Load Bearing Cob: Eco-Sense Home
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Cob is an ancient building material made of clay, sand and straw. Once compacted, it is durable, energy efficient and aesthetically pleasing. The cob Eco-Sense building, home to Ann and Gord Baird, was a labor of love. The Bairds used a variety of innovative strategies to complete the permitting process, including wall sensors to track moisture levels, custom engineering for lateral load requirements, and a rototiller for cob mixture consistency. The house is efficient and uniquely harmonized with the family’s lifestyle and values.
The codes applicable to permitting the cob walls were those requiring insulation, an air barrier, a vapor barrier, and lateral and vertical load requirements. The vapor barrier code was satisfied due to a schedule, signed by an engineer, to collect moisture data from sensors inside the walls. The lateral load requirement was met through a custom engineered cross-tethered bond beam wall design. Vertical load requirements were met due to samples of cob material that were lab-tested for compression strength.
|Code Requirement||Compliance Path|
Structural Design: Wind and Seismic Load - BC Building Code: Division B, Part 4
This requirement was met through a custom engineered cross-tethered wall system. Approval required a structural engineers stamped plans and submission of Schedule B Letter of Assurance to the overseeing building official, thus removing risk and liability from the city/district/municipality.
Thermal Insulation: 9.25.2 (IECC [RE] 402.2)
Air Barrier: 9.25.3 (IECC [RE] 402.4)
Vapour Barrier: 9.25.4 (IRC 603.1)
Thermal Insulation: Monitoring data showed an assembly insulation value of R21.6, well within the code requirement of R13.9
Air leakage: Cob is by its very nature is a monolithic mass wall, a continuous air barrier assembly. Special attention must be given to details where cob meets other assemblies to continue this barrier. Ecosense blower door test results: 2.12 ACH@50Pa
Vapour Barrier: Appendix A: A-184.108.40.206 states that local practice with demonstrated performance should be considered
Permitting Details and Project Description
The vapor barrier requirement is aimed at preventing moisture issues in a building, but in this case it would defeat the purpose of the construction method. Cob is a breathable wall assembly in which which the wall, exterior plaster and interior plaster are a single combined assembly with no vapor barrier.
Historical evidence has shown the effectiveness of the system, but to be sure, the Bairds installed moisture sensors on all four sides of their home, and scheduled a data retrieval program to ensure healthy moisture levels. Intended to be “load-bearing,” the Ecosense home is actually supported laterally by a “reinforced concrete bond-beam atop the ...wall” tethered “to the foundation using aircraft cabling.” Strength of the cob mix was tested by the University of Manitoba, as well as a geotechnical engineering firm in Victoria.. The Bairds sent in a dehydrated sample of their mix for compression testing.
The Baird’s intention in using cob was to create a home that was in harmony with the surrounding ecosystem, and with their lifestyle. They wanted to use a material that was abundant locally, and environmentally low-impact. They also chose cob due to it’s passive heating and cooling qualities. The project included a variety of other innovative materials and techniques which can be found on their website listed below.
Design / Build Process
After falling love with cob during a small building project using the material, Ann and Gord Baird chose to use cob to build their home. The Bairds home is comprised of many sustainable and innovative techniques. Permitting cob is challenging mainly due to the absence of the materials included in the codes from the International Code Council (ICC). The couple utilized various strategies to meet code requirements and to permit their cob home. They were determined to build according to their values, which required, at times, extra expense, yet the home came in considerably lower than other average new builds in their area.
With the intention to build a house that is connected with all aspects of the healthy and functioning ecosystem, the Bairds embarked on their home-making journey. Located in Victoria, BC, the couple sought locally abundant, recycled, non-toxic, renewable and low impact materials whenever possible in their building process. One of their guiding principles was to prioritize “lifestyle” over “life stuff”. The theory was to create a home that facilitated the lifestyle they wanted, which is based on sustainability and natural rhythms and patterns. A series of steps were necessary for the Bairds to build the way they wanted.
Since cob is not a familiar or recognized building material in the international building codes, they went through the process of testing and perfecting their cob mix, as well as custom engineering the walls for sheer strength. For the walls, a reinforced concrete bond beam and cross tethered cable system was custom engineered for the building. The cob mix was standardized by using a rototiller for consistency. They used horse manure (in plaster mixes) instead of a straw chopper for consistency in straw length. In order to reach energy efficiency standards, the Bairds added about 30% pumice to their cob mix. This increased R value of walls from roughly R-10 to R-20. Actual wall performance testing measuring energy inputs calculated an effective R-Value of R-22 per inch.
Although cob has been used as a building material for hundreds of years, and has withstood environmental damage in rainy climates for up to 700 years, the codes do not yet reflect this. In the permitting process, vapor barrier is one aspect of modern codes that poses a challenge to successful permitting of cob. In the case of the Eco-Sense house, the approving official was aware of the time tested quality and success of cob as a building material. The cob in the Eco-Sense home is not entirely load bearing. It’s walls were custom engineered to include a cross tethered system. While this element is not present in the traditional cob buildings from the UK, meeting the lateral shear requirement is an important consideration for modern U.S. builders.
Cost / Benefit
Due to the energy efficiency of Cob walls and other energy saving strategies, the Bairds home “uses 84.8% less electricity than the average single detached BC residence.”
Citations and References
|Title, description||File name, Web link or citation|
AFFORDABLE, SUSTAINABLE HOMES: Eco-Sense and the Future of Green Building, See Page 40: Policy Overview: Barriers and Opportunities
|By Cascadia Green Building Counci, Contributors: Ann and Gord Baird, Christine Goodvin, Goodvin Designers.
Standard Guide for Design of Earthen Wall Building Systems, Released in 2010 provides guidance for earthen building systems, also called earthen construction, and addresses both technical requirements and considerations for sustainable development.
|ASTM Standard 2392-10 "Standard Guide for Design of Earthen Wall Building Systems" ASTM International, West Conshohocken, PA, 2003, DOI: 10.1520/E2392_E2392M-10E01|
|Understanding Vapor Barriers: Report in Building Science Digest||
Lstiburek, J; Understanding Vapour Barriers; Building Science Digest, BSD 106, 2006.
|Owner: Ann and Gord Baird Eco-Sense http://ecosenseliving.wordpress.com/||Designer: Kris Dick & Tim Krahn Building Alternatives http://www.buildalt.com/index.htm||Other: C.N. Ryzuk Ryzuk Geotechnical 250-475-3131 & 1-800-249-7216|
|Approving Offcial: District of Highlands http://www.highlands.bc.ca/129/Building-Department|