Esoteric, expensive, unrealistic, inspired, essential.  A decade ago, these were common reactions to combining green building and affordable housing.  Today the reaction is quite different - expressed by the now frequently repeated aphorism: we can't afford to not build green. Green affordable housing has moved from a curiosity to a trend and is now rapidly becoming standard practice. Rapid growth in the adoption of green practices in affordable housing design and development over the past five years has brought us to a point where non-profit and for-profit developers across the country are embracing green building as an economically sound design approach. As the number of completed green projects grows each year, the discussion surrounding green building is shifting quickly from "doing the right thing" to a more substantial dialogue founded in project-related experiences and, increasingly, performance data colleted from those projects. But green building is just a part, albeit a critical one, of creating sustainable communities. To understand how green affordable housing can most effectively serve as a catalyst for a more encompassing transformation of urban neighborhoods, it is valuable to take a step back and revisit the aspects of affordable housing that are inherently sustainable.  In other words, how does affordable housing, before the integration of green practices, contribute to economic and social equity, and bring environmental benefits to tenants, neighborhoods, cities, regions, and the planet? The arguments for affordable housing center largely on issues of social and economic justice with environmental benefit often occurring as a fortunate side-effect if a project happens to be urban in-fill, adaptive reuse, or close to transit and services.  Economically, affordable housing ensures that low-income families do not have to go without essentials such as food and health insurance just to keep a roof over their head.  Providing affordable housing throughout a city or region ensures there is equity in where people can live, promotes the diverse, local work force needed to support the varied needs of a community, and adds a security of tenure that helps create neighborhood stability. Environmental benefits from reduced air pollution and carbon emissions accrue when housing built proximate to jobs and schools, cuts overall travel distances and allows transit to replace car trips. At a larger scale, reusing urbanized areas alleviates the pressure to sprawl. This is a good start, but unfortunately, all affordable housing is not created equal. According to Harvard University's 2008 State of the Nation's Housing Report, at least a quarter of the nation's affordable housing stock is in a state of disrepair¹ - bad news for both the residents and the environment. People living in sub-standard living conditions experience greater rates of asthma, elevated lead levels, and spend a much higher portion of income on utilities (up to 25% of income after rent) due to leaky buildings, inefficient heating and cooling equipment, and outdated appliances.  The environment suffers from wasteful energy use and the associated carbon emissions, unnecessary water use, and landfill impacts when buildings become so dilapidated that they must be demolished. Green affordable housing augments the core sustainability aspects of affordable housing while rounding out and expanding the environmental component.  Green building practices create healthier living spaces by improving ventilation and avoiding toxic materials, lowering utility costs, and improving durability. Funds not spent on health care or utilities can be redirected toward higher priority items such as education, or used to support local businesses.  Green affordable housing also mitigates the negative local, regional and global impacts of standard building construction and operation. The synergy created through the combination of green + affordable offers a microcosm of a sustainable community.  By simultaneously focusing on the details of a given project's design, systems, materials and orientation and considering how that project plays into a larger effort to create sustainable neighborhoods, we are forging the way for a new model of development that is truly green and sustainable. Sound too good to be true? Of course, achieving this ideal is not without its challenges.   Issues of financing additional upfront costs, capturing long-term benefits, and establishing an integrated design process remain complicated and time-consuming. But speaking from over 10 years of experience, it's only getting easier, as we have more built projects and more experienced practitioners to learn from.  Over the next 8 weeks I'll try do debunk some of the common misconceptions about green affordable housing development, tackle the thorny issues, share recent performance data, introduce emerging innovations, and attempt to situate the topic within a larger emerging theory around green urbanism. ---------- Walker Wells, AICP LEED AP, is Director of the Green Urbanism Program at Global Green USA and the editor and co-author of Blueprint for Greening Affordable Housing.