There’s an old saw that says: “you get what you measure.” It’s usually true that by focusing on a specific metric, you drive that metric toward a desired outcome. In the world of building energy use, we often use the phrase “energy efficiency.” While efficiency is a good thing, it’s not exactly the proper target or the best metric to guide the necessary action if our goal is to reduce greenhouse gases (GHG).

Efficiency is Relative

The term “efficiency” is used to express a ratio of one thing to another. Miles per gallon (MPG) refers to how far you can drive an internal combustion-powered vehicle on a certain amount of fossil fuel. While higher MPGs are certainly desirable, fossil fuel is still being consumed and carbon is still being emitted. To eliminate the GHG impact of driving, we must not only improve efficiency, but our fuel must become carbon neutral. “Efficient” diesel trucks are a good example. Yes, they are much more energy efficient than they used to be, but you can still see them spewing exhaust into the air we breathe and know that they are also emitting carbon into the atmosphere.

Home energy efficiency is somewhat similar to MPGs. An ENERGY STAR certified home with a low Home Energy Rating Score (HERS) will certainly be more efficient than a home built to the minimum code. However, a code-level home may use less energy if it is significantly smaller than a large “energy efficient” home to which it is being compared. In either case, both homes, if powered by natural gas or electricity sourced from fossil fuels, are still contributing greenhouse gas emissions and exacerbating the climate crisis, no matter how “efficient.”

Similarly, in the world of commercial buildings, efficiency is often expressed by energy utilization index (EUI). This is the amount of energy used per square foot of floor space in one year (kBtu/ft2/yr). It’s a useful metric because it allows designers and building operators to compare their progress against similar building types of very different sizes. The US Department of Energy publishes a list of EUIs by building type. The US median EUI for a K-12 school is 104, a hotel is 147, and a fast food restaurant is 886. Within each type of building, the median represents the midpoint between the highest and lowest energy used per square foot per year for all the buildings in that category. Because different buildings are used for different purposes, it makes sense that some buildings will use more energy than others. For example, a school will use significantly less energy than a restaurant due to the activities that go on in the building. If you design a restaurant, you’ll be moving in the right direction if your project has an EUI well below the national median value, but the restaurant will still be emitting large amounts of carbon.

Reducing Carbon Emissions is the Goal

What counts much more than “efficiency” is the amount of GHG emissions resulting from the construction of a building and those used to serve the needs of the people inside it. So it’s time to convert to a more useful metric for measuring progress to climate stability. We need to move beyond just a HERS Index for homes or the EUI for commercial buildings to measure how many tons of carbon dioxide are released by buildings – and to bring that number to as close to zero as possible.

Carbon emissions will depend on several factors. First and foremost is the amount of energy needed to erect and operate the building over its lifetime. Next is the type of energy used. Is it the direct use of fossil methane (natural gas) or electricity from coal or fossil methane power plants? Or is it clean energy from renewable sources? By measuring carbon directly we can move more rapidly toward the time when buildings are responsible for zero carbon emissions. To fully eliminate GHG emissions, the key is to design and build or renovate to use as little energy as practical AND acquire that energy from clean, renewable sources.

Design for Emissions

What should building designers, contractors, and consultants do to shift the metric from energy efficiency to greenhouse gas emissions? The first step is to use an energy model during the design phase to estimate energy use and the amount of each fuel type needed. Ideally, a building will generate as much energy as it uses through onsite renewable energy, most commonly solar panels.

If a building relies on natural gas or electricity from a utility, then the situation gets complicated. Some residential energy modeling software, such as HEED, includes GHG calculations in its standard reports. New software is now available for calculating a building’s or home’s carbon life cycle assessment and for calculating a building’s carbon footprint. For many, these may be useful tools for getting on the path to zero carbon. For those who want to keep it simple, using energy modeling and renewable energy sources to get as close to zero energy consumption as possible may be the better way to go.

Time is Running Out

If our goal is to maintain the essential functions and amenities of modern society while at the same time eliminating human-caused greenhouse gases (GHG), moving in the right direction is not enough. Time is quickly running out. We must swiftly transform the economy to carbon-neutrality. We must cut emissions to zero, and fast. Firm limits must be set on carbon emissions from both buildings and transportation.

Each new building that rises from the ground will exist for decades if not a century. And the energy use of that building is locked in for its entire existence. Some say that these buildings can be retrofitted to use less fuel. While that may be true, retrofitting is more expensive and less effective than building them properly the first time around. With thoughtful design and properly structured financing, any new building can reach zero carbon emissions and be a better building.

Since we know that fossil fuel use must reach zero within two decades or so, we must design and construct buildings TODAY with zero carbon emissions as our number one priority. The carbon measurement tools mentioned above are a good starting point. It’s time to use these or similar tools to move past mere efficiency and set our sights on zero fossil fuel consumption and zero carbon emissions.