After decades of building high-performance houses, we decided that we wanted to be able to quantify the things which have been intuitive to us. We wanted to be able to confidently advise our clients on which green building technologies would be the most effective in reducing their home’s impact on the environment.

As the evidence has come together about human impact on the planet, it has become clear that most environmental issues pale in comparison with the threat of climate change. Therefore, we have decided to express a building’s environmental impact in terms of the carbon dioxide emissions generated by its operation. We built a spreadsheet model which combines local climate data with estimated loads and heat loss to generate a monthly profile of a building’s energy requirements. We balance this against the available energy resources – fossil fuels and solar energy – to determine how much fossil fuel will be required to meet the heating, hot water and electricity needs of the house. An ordinary, code-compliant house will typically burn fossil fuels in both summer and winter for heating and cooling. The average American single-family home puts 40,000 pounds of carbon dioxide into the atmosphere each year. A tightly-constructed home with sufficient solar supply will burn a small amount of fossil fuel in the winter, but will deliver electricity back to the grid in the summer. With surplus photovoltaic supply, it becomes a net energy producer.

Green building techniques fall into two categories: conservation and supply. The conventional wisdom is that you should implement energy conservation strategies (insulation, air-tightness, and efficient fixtures and appliances) before investing in supply technologies (passive solar, solar thermal and photovoltaics). We wanted to determine exactly which techniques yield the greatest reduction in emissions per dollar spent, and find the point at which it becomes more cost-effective to buy supply than to continue with high-tech efficiency.

From our models, (based on incremental improvements to a hypothetical code-compliant, “ordinary” house with natural gas heat and hot water), we found a series of steps that represent the most cost-effective pathway from 40,000 pounds of carbon dioxide per year to zero emissions. We constructed a similar pathway for an all-electric house, (anticipating that electric heating systems will become cost-competitive in the near future). We found that the point at which it becomes cost-effective to add photovoltaic supply is also the point at which the house is halfway to zero emissions. This represented only a 3-5% increase in construction costs. We decided that this was an excellent target for a new building energy code minimum, and we have been advocating this goal to our local and regional building departments.

The next decade presents the best opportunity for a turnaround in the way houses interact with the environment. Understanding and action on the part of individuals, cities and nations can make the residential sector energy-independent, to the benefit of economy and ecology alike.