Are Windows and Walls Fungible?

Last month, I was part of a panel for a glass industry conference summer meeting reviewing the potential benefits and downsides of minimum thermal performance levels for building envelopes or their components in the performance compliance path of energy codes. There was not enough time to dig into the nuances of some of the points made against these so-called “backstops.” Because of the feedback I received afterward, I was persuaded that it is worth taking a deeper dive into one of the arguments against backstops: The fungibility of windows and walls.

Background

By way of background, there are two general kinds of backstop (see this link for more information):

• a maximum value for the area-weighted U-factor of the building envelope, which often is called a “UA” or envelope backstop, and
• a maximum area-weighted U-factor for a specific element, such as fenestration, which is called a “fenestration backstop.”

These maximum values only relate to the performance compliance path and are used to constrain the extent to which building envelope or fenestration performance, respectively can be degraded – compared to the prescriptive performance requirements – when trading off with better efficiency internal systems such as HVAC and lighting. The backstop performance levels are typically set at a level worse than the prescriptive performance to limit the very worst performing systems being used.

Please note: The prescriptive performance has already been judged to be cost-effective by virtue of it being in the code, and backstop performance levels are typically worse performance.

Envelope (UA) backstops have been driven by U.S. states and cities to ensure higher building performance and sustainability. Several jurisdictions – Washington state, Seattle, Massachusetts and New York City – have implemented an envelope backstop. British Columbia’s recently implemented step code has a minimum performance for the building envelope in the form of a maximum value of Thermal Energy Demand Intensity (TEDI). There is also a new addendum to ASHRAE 90.1 that includes another variant of envelope backstop.

The Lowest Cost Designs

As City of Seattle code official Duane Jonlin once said to me, “No one ever chooses the performance path to make the envelope better.”

Often, the cheapest design approach is to reduce the envelope performance and increase the HVAC system performance relative to their prescriptive baselines – despite the prescriptive envelope values being cost effective. By preventing the extremes of this trade-off, backstops are expected to increase the cost of construction on the cheapest buildings.

The Fungibility Argument: Walls are Equivalent to Windows

The main concern from the glass industry about backstops is rooted in the fear that the glass area will be replaced by an opaque wall if the cost of the minimum allowable performance fenestration is increased.

A panelist at the conference that I just attended introduced the concept of fungibility from economics. “Fungibility is the ability of a good or asset to be readily interchanged for another of like kind.” In an analogy to explain this concept, they likened windows to hamburgers and opaque walls to hotdogs and considered a situation in which hamburgers (windows) were forced to be made with wagyu beef (better windows) instead of regular ground beef and became more expensive as a result. They then predicted that the consumer would swap to buying more hotdogs (walls) than wagyu hamburgers (better windows). Therefore, they concluded that increasing the minimum performance of fenestration would result in less glass being used, which would thus be bad for the glass industry.

But let’s dig into analogy some more because there are some potentially flawed assumptions.

Arguable Assumptions?

Inherent in the fungibility argument is the assumption that windows are equivalent and indistinguishable from walls and are readily interchangeable. But we know that they are not. Walls cannot deliver daylight, views, natural ventilation and all the health benefits that result from windows.

Another inherent assumption is that the market wouldn’t be willing to pay more for a better-quality hamburger (windows). But we have evidence that they do, as illustrated by several examples below. And even if somewhat fewer hamburgers (windows) are purchased, but they deliver higher revenue and margins, that could be better business than selling more at lower prices and margins.

Also, Wagyu beef costs two to three times as much as regular ground beef and is considered a premium product. Backstops are not requiring premium windows to be used, just preventing the very worst ones.

Fenestration Backstops in Germany

Neither of the above assumptions appears to have been true in Germany, for example, where there has been a fenestration minimum performance requirement in their performance-based energy code for decades. German windows are recognized as very high quality and performance – typically commanding a higher price than in the U.S. Plus, the resulting buildings do not appear to be lacking in glass area judging by the skyline of Frankfurt and the plentiful daylight typical of German office buildings.

