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Build an affordable, off-grid, 100% solar powered tiny house as a model to advocate for the formation of carbon-neutral housing communities.



Cities around the world face the challenge of housing a growing population while the size, cost, and carbon footprint of homes continue to rise. Consequently, affordable and sustainable housing represents an emerging market that is in desperate need of innovation.

Tiny houses stand out as a unique response to this need.

The tiny house movement is a growing trend across the country, favored primarily by those looking to do more with less and, in many cases, to live closer to nature. Thanks to their small size and emphasis on efficient usage of space and materials, tiny houses are both more affordable and more sustainable than traditional homes, demonstrating the compatibility of environmental sustainability and cost effectiveness objectives. This convergence is particularly salient in the Bay Area region of California, where the impact of climate change on water availability and reliable energy generation meets with a crushing housing crisis.

Tiny House in My Backyard (THIMBY) is an interdisciplinary group of UC Berkeley students designing and building a budget- and climate-conscious tiny house on a trailer — adding mobility to the unit’s list of advantages that includes grid-independence, a biofiltration-based water recycling system, and an appealing price tag for low-income and first-time home buyers.

The house will be displayed at the Sacramento Municipal Utility District (SMUD)'s Tiny House Competition in October 2016, and will thereafter remain in the East Bay, serving as an educational tool, a living laboratory, and the first of a series of sustainable and affordable housing projects that will form a community in either Richmond or Oakland, CA. With this project, we are pushing the boundaries of sustainability within the tiny house movement and investigating both the advantages and challenges of tiny homes as one solution to the lack of affordable and sustainable housing options.

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What actions do you propose?

Our plan is to build an affordable, off-grid, 100% solar-powered “tiny” house on the site of the planned Berkeley Global Campus in Richmond, California. The house, built on a trailer, will be small enough (<200 sq. ft.) to fit in the unused portions of urban lots, yet will provide a comfortable and inviting home environment with a focus on energy and water efficiency. We plan to integrate the following systems, among many, to achieve our goal of cost-effectively minimizing the life cycle emissions of the home: Rooftop solar + Li-Ion battery storage and EV integration, heat pump water and space heating, rainwater collection, human waste composting, household greywater treatment via a living wall biofilter, and weather forecast-based optimal control of heating loads.

Once built, our house will:

  1. Demonstrate the compatibility of financial and environmental objectives in a mobile, off-grid, 100% solar-powered tiny house.

  2. Serve as a “living lab” on the Berkeley Global campus, allowing researchers to test novel residential energy- and water-efficiency technologies, as well as strategies for residential design and systems integration.

  3. Serve as a seed for additional sustainable and affordable units, with the long term goal of scaling to form a carbon-neutral community of small and/or “tiny” homes.

By building this house, we will explore the viability of tiny homes as a response to the pressing, simultaneous needs for reductions in building emissions and increases in the availability of affordable housing. Our unit will represent a starting point from which students, scholars, homeowners and policymakers are able to interact and experiment with this emerging design approach, one day resulting in the creation of THIMBY-inspired carbon-neutral communities in the East Bay and beyond.

Our goal of generating all of our own energy will be achieved by minimizing electrical demand while storing energy generated by photovoltaic panels. The tight building envelope and carefully planned solar gain will minimize heating needs. We will use a heat pump to efficiently heat water at times when we are harvesting electricity: this will serve as both a source for domestic hot water and for space heating through a radiant floor system. Thanks to the high coefficient of performance of the heat pump, this will consume  only 1 kWh/day, even when space heating is needed. All other appliances will be chosen to use minimal energy while providing the comforts of a typical home. Total daily energy use will only be 4-5 kWh, including a discretionary budget of 1.5-2 kWh for cooking. Given the location of our house in Richmond, CA (Climate Zone 3), summer cooling loads will be minimal. We will meet these needs through ventilation, ceiling fans, and the use of phase change materials in our roof assembly. However, a key benefit of the Tiny House On Wheels (THOW) is mobility, and a user may wish to travel to areas with hotter summers. Because our off-grid electrical system is sized to meet winter heating needs - when PV production is significantly lower - ample electricity will be available in summer months to power a standard window A/C unit in such situations.

We will have over 2 kW of solar panels that will generate an estimated 6.9 kWh/day in December and 11.9 kWh/day in June. The panels will be tilted to optimal angles for each season. Power will be stored in a 6 kWh Tesla Powerwall. This battery is uniquely lightweight, compact, and affordable - key requirements for a house on a trailer. An inverter system will supply power from the battery in a form that is compatible with standard North American appliances and will allow us to give extra power back to the electrical grid when we are in a location that allows grid access while also functioning in completely off-grid settings.

The size of the photovoltaic and battery system will allow the house to be off-grid, even in the winter. We have modeled the energy usage of our house as well as photovoltaic generation in Richmond, CA throughout the year. From this, we plot below how full the battery will be over the course of the year (plotted in hours), as well as weather data. Even on winter days, when power is needed for heating but the photovoltaic panels do not produce well, the battery still almost never needs to be fully discharged. The house can function comfortably through cold and cloudy stretches.

