A Carbon Challenge contest to design a low-budget low-carbon-footprint house was recently held in Providence, Rhode Island. The 144 entries were judged on four criteria, with the emphasis on a carbon footprint measure that looked at the entire life cycle: building materials and the energy to ship them, the energy to heat and operate the house, and the processing of materials when the house comes down some day.
This approach yielded a lot of interesting details. It did not produce stunning visual statements—no surprise there, given a construction budget of just $80,000 to $120,000. The houses are designed to be Habitat for Humanity infill within an older neighborhood of simple saltbox houses, and the winning designs would fit right in.
In a nod to the emphasis on embodied carbon (that’s the global warming potential of the building materials) several winning entries use innovative foundation systems to reduce the volume of concrete.
The grand prize design—the “Little Green Rhody” from ZeroEnergy Design in Boston—uses a 7.5-kW rooftop photovoltaic array to get to Net Zero energy. If I’m reading the statement correctly, the solar panels do not actually fit within the budget.
ZeroEnergy has experience with Passive Houses, and incorporated several typical PH features like European triple-glazed windows, taped ZipSystem wall sheathing, and an HRV with air-source heat pump for both heating and cooling. They aimed well short of PH in an area usually thought of as easier on the wallet: the thickness of walls and roof. These are just R-27 and R-52, respectively. Indeed, the wall studs are 2x4s on 16-inch centers—numbers from the old days!—with 3 inches of mineral wool batt insulation added on the outside.
If you’ve ever looked at a cross-section of those high-end German window frames, it’s downright Byzantine. They’re very complexly structured, in order to prevent solid frame material from conducting heat from the inside of the house to the outside. Some builders have turned to new Canadian windows that are slightly less efficient and considerably less expensive. In ZeroEnergy’s calculations, however, German windows came out within budget and on top, partly because of less energy going into shipping them. Both the building site and the manufacturer are close to Atlantic seaports; a long Transatlantic passage by ship plus two very short deliveries in trucks uses less energy than one medium-long trip in a truck.
A Habitat for Humanity Passive House went up in Vermont last year. It roused some controversy surrounding its cost, estimated at $50,000 above that of a similar house built to code. (“Habitat shouldn’t build ritzy houses! Harrumph! How many years does it take to pay back that premium?”) Little Green Rhody’s architects made sure they didn’t follow in those footsteps. They estimate an $8,000 cost premium compared to a similar design built to code, paying for itself in a little over five years thanks to $1,500 annual savings in heating costs.
Just to be clear, Little Green Rhody was not designed to meet Passive House standards. By utilizing some of the typical PH elements, it comes fairly close and it makes the point that really good insulation and airtightness can pay for themselves in just a few years. I’m still a bit puzzled by those thin walls, which may be in the design just because it was running up against rigid budget constraints. Wouldn’t a few more inches in the walls and roof pay for themselves soon enough, as well?
The US Forest Service and APA, The Engineered Wood trade association, sponsored this contest and a simultaneous one in Baltimore. All designs had to use wood, and were judged partly on their use of wood, but the winners don’t look unduly woody. Whereas the Providence designs were for families of five, Baltimore called for row houses for three adults living as roommates. The life cycle assessment of each design’s carbon footprint was calculated using software from the Athena Sustainable Materials Institute.