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Space as a Green Metric

The average size of a U.S. house is going up, but the number of occupants is going down

Posted on Jun 1 2017 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 (from activities like 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.

Sometimes additions are unavoidable

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.

How do we measure 'comfort'?

But what does comfort really mean? The amount of space that feels comfortable to a person depends on any number of factors: 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 2,392 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.

Size is not a sustainability metric

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 HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. 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 LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. 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.

Kristina Eldrenkamp recently completed her Master of Architecture degree at the Massachusetts Institute of Technology. She has worked in Minnesota, Massachusetts, New York, and Tanzania. Her father, Paul Eldrenkamp, is the founder of Byggmeister, a design/build firm specializing in house renovations. This post was originally published at Byggmeister's website.

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Image Credits:

  1. Byggmeister/ U.S. Census Bureau

Jun 1, 2017 8:11 AM ET

It's really about basic economics.
by John Clark

The economics revolves around location and by extension the cost of land which impacts the cost to build. It's simply too expensive to build a small detached single family house. Developers adjust by building a townhome or multi-family condominium. Moving towards multi-family is arguably a "greener" approach and in my opinion ultimately what "green" urban-planners have in mind for everyone but the upper class.


I also think we shouldn't be so naïve to believe that our grandparents or great-grandparents who bought a small post WW2 bungalow on a narrow lot because it was all that they needed when in fact it was more likely the most they could afford.

Another thing is that the 3/2 (bdrm/bath) is basically the minimum for the vast majority of homeowners under the age of 55 simply because people have large extended families that visit. In the 1950's everyone lived relatively close by so you didn't really have a need for a "guest bedroom".

My two cents.

Jun 2, 2017 1:53 AM ET

If these trends continue
by Alan B

By the year 2206 the average house will be 6994 sq/ft and have zero occupants :D

Jun 7, 2017 11:58 AM ET

smaller housing is low hanging fruit
by Curt Lyons

Thank you for your article. In an age of increasing conscientiousness of energy use, it is ridiculous that we are making more space for less people. If you would like some back-up for your 500 s/f per person, Jason McLennan, who developed the living building challenge proposes that, regardless of "green" features large houses simply cannot be environmentally sounds. He says that 450 s/f per person is as big as we should go, and smaller is better. Yes Magazine, issue 62, summer 2012.

Jun 7, 2017 10:30 PM ET

Homes for 1?
by Paul W

500 sq.ft./person may work until you hit the single buyer. Functional kitchens, bathrooms, mechanical rooms are unrealistic for most home buyers in those dimensions. In theory you need only one bedroom/bathroom, but that's just nit going to work for most buyers either. I would maybe amend the formula to something like 500/person + xxx utility room/mud room/other space that doesn't change if residents increase.

Jun 18, 2017 4:02 PM ET

guest space
by Charlie Sullivan

It's common to want a house big enough to provide guest space. From an economics perspective, that doesn't make a lot of sense. It would be cheaper to own a smaller house and rent hotel rooms when guests visit. But there are two reasons why that doesn't actually meet the objective. One is that you'll spend less time with your visitors if they are off in a hotel room, and the other is that offering to pay for a visitor's hotel room involves awkward conversations about money than can be avoiding by offering a room that you've already paid for (or perhaps already committed to pay for if you have a mortgage).

Here's a proposal: A company offers the service of parking a travel trailer in your driveway whenever you need space for guests. This way, the guests can stay very close to the hosts. The company could offer a subscription plan that provides 10 "free" nights per year, to enable the hosts to tell the guests that they can offer them a bed at zero incremental cost. I'm not actually sure how the energy consumption and cost would compare to adding a guest room to a pretty good house--it might not be a slam dunk for people who have regular visitors. But the real advantage might be avoiding construction of a way-too-big house based on the possibility of guests who turn out not to come that frequently.

Jun 18, 2017 4:28 PM ET

Edited Jun 18, 2017 4:29 PM ET.

by Malcolm Taylor

Another problem with not having additional sleeping areas is it precludes live-in care in the event of illness, accidents or aging. Perhaps a better solution is a space that can be used for other purposes when not a bedroom?

Because the majority of the projects here in GBA are single family suburban or rural houses, the discussion often precludes easier answers that are available with other housing types.

So for example, with appropriate zoning, an urban lot can include a single family house, carriage house over a detached garage, and a basement suite. Depending on their situation, as time passes, the owners could live in whatever sized unit met their needs and rent out the others, use one for other purposes or leave it vacant as shared guest accommodation. Condo units now often include shared guest suites - which seems like the best of both worlds.

I'm not sure about how I feel about architects having a fixed square footage number as part of their practice that is independent of the specific circumstances of the client's brief. I guess as a bare minimum it should be clearly stated upfront as a design criterion before the process begins..

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