THIMBY’s low power demand, combined with the incorporation of significant thermal energy storage within our hot water tank, which provides space heating through the radiant floor, facilitates the use of a battery that would normally be considered under-sized for off-grid use. This reduces THIMBY’s embodied emissions and the raw materials needed to manufacture large batteries.

The water system for the house is designed to recycle greywater through a vertical “living wall” combined with an activated carbon filter on the west facade, and then capture filtered greywater for non-potable reuse in the sinks and shower. Water quality testing results will determine the ultimate uses of recycled greywater. Throughout the house, water outlets are designed to minimize waste, incorporating low-flow faucets and a vaporizing shower head that has flow rates of approximately 0.7 gpm. The later-stage filtration planters in the living wall will contain edible plants, providing a portion of the residents’ food needs. The wall will also take advantage of compost generated from on-site human and food waste, thus forming a closed-loop system that channels “waste products” back to productive uses on-site.

Given that it can be difficult to maintain “living” systems in a house when occupants have guests or go on vacation, back up plans are in place to allow the house to be flexible of user lifestyles. For example, the greywater treatment loop through the living wall can be disconnected and diverted to a “black water” tank when users are sick, have guests that might dramatically increase water load on the plants, or don’t want the system to be recirculating. The black water tank can be removed and emptied at an off-site location. When occupants go away on vacation for extended periods of time, it would be advisable to have house-sitters keep the systems running, or to empty the water tanks so that bacteria doesn’t build up in any tanks. The living wall plants would have to be looked after and watered by friends and neighbors if time away from the house extended beyond a week.

Although the actual tiny house is the central focus of our present work, the long-term goals of this project are to investigate the potential of this form of sustainable and affordable housing at scale. Thus, to promote improvements upon our initial design and to encourage the incorporation of successful components into larger-scale developments, we also want to share the process behind the project with as many people as possible. It is our belief that at the heart of sustainability is education. In light of this, we taught a faculty-sponsored peer-to-peer class at UC Berkeley about Sustainable Design and the implications of Tiny Houses this past semester. We engaged our students about the social aspects of the tiny house movement, taught them about several different components of sustainable design, and granted them the opportunity to undertake hands-on prototyping projects directly related to our tiny house. In the end, their work gave the THIMBY team valuable insight into the design process while our course material helped spark their interest about the tiny house movement and amplified the conversation about affordable sustainable design.  

To continue our educational programming this summer, our team will hire several Richmond-area high school interns to work on the project alongside UC Berkeley mentors, building college pathways and career training opportunities for these students. Additionally, we will engage the community by hosting group build days, in which community volunteers will be able to join us and undertake a variety of activities such as putting up walls, installing solar panels, and integrating the living wall system.

Who will take these actions?

An interdisciplinary team of UC Berkeley graduate and undergraduate students is designing and building the THIMBY project. Students hail from numerous departments and colleges across campus, and project advisors Daniel Kammen (Professor of Energy) and Harrison Fraker (Architect and Energy and Resources Group (ERG) Department Chair) provide support and mentoring for the energy systems and architectural design, respectively.

The design of THIMBY systems will take particular advantage of student research interests and expertise. For example, team members have used a class project to prototype the Home Energy Management System (HEMS) that will coordinate THIMBY heating loads; other students studying sustainable food and water systems have designed the living wall for biofiltration, and architecture students with experience in passive house methods are leading the windows and insulation material plans.  A student researching fog capture will integrate this technology onto the roof, demonstrating an innovative water access option for increased drought resilience in California.

Achieving our scalable sustainability goals requires the collaboration of stakeholders beyond the university setting — Richmond City Planners, developers, government agencies, and local professionals will all contribute to the action of scaling the tiny house design into a net-zero housing community. The THIMBY team has already established many of these partnerships, integrating members of the Richmond Planning Commission, the Government and Community Relations arm of UC Berkeley and various industry partners into our team. Community contacts are interested in engaging organizations, youth, and local expertise, while industry partners are interested in making their energy-efficient products available to a larger market. By leveraging synergistic goals between the university, local government, and industry, the objective of a THIMBY-inspired carbon neutral housing community can best be realized.

Where will these actions be taken?

The initial tiny house will be constructed at the Berkeley Global Campus (BGC) in Richmond, proposed by the Office of the Chancellor as “a focal point for an international coalition of leading academic institutions and private sector and community partners [who will] collaborate on research and academic initiatives addressing complex global challenges of universal relevance to academic and industrial researchers.” This site is an active area of negotiation between the community and university, with the Richmond Community Working Group advocating for the project to provide community benefits such as local hire, materials procurement, and education. THIMBY members have been attending Community Working Group meetings to best understand the desires and needs of those in Richmond specifically seeking affordable housing and green building skills training, which the project seeks to address.