Hurricane Glazing

What’s hurricane glazing got to do with backstops, you might ask? Valerie Block, executive director of the Façade Tectonics Institute, reminded me of the glass industry’s concerns following Hurricane Andrew’s devasting results in 1992. The Florida government, code officials and other interested parties initiated work to change the building code to require impact-resistant glazing to protect life and property during future hurricanes and tropical storms. At the time, the Primary Glass Manufacturers Council vehemently opposed these improvements to fenestration performance. Their belief was that any new requirement for higher-cost fenestration would result in less glass being sold, lowering their sales and profits.

Did their fears come to pass? No. In fact, a very large and profitable laminated glass industry has grown around impact glazing requirements. In fact, glass sizes are getting bigger than ever in hurricane zones, with more glass sold because of the two lites in the laminated glass needed in the construction. There is certainly a higher value-add, increasing revenue and profits for the industry. According to the Fenestration and Glazing Industry Alliance’s FGIA 2021/2022 Study of the U.S. Market for Windows, Doors and Skylights, the share of hurricane glazing continues to grow in Florida, as does the total volume of fenestration sold there.

Arguments for Backstops

There are several reasons why jurisdictions are implementing minimum performance levels:

• Long service life: The replacement cycle on the building envelope (30+ years) is much longer than mechanical (~15 years) and electrical systems. It is important to ensure initial performance is as good as possible. The consequences will be with the building for its life.
• Load reduction: For success in electrification strategies (e.g. transition to heat pumps) and renewable energy transitions, the loads on HVAC systems must be reduced. This means a better building envelope is needed.
• Resilience and passive survivability: Better envelopes improve building resilience and passive survivability in severe weather, especially during power outages. As recent examples, the excessive heat in the Pacific Northwest and the excessive cold in Texas caused significant excess deaths.
• Energy use (carbon emissions) reduction: There is an argument that occupant behavior causes higher energy use than expected in buildings with poorer envelopes because they will be uncomfortable next to the façade. They will then adjust the thermostat to achieve comfort, which will drive up energy use.

Final Thoughts

The potential impacts of backstops on the glass industry are nuanced and depend on the specific criteria used. We have examples of how fenestration backstops have already worked effectively outside our borders without negatively impacting the local glass industry and resulted in higher performance business-as-usual fenestration.

While there are conflicting opinions on whether backstops will either positively or negatively impact the glass industry, we also need to look to the already-existing North American ecosystems using envelope backstops to really understand how the market will respond and adapt.

These new requirements are already driving innovation in spandrel system performance to support the continued use of curtainwall and unitized wall systems under tight backstops. Flynn’s Speedwall and MiTek’s ONEWALL are two examples. More triple-pane glazing (50% more glass per unit) is reportedly being specified in British Columbia, too. Additional data is certainly needed to understand if, on average, less glass is being used on buildings, but more glass through triple panes and higher value-added products seem like wins.

Bertrand Cazes, Secretary General for Glass for Europe (GFE), the trade association for Europe’s flat glass manufacturers, spoke at a 2019 conference in Argentina about their approach to building sustainability. His statements have stayed with me because of their forward-thinking view and understanding of the critical role and responsibility that the glass industry has in driving sustainability in the built environment. He conveyed that GFE recognized the need for increasingly higher performing, sustainable building envelopes. He acknowledged that they may, as a result, have to cede some glass area, but they believed that they would make up for this in increased revenues and profits from selling higher performing, higher value-added products. A win for building occupants, our environment and the economics of the glass industry.

As we saw with hurricane-impact glazing, history has shown that even in the face of higher cost fenestration, the U.S. market is willing to invest for the quality of the environment and the aesthetic that glass provides. I encourage our industry to seek out the triple bottom line by working with jurisdictions to create codes that simultaneously improve buildings’ performance for occupants, put us on a path to net zero carbon buildings by 2030, and support the economic health of the building industry. People, planet, profit.