Backstops seem to be all the rage these days, what with the “Brexit backstop” in European politics, financial backstops and the many backstops in the sporting world. The term has even crept into building energy codes through the creation of the “envelope backstop” the intent of which is to prevent poor performing building envelopes. While some oppose this mechanism because of the potential to reduce glazing area, others see significant opportunity for more sustainable and comfortable buildings, and for business growth through accelerating the adoption of high value-added fenestration.
During a recent visit to Washington, D.C., and discussion with my friend Vicente Montes of Curtainwall Design and Consulting, I learned that many buildings are routinely being built across the country with fenestration of lower thermal (U-factor) performance than even the prescriptive code compliance path requires. For example, where the prescriptive code may require fenestration with a U-factor of 0.38 btu/of.hr.ft2, the actual thermal performance of the fenestration delivered is, say, 0.43 btu/of.hr.ft2 or higher. This is because when following the performance code compliance path, the performance of the envelope can be “traded off” with improvements in the performance of internal systems such as HVAC and lighting to deliver the same overall energy performance as the base prescriptive building.
Many people argue that it doesn’t matter how building energy target is achieved – that’s what the performance path is all about and it gives the architect design flexibility. If a strategy using poorer performance windows with higher performance HVAC and lighting delivers the required energy performance, this is considered acceptable.
I would argue that it is does matter how you achieve the target. In the absence of complementary mandatory targets for occupant comfort or other measures of building sustainability, a sole focus on achieving energy performance can lead to unintended consequences of poorer occupant experience and lack of resilience to severe climate events and longer-term climate change.
Not all designs that meet the same energy target result in the same indoor environmental quality. Those with a lower performing envelope will probably have a more uncomfortable perimeter zone. And in colder climates, condensation issues and subsequent indoor air quality issues are more likely.
Unfortunately, from an upfront cost perspective, it is probably less expensive to invest in improved internal systems than to invest in a higher performance fenestration.
Enter the “envelope backstop.” To prevent these unintended consequences, some states have enacted requirements that restrict the trade-offs allowed. Discussions are also ongoing within the ASHRAE Standard 90.1 development process.
The way backstops work is that when design teams follow the performance path the energy target must be met and, additionally, they must ensure that the envelope thermal performance is no more than a defined percentage worse than that of the prescriptive envelope.
Often the performance path is used to allow the window area to be increased over the prescriptive limit (40% in ASHRAE 90.1), so backstops will mean that opaque envelope and fenestration products with performance higher than the prescriptive values will be required for designs with a higher window to wall ratio. According to Tom Culp of Birch Point Consulting, New York City (NYC) is implementing a backstop that will allow window areas up to around 70% using fenestration meeting the prescriptive requirements and up to approximately 90% glazed area if better performing windows, optimum orientation and shading are used as part of NYC’s new energy code. The same backstop is being considered for future revisions of ASHRAE Standard 90.1.
Washington State has a more stringent backstop. According to Culp, its code will allow approximately 50% window to wall ratio using fenestration matching the prescriptive requirements, and up to 70% window area with better performing windows, but with no allowance for orientation, shading or daylighting.
Massachusetts is going a step further, enacting a backstop that provides no ability to reduce overall envelope area-based performance compared with the prescriptive path. Any window area above 40% will require higher fenestration thermal performance and/or higher wall performance.
Envelope backstops won’t necessarily make highly glazed buildings harder to achieve because of lack of product options. There are many high-performance curtainwall and window systems readily available that meet these needs. However, backstops will likely increase upfront costs.
While we should be aware of the potential risk of constraining window area in our buildings, highly glazed buildings are still highly desired by owners and developers – not just for leasable value, but also for the occupant wellness and productivity benefits that comfortable daylighting brings.
These risks should be balanced with the opportunity to deliver higher value-added fenestration products and to create buildings with a higher level of comfort, sustainability and resilience. And if envelope backstops can increase the adoption rates of high-performance fenestration, economies of scale can be achieved which typically result in product cost reductions. That would be a win for building performance – for people and the planet – and a win for the building owner’s budget.
Agreed. How are we going to achieve Net Zero Carbon Emissions buildings if we do not move the u-value lower and lower? In Mexico and vast majority of Latin-America we have the issue of most commercial buildings not using thermally broken aluminum. We are working to educate developers and architects of the u-value and SHGC benefits of thermally-broken aluminum. To achieve net zero emissions buildings it is cumulative. Every product performance matters. No trade offs. Only the best. Greta is coming to hold a Friday School Strike at a building near you!
Great read, Helen! And required reading by all of in the industry. After all ASHRAE has done to force us (in some degree) down this path, one would think they’re giving in to let the wall go larger in WWR or poorer in performance. What a world we live in!
As Helen suggests, ASHRAE 90.1 is not near enough to assure a truly high performance facade system. It seems to me that one thing missing in all this is convergence on a clear set of comprehensive performance metrics for the building skin (that includes embodied carbon and other impacts in addition to operational carbon considerations). These could then be the basis for the development of some form of rating system for facades and guidelines for achieving those ratings, which could ultimately become a standard that could be used by States and other entities struggling with how to improve the performance of their building stock and the role the facade system plays in this. Thinking about this compels me to wonder what we have all been doing that we don’t already have this!