THIMBY will begin at the BGC, but its goal is to explore the potential of tiny house communities in a broader context. Because of the unique nature of the BGC site, the tiny house has high visibility as a demonstration unit for climate-smart urban infill housing and a community-scale housing project. The house will be showcased at the Sacramento Municipal Utility District (SMUD) Tiny House Competition in October 2016, and from there will be available for public viewing in cities such as Richmond and Oakland. THIMBY project advisors are simultaneously developing a retrofitted energy-efficient neighborhood in Oakland as part of the California Energy Commission-funded “Oakland Eco-Block” project; this project will include designated lots for tiny houses, with THIMBY providing the initial prototype for these homes. The THIMBY house, taking advantage of its mobility as it is built on a trailer, can furthermore be placed in strategic exhibit spaces as it seeks a more permanent “home” beyond the building cycle at the BGC.

How much will emissions be reduced or sequestered vs. business as usual levels?

The CoolClimate calculator (CCC) estimates electricity/gas usage-related emissions for a low-income 2-person household in Richmond, CA at 100 tons CO2e over a 20 yr lifetime. These emissions are avoided with an off-grid system.

In partnership with the Bay Area Environmentally-Aware Consulting Network, we have also developed a tool to estimate THIMBY’s embodied emissions (~18 tCO2e). The CCC estimates embodied emissions of a 192 sq ft house at 9 tCO2e (82 tCO2e for 1750 sq ft).

This suggests that THIMBY has a ~2 yr “relative emissions payback period (EPP)”, whereby additional emissions associated with off-grid energy generation, storage, and efficiency equipment are offset by avoided natural gas and grid electricity use. A ~4 yr “total EPP” represents the timeline for full offset of embodied emissions. Further energy savings will likely exist due to greywater recycling (CCC estimates emissions from water use at 1.29 tCO2e/yr for a 2-person house).

What are other key benefits?

The THIMBY project contains an array of co-benefits to both students and community. Working on an interdisciplinary team that extends beyond university classrooms into real-world settings and gaining hands-on engineering and policy experience affords students invaluable career training and networks. This project is directly aligned with the kind of 21st century environmental education needed to create leaders prepared to address and mitigate the impacts of climate change.

Richmond has a long history of environmental injustice, with residents suffering high levels of pollution from the legacies of munitions manufacturing and oil refining. By providing residents with affordable, safe, and pollution-free living spaces, this project creates the kind of solutions needed to redress this unjust history. By including community members alongside students in the design and construction of the initial tiny house, the community is empowered to scale up this project to best suit their needs. 

What are the proposal’s costs?

The goal of the project is to create a self sustaining tiny home for less than $40,000 in materials costs. This figure does not include the cost of labor to build the home (~$15,000) or the cost of the trailer (~$6,500), but does represent the full retail cost of any materials donated to the group. To date, our team has received close to $42,000 through stipends, grant awards and a crowdfunding campaign. Additionally, we have received around $10,000 in materials donations from companies such as Platt, Pella, Taco, Warmboard, and Simpson Strong Tie. The largest components of the overall cost include the battery and solar panels (>$10,000) and the water system ($5,000). Other major expenses include energy-efficient doors and windows, roofing, radiant floor system, and hardwood flooring. As the Bill of Materials for the house is still being finalized, it is difficult to place an exact number on the cost; however, we intend to produce a carbon-neutral house that is replicable for <$65,000, including foundation and labor costs. 80% of this estimated project cost has been raised to date. It is important to note that the off-grid nature of THIMBY means that homeowners will pay no electricity or gas bills, resulting in ~$2,000 in annual avoided costs for the average U.S. household. Efficient and/or sustainable designs and materials are often not utilized due to high capital costs. It is our hope that the low price tag on THIMBY helps demonstrate a feasible pathway to unite goals of affordability and sustainability. With the current target cost, this house represents a quick pathway to homeownership for low-income and first-time homebuyers while simultaneously meeting emissions reductions goals.

Time line

We began this project in October 2014, growing the team from 4 co-founders to 20 current members and fundraising through grants, donations, and crowdfunding campaigns. The conceptual design phase was completed in Summer 2015, and initial construction drawings were submitted to the SMUD competition in November 2015. We have been iterating on these drawings and procuring materials and equipment since that time, in addition to managing continued team growth, incorporating participation and direction from local community and university entities, and raising additional funds.

We are currently finalizing several components of the design, and construction is beginning on May 23. It will continue through Summer 2016, including several build days in which volunteers can join in the effort and learn about building techniques. While the frame of the house is being built in early summer 2016, we will simultaneously begin testing water and electrical systems of the house.

As the house is being completed in September 2016, we will also finish documentation of the construction process, explaining many of our design choices and providing a guide to others who wish to incorporate components of the systems, technology, and design of our house into future projects. On October 10, 2016, the tiny house will be entered into the competition in Sacramento, CA, to be judged against seven other houses built by teams across California.

Following the competition, the house will be exhibited at a site in Oakland in collaboration with the aforementioned EcoBlock project, before reaching it’s final home either on the Berkeley Global Campus or as the first house of a development project for an empty lot in Richmond, CA. This latter option is still under negotiation.

For further timeline details, please visit

Related proposals


Citations have been included as links in the proposal. Uncited, but relevant, reports include the following: