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Ensley Street Strawbale ADU

This project was primarily challenging due to innovative insulation technique (straw bales); Tumwater and Thurston County jurisdictions were previously inexperienced with this type of engineering. Due to material choices based on ecologically based decision making, straw bale walls were approximately 2’ thick, this required extra discussion to verify footprint, whether to count sq ft from internal or external footprint of house—an important qualification to meet ADU permitting guidelines.
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Ensley Street Strawbale ADU
Building Envelope
Insulation
Mixed Post and Beam/Strawbale Construction
Tumwater
John Darnall, City of Tumwater
Jim Dawson
Residential
795
N/A
Jim Dawson
N/A
Shay Hohmann, NWEBG Research Volunteer, Winter 2011
Joseph Becker, NWEBG Project Implementer, Editor 2013

Abstract

Straw Bale insulation is an ecological solution that provides a high insulation value. Washington State Energy code requirements were satisfied by using default values determined by other state agencies. A custom engineered post and beam structure (with embedded sheer walls) was used to satisfy structural requirements. The thick walls of this Accessory Dwelling Unit (ADU) was deemed to not count against the max square feet allowed for ADU's.

Approval Process

The Owner Builder/Designers worked closely with the City of Tumwater to ensure code compliance. This project was primarily challenging due to innovative insulation technique (straw bales) that city and county jurisdictions were previously inexperienced with. The California Energy Commission officially recognizes Straw bale construction using 3- string, 23” bales as having an insulation value of R-30. The thick walls required extra discussion to verify footprint and whether to count sq ft from internal or external footprint of house—an important qualification to meet ADU permitting guidelines.

Independent structural engineering was required, with additional review by a structural engineer on behalf of the jurisdiction.

Code RequirementCompliance Path

Structural: Lateral resistance

in Post and Beam construction

Custom Engineering utilizing embedded sheer walls
WA State Energy Code

Effective default R-Value of 3 string straw bales

as provided by California Energy Commission

Vapor Retarder

R 601.3 Vapor Retarders

Vapor Permeable wall system

Accessory Dwelling Unit (ADU)

Max footprint of 800 sq ft

Extra discussion to verify footprint, whether to count

sq ft from internal or external footprint of house.

Project Description

As straw bale construction is still somewhat new to NW building officials, Jim decided to make the straw bales in his house non-load bearing, leading them to use post & beam construction for the main frame of the house. In order to get this project permitted Jim and wife Heather had to get the house professionally engineered, these plans were reviewed once again by a structural engineer on behalf of Tumwater’s permitting office. A key detail to this particular projects final design included substantial sheer support to account for the weight of the house under catastrophic circumstances.

This project was inspired by owner Jim Dawson's intention to build an ecological Accessory Dwelling Unit in his own back yard to provide additional income while increase suburban density. Guidelines in Jim’s decision making largely focused on overall material life cycle costs; these included embodied energy (energy required to manufacture and transport products to building site), and biodegradability/ reusability of products in the eventual decay/deconstruction of the building many years in the future.

Strawbale is a building technology pioneered in Nebraska due to the lack of trees for home building around the turn of the last century. It has been proven through cold winters of the northern plains Over the last 20 years straw bale homes have re-gained popularity in North America and around the world.

More on the project:

In the building of this home they made a number of other ecological design choices. This included reusing much of the lumber from the deconstructed garage which had sat where the new straw bale house was built, as well as all new wood was FSC certified. In finishing and furnishing lots of other reclaimed furniture, cabinetry, fixtures and other reclaimed details were incorporated. The walls were finished with natural plaster, lime finishes, low VOC paints, and natural wood finish (OSMO hard wax). To increase home heating efficiency and reduce natural gas emissions they chose a condenser boiler hot water heater (M2 emission condenser) for home use and radiant heat floors with increased slab thickness for thermal mass and dark finish; key features included in the passive solar house design.

Foundation walls are Rastra (80% recycled Styrofoam/20% concrete) and all windows are fiberglass framed. Aside from the major design characteristic and details, at the core of this house are the principles of a small footprint and shared space. Living in a smaller home reduces energy consumption and construction costs, both economically and environmentally. Considering the overall lifecycle costs is a major innovation outside of the codes that is important to living a non-toxic life. Jim & Heather chose not to put a fence in between the old house in front and their new home out back, sharing a yard with their new tenants. This contributes to a sense of community and shared resources, truly innovative in an old (and simple) way.

 

 

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