Space as a "Green" Metric

Space as a "Green" Metric

  • U.S. house and household sizes from 1950 to 2010
by Kristina Eldrenkamp

“Green” can be an empty term if it’s not defined in measurable ways. This conviction has informed our efforts to assign performance metrics to projects and to monitor progress towards meeting these over time. Some of the ways we document performance include our energy and water use tracking program and our protocols for protecting occupants against indoor pollutants (such as cooking). One aspect of “green” building, however, has eluded standardization: house size. How big is too big to be sustainable? Is there a point at which high-performance building practices are undercut by square footage?

Most of our work is remodeling, so we don’t encounter this every day. Our square footage is usually predetermined by an existing building. In many projects, however, we are confronted with the decision either to build an addition or reconfigure a space within the existing envelope to address a client’s needs. Because we ultimately want to leave a project knowing that it uses less energy than it did before, we have to think beyond energy use intensity (EUI, the average energy use per square foot per year) to the total energy expenditure of the whole house. Even if we add insulation and air sealing to make a house more energy efficient, tacking on a big addition could increase the overall energy use. For this reason, we always start by looking for solutions that don’t require an addition.

For projects where an addition is unavoidable, we have adopted 500 square feet of finished floor area per person as a rough guideline. This number is somewhat arbitrary in that it has no empirical research backing it, but we’ve found from our many years working on houses that this strikes a reasonable balance between extraneous energy consumption and comfort—which isn’t to say that we haven’t ever exceeded this threshold (or even that most of the existing homes we renovate conform to it).

In our experience—whether or not we’re able to hold to this threshold—houses beyond 500 square feet per person often don’t add functionality. Instead, they have rooms that sit largely empty, such as a formal dining room, living room (when a family room or den exists as well), or guest bedrooms. When we have more rooms than we need to meet daily needs, we are inefficiently consuming both space and the energy needed to condition that space.

But what does comfort really mean? The amount of space that feels comfortable to a person depends on any number of factors, among them where we live, how we grew up, and what our socioeconomic status is. In a statistic we've referenced before, the average American house in 1950 was 983 square feet and the average family size 3.37 people, while the average American house in 2010 was 2392 square feet and the average family size 2.58 people. Put another way, the average living space per person rose from 292 square feet to 927 square feet in under 60 years. Other research has shown that younger generations are more likely to have grown up in their own room than previous generations. In other words, Americans are growing accustomed to more private space.

Shifts in cultural attitudes towards private space makes a universal definition of comfort elusive. A house’s layout complicates this further. Older homes may have small or inaccessible kitchens, a formal dining room, or a now-redundant “servant” stair. These houses can feel cramped in our current cultural context, no matter the overall house size.

Comfort is a necessary component of sustainability because a dose of practicality lets us implement these practices at a large scale. There are always people willing to make personal sacrifices for the environment (the tiny house movement is one example) but most of us gravitate toward the middle rather than the extreme. A house size sustainability metric therefore has to negotiate a reasonable expectation of comfort while considering evolving cultural expectations about private and shared spaces in the home.

The standards used by the building industry do not incorporate size as a sustainability metric, and in some instances they inadvertently privilege larger buildings. Passive House certification, for example, uses a primary energy limit that is calculated as a rate per occupant per year, but it derives this number from design occupancy (the number of bedrooms plus one) rather than actual occupancy. A three-bedroom house is given the same annual energy allotment whether a multi-generational family of five or a couple that occasionally hosts guests lives there. The Passive House methodology also unintentionally produces something called the “small building penalty.” Small homes typically have a greater surface area-to-floor area ratio than large ones, and since heat loss is a function of surface area, a small house will lose more heat relative to its living space than a larger one, making it harder to reach the Passive House standard. On the other hand, the LEED rating system includes safeguards so that larger homes have a harder time achieving higher ratings.

In many of the metrics monitored by industry standards, square footage is used as a constant, a unit to measure other units by (energy use as a rate by square foot, for example). It’s surprising, then, that square footage itself hasn’t been more regulated for its impact on energy usage. This gap in measurable size metrics speaks to the deeply personal nature of space. We’ll continue striving for minimal environmental impact in our projects, aiming to build 500 square feet per person until we can get away with something smaller.