Energy Use Intensity Myopia: Moving Forward to Build Better Buildings

By Helen Sanders

People often assume that it does not matter how a building achieves its energy use intensity (EUI) target to achieve energy code compliance through the performance path approach. This flawed assumption has led to the widespread trade-off of lower performing building envelopes with higher efficiency internal systems such as lighting, HVAC, etc., typically the least expensive compliance route.

Matters of Concern

First, energy simulations used to calculate EUI are not accurate, do not account for all energy losses nor installation quality, and are not calibrated against real outcomes. In addition, the performance of façades, especially in spandrels and opaque panel assemblies, is already overestimated. The lack of correlation between as-designed and as-built energy performance is called the “energy gap.” As a result, two buildings with the same simulated EUI, one with a better envelope and one with a poorer envelope, will likely not have the same actual energy performance.

A single focus on EUI can lead to sub-optimization of these important parameters. An efficient HVAC system is of no use during power interruptions, such as the brownouts experienced in California. The building with the better envelope will maintain a human survivable indoor environment much longer than one with a poor envelope.

What About Backstops?

While some jurisdictions are implementing envelope backstops to limit the trade-off with internal systems, they risk constraining  the window area. This is because the least expensive compliance solution tends to use more opaque area.

California is considering a fenestration backstop for Title 24 and CalGreen, which doesn’t have the same unintended impact on window area. It just limits the ability to degrade fenestration U-factor by more than a defined percentage above the prescriptive level when using the performance path.

Learn from Others

Other countries have recognized the limitations of a sole EUI focus and the importance of envelope performance in achieving their net zero carbon goals. Canada’s British Columbia has implemented requirements for “Thermal Energy Demand Intensity,” which is primarily influenced by the building envelope and ventilation system. Singapore has introduced requirements for “Envelope Thermal Transfer Value,” which measures heat conduction through opaque and transparent areas, and solar heat gain through windows.

These metrics provide strategies for reducing the unintended consequences of EUI myopia, driving high-performance fenestration, and getting to net zero. They also provide proof points to advocate for adoption of similar envelope metrics in the U.S.

Helen Sanders is in strategic business development for Technoform North America Inc. based in Twinsburg, Ohio. Read her blog each month at usglassmag.com/insights.

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