans show a proposed structure. Point loads are indicated by black triangles in the drawings.
3D rendering of this floor structure
Ground Floor
Upper Floor
Floor specifications require 10 pounds per square foot (psf) dead load (weight of the floor itself) and 40 psf live load (furniture, people, etc.). We anticipate using softwood 2"x6" tongue and groove decking resting on solid wooden floor joists spaced approximately four feet on centers. The underside of the flooring will double as the finish ceiling of the level below.
Roof and Attic / Solar Utility Area
Roofing material will consist of sheets of corrugated or standing seam painted galvanized or stainless steel running in a north-south orientation. The attic space will function as utility area, similar to a conventional basement. Due to the shallow bedrock on this site it is not feasible to excavate into the hillside
Walls
Eight foot walls on each floor will be constructed of stacks of two-string rye grass or wheat straw bales 16" thick and high by about 42" long, weighing 60 pounds each, of approximately R-40 to R-50 insulating value. There will be about 7,000 pounds of straw used for each floor (about 100 pounds per lineal foot of six bales high). They will be covered with an unsealed earthen plaster directly over straw on both interior and exterior walls. Bales will be stacked in a running bond, as is typical of straw bale construction, and secured to stiffening members such as steel rebar or recycled iron pipe spaced at 2' intervals on the inside of the bales. Wide impermeable balcony overhangs will minimize rain splash onto exterior walls. The structure will be elevated above ground to avoid any moisture from that source. Natural pigments such as iron oxide (ochre or burnt sienna) and lime may be added to or over plaster. We hope to include small amounts of decorative elements on the wall such as ceramic tile for baseboards and window and doorframes. Using slip (liquefied clay) and earthen plaster (a mixture of clay, sand, and chopped straw) as undercoat and lime plaster as a finish will allow the use of natural clay as the plastering material. We have tested the on-site clay subsoil and found a 1:1 mix of clay to sand to make a satisfactory earthen plaster.
Roof and Attic / Solar Utility Area
Roofing material will consist of sheets of corrugated or standing seam painted galvanized or stainless steel running in a north-south orientation. The attic space will function as utility area, similar to a conventional basement. Due to the shallow bedrock on this site it is not feasible to excavate into the hillside
Walls
Eight foot walls on each floor will be constructed of stacks of two-string rye grass or wheat straw bales 16" thick and high by about 42" long, weighing 60 pounds each, of approximately R-40 to R-50 insulating value. There will be about 7,000 pounds of straw used for each floor (about 100 pounds per lineal foot of six bales high). They will be covered with an unsealed earthen plaster directly over straw on both interior and exterior walls. Bales will be stacked in a running bond, as is typical of straw bale construction, and secured to stiffening members such as steel rebar or recycled iron pipe spaced at 2' intervals on the inside of the bales. Wide impermeable balcony overhangs will minimize rain splash onto exterior walls. The structure will be elevated above ground to avoid any moisture from that source. Natural pigments such as iron oxide (ochre or burnt sienna) and lime may be added to or over plaster. We hope to include small amounts of decorative elements on the wall such as ceramic tile for baseboards and window and doorframes. Using slip (liquefied clay) and earthen plaster (a mixture of clay, sand, and chopped straw) as undercoat and lime plaster as a finish will allow the use of natural clay as the plastering material. We have tested the on-site clay subsoil and found a 1:1 mix of clay to sand to make a satisfactory earthen plaster.
Other windows
Windows on the north, east, and west sides will be operable or have vents incorporated into the wooden frame to provide cross-ventilation as needed. Movable, form fitting, four inch thick foam pads (used as seating cushions during the daytime) will assist insulation of the smaller windows without retractable quilts. Although the drawings do not show this, bales will be beveled around the windows to allow better light penetration. All the windows and doors will be framed with 2"x4" lumber connected to the floor and ceiling for support.
Water system
A rainwater harvesting and purification system consisting of screened gutters, roof washer, 20- and 5 micron particulate filters, ultra-violet sterilizer, and 1/2 horsepower pressurizing pump will provide potable water to all plumbing fixtures. The site already has 3 cisterns approved for storm water management that is approved for irrigation. We plan rainwater cistern of 4500 gallon capacity which will provide 75 days use for our family. (We have measured the daily indoor use for our family of five at less than 60 gallons per day.) Assuming a 1400 square foot roof area and 36 inches of annual rainfall, 4200 cubic feet of rain will fall on the roof each year. This is approximately 31000 gallons, well above the 21900 gallons our family requires per year.
Sanitary system
We initially requested the city to approve an on-site constructed wetland for gray and/or black water processing, whose output would be suitable for garden irrigation. However, due to the steep slope, any on-site system was rejected. We then requested a composting toilet and greywater recycling system that produces no effluent requiring disposal. Unfortunately, this system and the much simpler and more elegant sawdust toilet are not approved for use in Oregon. The city is requiring connection to the municipal sewer system. We are avid organic gardeners and compost as much household waste as we can to close the nutrient cycle.
