As of January 1, 2020, Massachusetts has adopted one of the most stringent U.S. building energy codes. Not only is it using the very latest, 2018, version of the International Energy Conservation Code (IECC), it has gone one step further and implemented a strict “envelope backstop.” Many fear that this provision will result in less glass and, thus, poorer daylighting in buildings. So, on a recent trip to Boston, I decided to find out more.
An envelope backstop is a provision that limits the ability of a design team to trade better performing internal systems (HVAC, lighting, etc.) for envelope energy efficiency in the performance compliance paths. Depending on the local code, these trade-offs often allow larger window areas compared with the prescriptively allowed 30-40% window area without having to increase fenestration thermal performance. As I reported in November’s blog on this topic, these trade-offs have facilitated the use of fenestration U-factors lower than those allowed in the prescriptive compliance path, even with larger window to wall ratios (WWR). On paper, the energy budget of the building meets the target, but a poorer performing envelope results along with the unintended consequences of reducing occupant thermal comfort in the perimeter zone and increasing the risk of condensation related issues.
In Massachusetts, the envelope backstop enacted requires that the area (A) multiplied by the U-factor (U) of the envelope (often referred to as “UA”) equals the UA of the prescriptive building. If a design team wants to increase the WWR, it must improve the thermal performance of the fenestration it uses significantly and/or increase the insulation performance of the wall to achieve the same overall UA.
For example, in IECC 2018, the prescriptive WWR maximum is 30% (IECC 2018), the fenestration U-factor maximum is 0.38 btu/of.hr.ft2, and the wall R-value minimum is 13 (U-factor 0.08 btu/of.hr.ft2). To double the WWR to 60% without changing the wall insulation, the fenestration U-factor must be 0.23 btu/of.hr.ft2.
The challenge is mitigated somewhat if daylighting controls are used in more areas, in which case the prescriptive WWR is 40%. Achieving a 60% WWR then requires a window U-factor of 0.28 btu/of.hr.ft2. There are several fenestration systems available that meet these targets by using high-performance wide thermal breaks, triple-pane glazing and warm-edge spacers.
It will be difficult to achieve an overall window area above 50% without going to high-performance, thermally broken fenestration with triple-pane glazing. The challenge increases further with curtainwall designs because even the best spandrel areas do not achieve the same thermal performance of a traditional opaque wall (0.08 btu/of.hr.ft2).
Given this, what is the architectural community saying? Overwhelmingly, the feedback from Boston architects on the envelope backstop was positive. One architect stated that they thought it was a “leveler” for developers and would force the real estate market to move to higher performing buildings and avoid the need for envelope remediation after construction. Others acknowledged the design challenges that the backstop presented yet asserted that it was necessary for higher performing buildings and for their clients to invest appropriately in the envelope.
Will glazed areas be reduced? I heard one report of a curtainwall being replaced with punched openings because of the cost of achieving the needed performance. Another individual wondered if the new code would spell the end of the curtainwall as we currently know it. But others noted that because the real estate market loves all-glass buildings even though costs may increase, they didn’t believe that the envelope backstop would prevent this design style from continuing.
Will highly glazed buildings be a thing of the past in Massachusetts given the increased cost to achieve higher glazed areas, or will higher glazed areas along with high-performance fenestration prevail? Time will surely tell.
For now, it is clear that design teams are hungry for information on how to specify and source higher performing fenestration systems that meet the lower thermal performance requirements. While building teams may naturally focus on the center of glass, they also may overlook strategies to improve thermal performance of the frame and edge of glass which can provide more cost-effective and balanced performance. This provides an excellent opportunity for our industry to promote the use of high-performance fenestration.
Author: Helen Sanders