The Value of Benchmarking Buildings with Water Use Intensity
All images courtesy of author.
Tracking performance data is a key aspect for validating the operational outcomes of high-performance buildings. As the adage goes, you can't improve what you don't measure. Through public resources such as the U.S. Environmental Protection Agency's (EPA) ENERGY STAR Portfolio manager, property owners have the ability to benchmark the performance of their building assets against other buildings of the same property type and region. Regarding energy, this entails reporting at least a year of metered usage from utility bills. In the case of ENERGY STAR Portfolio Manager, the utility information is processed and the property is attributed a percentile-based ENERGY STAR score resulting from the building's relative standing to comparable properties based on energy use intensity (EUI) using units of kBtu/sf/year.
EUI has emerged as a "common language" by which the building design and construction industry communicates annual energy performance for projects.
Is There a "Common Language" Metric for Water Performance?
The most common units for water supply reported on utility bills are gallons and centum cubic feet (CCF). The word “centum” is derived from the Roman word for hundred. As such, a CCF may also be identified as a hundredth cubic feet (HFC). One CCF (visualize a 5-ft by 5-ft by 4-ft container filled with water) is equal to 748 gallons.
Should a sufficient number of building owners report metered gallons or CCF data, such that the information can be aggregated and cataloged like energy data are through ENERGY STAR and other programs, the building design and construction industry could benchmark water performance for a range of property types in much the same way that we do for energy.
Luckily, an increasing number of property owners are already doing this.
The Emergence of Water Use Intensity as the de facto Water Performance Metric
According to the EPA, around 40 percent of the buildings in their ENERGY STAR Portfolio Manager report water data in addition to energy data—a proportion that has roughly doubled over the past decade due to increasing water conservation concerns and as more cities and states have implemented benchmarking ordinances. This has provided the EPA with data for tens of thousands of buildings, upon which validated benchmarks for several distinct buildings types have been established (see the figure below). As a result, the building design and construction industry now has useful water use intensity (WUI) data per property types, upon which property owners can now benchmark and improve their building's water performance.
With increased frequency, WUI is being used to measure, track, and compare building water consumption much like how EUI is used for energy consumption. WUI basically refers to the rate at which water is used on a property on per building unit area basis. Measured in gallons per square foot per year (gal/ft²/year) or liters per square meter per year (L/m²/year), WUI is an indicator of how much water a building consumes per year under typical operation. Relative to a water budget or a comparable baseline, WUI can serve as a gauge of a building’s water efficiency. Building performance simulation tools can help project teams develop a predicted water use intensity (pWUI), while metered data will reveal a building’s actual WUI.
Due to the highly variable relative magnitude of outdoor water use compared to indoor water use, scoping should be clarified when assessing WUI figures. General WUI indicates a consideration of all water sources by a building and site excluding parking and irrigated area. Indoor WUI considers only indoor water usage (i.e., showers, toilets, lavatories, kitchen equipment, and other indoor equipment), which is also divided by the building’s area. If unspecified, assume WUI figures are general.
Figure: Water Use Intensity (WUI) by Property Type via the EPA's ENERGY STAR Portfolio Manager. Figure by author. |
