Envelope backstops in action: what's the verdict?
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...to continue reading click here.
(the full blog, as well as previous posts, are hosted on usglassmag.com)