Stretch code or breaking point? Designing high-performance fenestration for Massachusetts
Massachusetts’ stretch code, adopted by more than 85% of its communities, is arguably the most stringent building energy code in the United States.
The stretch code responded to Massachusetts’ 2021 Next Generation Road Map Act. It is designed to “crush heating” loads to facilitate building decarbonization through heat-pump deployment.
Its main envelope-related features include a performance compliance path that sets quantitative targets for envelope performance (thermal energy demand intensity) and building energy use intensity. It also has a stringent minimum envelope thermal performance limit (backstop) to prevent performance degradation through tradeoffs with HVAC and other internal systems.
Furthermore, an additional envelope area-weighted thermal performance (UA) limit for buildings using glazed walls with spandrels (window wall or curtainwall):
- For buildings with glazed walls covering less than 50% of their wall area, the UA must not exceed 0.1285 BTU/oF.hr.ft2.
- For those with glazed walls covering more than 50% of the wall area, the UA must not exceed 0.16 BTU/oF.hr.ft2.
- A mandatory minimum area-weighted vision glazing U-factor in the 0.25 BTU/ oF.hr. ft2 glazed wall also applies.
Uncomfortable but not impossible
According to Paul Ormond, a senior clean energy engineer with the Massachusetts Department of Energy Resources, the intent was for the curtainwall to be considered “uncomfortable but not impossible.”
For instance, there are requirements to mitigate 11 types of thermal bridges, including spandrel, fenestration-wall interfaces and cladding attachments. There is also a prescriptive compliance path that can only be used for commercial buildings under 20,000 ft2. The prescriptive fenestration U-factors are 0.30 and 0.32 BTU/oF.hr.ft2 for fixed and operable fenestration, respectively.
Ideal U-factor of new systems
What should the target U-factor for new systems be to ensure they will be adequate for use in Massachusetts? For curtainwalls and window walls, 0.25 BTU/oF.hr.ft2 might seem to be the target. Based on the prescriptive values, the maximum limit for windows is 0.30-0.32 BTU/oF.hr.ft2. But these will likely not be sufficient.
In practice, the fenestration U-factor target must be significantly lower than 0.25 BTU/oF.hr.ft2. Enclosure consultants report they need to source fenestration with U-factors approaching 0.16 BTU/oF.hr.ft2 to comply with the stretch code.
This is because the derating from thermal bridging in spandrels and other envelope system transitions and connection points negatively impacts the area-weighted envelope U-factor. Even for modest window areas and sound thermal bridge mitigation, a transparent glazing 0.25 BTU/oF.hr.ft2 U-factor in glazed walls and windows elsewhere on the envelope is insufficient to meet the overall wall UA requirement.
Targeting fenestration U-factors of at least 0.16 BTU/oF.hr.ft2 is recommended.