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The Strawtegi C02ttage

By March 30, 2022May 11th, 2022Eco-Technics, No. 71, Winter 2022

We think this project helps push low-carbon building forward to mainstream use. The Strawtegi vision tackles the urban scale, which is much less explored in the U.S. than elsewhere and also expands our knowledge of straw panels.

 

The Strawtegi CO2ttage broke ground on October 21, 2021, as the first permitted structure in the U.S. insulated with dense-packed chopped straw. This net-zero carbon, modular, straw panel Accessory Dwelling Unit (ADU) in Salt Lake City, Utah, is the first demonstration project and test build of Strawtegi, Inc.  The 650 square foot building demonstrates how we can build a home with rapidly renewable, bio-based materials, such as wood, straw, and hemp. These materials sequester atmospheric carbon within the structure, safely locking it in for the life cycle of the building, which could be 100 years or more. Using only FSC sustainably harvested wood products, this home design will sequester 5.5 more tons of carbon than were emitted in the production and assembly of the building’s envelope. We can achieve all of this while creating a beautiful and comfortable home that exceeds standards for health, safety, comfort, durability, and environmental sustainability. The Strawtegi CO2ttage project has been quite a journey so far and these remarkable times are sure to bring many more construction adventures in the days ahead. 
Visualization of the CO2ttage

 

The Spark of Inspiration
In 2015, 150 natural builders from all over the world gathered to demonstrate their skills, new projects, and emerging technologies. The week-long event at the Black Range Lodge in Kingston, New Mexico was a 20-year anniversary gathering of the first Natural Building Colloquium. The beautiful remote location, the charm of the historic lodge, and the company of amazing people doing outstanding natural building work all over the world were truly inspiring. As I said goodbye, my immense gratitude for the experience was strangely obscured by a frustrating realization. In the 10 years since I had attended a previous colloquium, very little mainstreaming of natural building had occurred. I had expected to see more straw bale housing developments, more natural building construction companies, and new multi-family housing projects. Although there was amazing and diverse work being done, there was very little focus on scaling up production, increasing consumer awareness, and expanding the workforce of builders skilled in natural building trades. As I drove away full of inspiration, I committed to doing my part to advance urban natural building solutions. I had no idea what that meant, except that I would apply myself toward change.   
Big Dreams
Armed with motivation, I began dreaming of large scale naturally built planned urban developments with a range of housing types to suit buyers of all ages and income levels. I studied Chris Magwood’s book on building straw panels. I visualized a beautifully designed, practical, and unique housing brand that would capture carbon and embody my values all the way to the bank. This brand would also scale up and franchise to provide ongoing job opportunities for tradespeople skilled in natural building methods. I imagined providing open-source details and resources online, inviting others to replicate, innovate, and advance natural building further. Surely we could find a developer willing to invest in the opportunity of advancing natural building at scale and providing consumers with a unique housing product that matched their values for a sustainable future. Surely proactive marketing campaigns could drive consumer awareness and desire for housing that helps to mitigate climate change and provide beautiful, healthy, and hip living spaces in the heart of the city. Enthusiastically embracing the vision, we created Strawtegi: a business focused on the development of natural sustainable urban housing. I brought 20 years of experience traveling in natural building circles and supporting Builders Without Borders. My business partner Soren Simonsen brought experience as an architect, city planner, innovator, developer, and two-term city council member in Salt Lake City. As an organization, Strawtegi is focused on education, promoting bio-based building technology, mainstreaming the values of natural building, and inspiring others to follow with new innovations in smart carbon sequestering building practices.  Our business model was based on a triple bottom line of supporting people, the planet, and profits. Throughout 2016 and 2017 we explored its potential, met with natural builders, developers, and investors, but life got in the way and progress came slowly. When Salt Lake City approved ADUs throughout the city, I decided that our initial test build project would be built on my own property and at my own expense. 
Axon drawing of straw panel design

 

Proof of Concept for Chopped Straw
We consulted with Chris Magwood regarding straw panel development and Craig White regarding his recent work using chopped straw in Europe. Chris Magwood also suggested SONOclimat ECO4 wood fiber insulation produced in Canada as panel sheathing.  Exploring this uncharted territory took time but seemed like a great way to build a narrower footprint wall insulated with straw. Straw bale walls take up a lot of real estate! Straw insulation was not a concern for code officials. Salt Lake City uses the 2015 International Residential Code (IRC), which has no fire rating required for wall cavity insulation in single family dwellings. Regardless, we conducted our own 2-hour fire rating test to verify that dense packed chopped straw is a safe insulation as applied in our modular panels. We were extremely pleased with the results. Preliminary designs were submitted to Resource Engineering Group (REG) for proof-of-concept analysis, verifying basic structural, insulation, and code compliance for our proposed wall panel system with chopped straw insulation. Carbon Footprint Calculations were developed by Builders for Climate Action, verifying a Net Zero Carbon Footprint for the ADU building shell, meaning that the basic envelope of this structure sequesters and locks in more carbon than was emitted in the overall building of the structure. 
Mocking up a chopped straw panel

 

