Energy use intensity myopia
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 (lighting, HVAC etc.), because this is typically the least expensive compliance route.
Firstly, energy simulations used to calculate EUI are not accurate, do not account for all energy losses (e.g. thermal bridging) nor installation quality, and are not calibrated against real outcomes. In addition, the performance of façades, especially in spandrels and opaque panel assemblies, already are overestimated. The lack of correlation between as-designed and as-built energy performance has been documented globally for many years and 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. These two buildings also will not have the same thermal comfort, condensation resistance and passive survivability 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 experienced during acute weather events, such as the brownouts experienced in California this year. The building with the better envelope will maintain a human survivable indoor environment for much longer than one with a poor envelope.
While some jurisdictions, such as Massachusetts, New York City and Washington state, are implementing envelope backstops as a way of limiting the trade-off with internal systems, they risk an unintended consequence of constraining window area. This is because the least expensive compliance solution tends to use more opaque area, especially since simulations still ignore the degradation due to thermal bridging.
California is considering a fenestration backstop for Title 24 and CalGreen. This backstop does not 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. It promises to be a better way of driving higher performance fenestration within the constraints of an EUI focus and warrants industry support.
Other countries have recognized the limitations of a sole EUI focus and the importance of driving 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.
Use of these metrics provide excellent strategies for reducing the unintended consequences of EUI myopia, driving high-performance fenestration, and getting to net zero. They also provide proof points which our industry could use to advocate for adoption of similar envelope metrics in U.S. energy regulations.
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