Envelope Backstop: what is it? How is it implemented? What's the impact?

For the first time ever, ASHRAE 90.1 2022 introduced a concept called "envelope backstop", which is a common concept we see in WSEC, CA Title 24 Part 6, NY Stretch, and MA Stretch.

ASHRAE 90.1 2022 ECB Backstop

In the past, if a project picks the performance path aka energy modeling, there is almost no limitation how trade offs are made between MEP and envelope.

Now with this extra layer, project teams need to make sure they meet the backstop requirements AND pass the energy model.

ASHRAE 90.1 backstop uses its AppC - Methodology for Building Envelope Trade-Off Option in Section 5.6. It's basically a whole building energy model looking at predicted "energy cost" as its proposed and baseline building envelope performance factors. The modeling is extremely simplified and automated in COMcheck.

So project teams following 90.1 2022 performance path now will have to perform COMcheck as well. This process is quite confusing and not efficient because if a detailed code compliance model is being built, why should the modeler use the simplified AppC again?

What's the impact? The backstop is very weak, it's a starting point so it's not stringent at all. AppC is whole building annual simulation that looks at a period of time energy cost, not a point of time. With that being said, it's drastically different from the total UA approach used in IECC.

What kind of project will be affected? Probably only warehouses, very high WWR offices, and projects in cooling dominant zones where laminated glass is being used with a high U value.

It's not in IECC C407 Simulated Building Performance, but its PAEC calculation made the performance path unusable anyways.

Let us know your thoughts!