Forging Ahead
Once approved by the Planning Commission, we recruited a contractor who was excited about the unique and custom nature of this project. Unfortunately, he took all summer to come back with an unworkable budget of $350,000 for our small building.  It took several months more to locate Parker Peterson who embraced our project values, estimated project construction at less than $250,000, and committed to seeing the project through. Although this was still high, it was within the acceptable range of funding available through cash-out equity on my existing property. I refinanced with a better rate and got cash to build. 
Breaking Ground
The permit was approved on Oct 13, 2021. We broke ground one week later, only to discover a new six-month delay in the production and shipping of our SONOclimat wood fiber sheathing panels coming from Canada. Unfortunately, there’s no comparable product manufactured in the U.S. at this time. The delay has allowed us to increase the concrete footing and foundation wall curing time to 56 days and increase fly ash content up to 40%, which significantly reduced the carbon footprint of the foundation and slab. Although delays, supply chain issues, and costs in these unprecedented times definitely try our patience, we trust that it’s all happening in perfect time, allowing us to explore salvaged wood and other material options, and to cast our net of community engagement wider.  
Ground floor plan
Loft plan
The CO2ttage is designed to affordably lower the embodied carbon footprint of the home while maintaining luxuries, conveniences and comforts of modern urban life. The ADU’s aesthetic is consistent with the charm of an older Salt Lake City neighborhood, reinforcing the resident’s sense of belonging and permanence on site. It is two stories to maximize the small building footprint, but the first floor is still designed to be accessible. The 4×8 modular wall panel system has little material waste and it can be constructed by the owner or with community volunteers which can greatly lower labor costs.
Natural Material Building Envelope 
The CO2ttage is oriented for year-round comfort using passive solar energy. Alpen’s quadruple glazed high performance windows are tuned to each elevation by AE Building Systems to optimize heat benefit in the winter while still achieving a super high insulating value (U-Factor, as low as 0.10). In the summer, when the sun is higher in the sky, the windows are completely shaded by overhangs and awnings, and still function to insulate interior spaces from ambient temperatures outside.
CO2ttage cross-section
CO2ttage long section
Chopped straw in wood fiberboard structural sheathing by MSL SONOclimat, is key to the R-30 modular wall design. The CO2ttage uses 42 prefabricated panels, that are delivered and assembled on-site with the support of a lift or crane. The roof assembly is insulated with Hempitecture’s hemp wool batting to meet R-49 code requirements. All concrete on this project is mixed with the maximum fly ash content and allowed to cure for 56 days, to reduce the amount of concrete needed to achieve 3,000 psi structural specs. Locally mined, natural perlite from Hess is used to insulate the foundation and French drains are designed to move water away from the structure as quickly as possible during heavy storms. 
Natural Material Finishes 
Wet areas like the bathroom and laundry area are glazed all around in beautiful, waterproof tadelakt lime plaster produced locally by Limestrong. Healthy, natural, non-toxic materials are emphasized throughout the CO2ttage, including natural and reclaimed wood, Zero VOC paints, butcher block counters, Eureka strawboard cabinetry, and a naturally stained and polished concrete slab. Beautiful and highly repairable natural clay & lime plaster walls by Limestrong and American Clay highlight the hand-crafted aesthetics of the home.
Visualization of C02ttage loft from below
Visualization of C02ttage living room
Visualization of CO2ttage kitchen
Efficient Mechanical Systems
Two energy-efficient Fujitsu Halcyon mini-split systems (33.1 SEER) will operate independently to heat and cool the upper and lower floor. Indoor air quality is managed by the Heat Recovery Ventilator system which also provides passive nighttime cooling with filtered cool air when windows cannot be opened due to poor outdoor air quality. The Rheem ProTerra hybrid heat pump water heater is 400% more efficient than standard electric water heaters.
Reducing upfront carbon emissions has become the primary point of concern in designing new buildings at this time of intense focus on the climate crisis. Building industries are responsible for 40% of global CO2 emissions. Common building materials used to achieve net-zero energy, like spray foam & XPS foam are extremely high in embodied carbon, doing more damage than good, even before a building is occupied. Now more than ever, we have the potential and responsibility to mainstream natural building solutions.  Building with plants is the simplest, cheapest, and fastest way to immediately reduce atmospheric carbon emissions. When we build with biogenic materials, we lock in carbon for the life of the building. At the end of the CO2ttage’s long life the building shell can be fully recycled and composted, returning nutrients to the earth to be recycled again. 
If you’d like to support the Strawtegi CO2ttage and help push natural building further, we welcome construction volunteers this summer and are accepting donations for educational activities through our collaboration with Builders Without Borders. Workshops planned for this summer include wall panel assembly with chopped straw, roof insulation with hemp wool, clay plaster, lime plaster and tadelakt wall finishes, and more.  Please subscribe to keep up with the latest updates at https://strawtegi.org/. Our preliminary fire testing has yielded promising results, but the precision of a university or laboratory would go far in validating the safety of chopped straw insulation in wall cavity applications. If you can help make connections to university staff, labs, or individuals who may be interested in funding lab testing, please contact Susan Klinker directly, at strawtegi@gmail.com. We’ve already learned a lot in designing the Strawtegi CO2ttage and will undoubtedly learn more as we continue the build. 

 

Susan is a passionate advocate for mainstreaming natural building practices. She has designed and built a passive solar home in Tetonia, Idaho, and is currently building a straw panel ADU in SLC. She is a contributing author of 2 books, The Art of Natural Building, 2015, and Building Without Borders: Sustainable Construction for the Global Village, 2004. On a daily basis, Susan works in arts education, creating opportunities for students and community members to engage in the arts as a mechanism to develop a rich community life.  She is a founding member of Strawtegi, which is focused on promoting and building naturally built homes for urban applications.

Read the full issue no. 71 to see Susan’s BEAM calculations and fire tests on her dense packed chopped straw panels.