Passive and Active Soler Design
Passive and active solar design is achieved with:
south facing glazing collecting solar energy only during the cold seasons;
earthen plaster thermal mass on the inside of the straw bales to store this energy;
the attic "solar furnace" will provide hot air for space heating and clothes drying;
a solar collector secured to the roof will assist heating of hot water that can be used for both washing and hydronic space heating.
Hydronic Heating
This thermal mass may be difficult to heat passively either on cloudy days or because it does not receive direct sunshine. Therefore, we plan to incorporate a backup tankless propane or natural gas hot water heater which will supply hot water to conductive pipes imbedded in the lower portion of the wall on each floor. Hydronic heating systems have multiple advantages: automatic operation, including programmable thermostat with nocturnal setbacks, easy zone control (turning off heat to an area is effected by closing a single valve), the ability to use solar heat when available, and economy (the house's hot water heater serves double duty as back-up space heating source). The system can also be designed to recirculate hot water to faucets distant from the water heater, providing instant hot water at all hot water faucets in the house, no matter how far from the heater. A small wood stove will serve as an additional backup.
Natural Cooling and Ventilation
Operable, shaded windows placed in east and west sides will be able to catch natural breezes to maintain ideal summertime temperatures and provide fresh air as needed. The south windows are shaded to prevent excessive summertime heating. The hot water heater is to be of sealed combustion type to prevent leakage of combustion byproducts into the living space. A carbon monoxide detector will be installed in the utility area to monitor its performance. 80cfm exhaust fans will ventilate all bathrooms.
Wrap-around balcony
Balcony decking on the roof will be 3/4" marine plywood sealed with a waterproof deck membrane. Other levels, being covered, do not require waterproof decking. Balcony structure will consist of inexpensive but weather resistant galvanized industrial shelving components. Decking will consist of either conventional materials such as 1.5" thick lumber or steel grating. Deck railing will consist of tempered glass panels to enhance light penetration and views and minimize weathering
3D Rendering
Elevation Sketches
In addition to minimizing splash onto the plastered bales, the balcony, which will provide access to the entire structure without using a ladder, serves several additional purposes:facilitates plastering of the bales making scaffolding unnecessary for construction or repairs, provides an appropriate shading of the south-facing glass, reducing unwanted summer heat gain, provides a safety escape in case of fire or other emergency easy access to the roof area for servicing the rainwater system or solar collect or soffers a pleasant transition to the outside from all levels, and excellent views from the roof, provides a trellis upon which a living wall will grow, such as for grape or kiwi vines.
Features unique to this house
As far as we are aware, no one has ever constructed a straw bale structure more than 2 stories high or on a steeply sloped hillside. Other unique features include the wrap-around balcony on each level and hydronic heating incorporated into the stucco/plaster finish of the walls.
Utilities
Electrical (romex), telephone, and Ethernet cables; and piping will be mounted on the inner surface of the bales and covered with a protective sheath of earthen plaster when not centralized into easily accessible wall cavities for maintenance.
Plan preparation
The site and floor plans were prepared with AutoCAD LT. Three files were used: site plan; floor and roof plans; and window/door details. Extensive use is made of this program's layering feature for the floor plans. For example, this file contains layers that represent the different floor plans, roof plan, straw bale wall, balcony footprint, post placement, floor structure, foundation plan, and dimensions. To produce a final drawing the appropriate layers are turned on. This allows redundancy to be minimized. Thus, the stairway layer is shared by all the floor plans and is therefore only drawn once. Likewise, the dimensions and titling are shared by all the drawings. Output is sent either to a plotter or gif file for viewing over the Internet.
Three-dimensional renderings
John Salmen of Terrain Environmental Design Services prepared the structural plan, elevations, and 3-D renderings of the house and balcony structure using his own CAD tools.
Construction materials
Materials imported to the site for the shell and structure of the home will consist primarily of steel posts, steel and/or wood trusses, wooden floor joists and roof rafters, 1.5" thick floor decking, 2"x4" lumber (or equivalent steel studs) for non-load bearing walls, window and door frames, steel roofing, glass, sand, lime and straw. Most of these construction materials are produced locally; some, such as steel rebar, have substantial recycled content. We have attempted to avoid manufactured lumber products to avoid potential out gassing concerns. Both recycled hard and soft woods (such as from discarded pallets or deconstructed buildings) could be used for balcony decking, door and window frames, and cabinetry. Toxic materials can be avoided in the small area of weather-exposed decking on the bridge by the use of recycled plastic composite lumber.