IECC prescriptive-based vs performance-based compliance methods

[Tim Mailloux]

I am looking for a clarification on the IECC prescriptive-based compliance method vs performance-based compliance method. When using the prescriptive method you simply make sure your buildings kit of parts (walls, roofs, glazing, etc) all meet the minimum R or U values listed in the IECC. But the performance-based compliance method uses a whole building energy model to determine overall energy usage and as a designer I am not locked into the rigid prescriptive requirements for any given building component when using the performance-based compliance method. As an example, I have a very energy efficient building in the late stages of design for which we have performed a whole building energy model. I would like to use some very large skylights in one space, but any skylights of this size does not meet the minimum U value and SHGC requirements of the IECC. But since my overall building is so energy efficient (NetZero ready) its my understanding that the skylights don’t have to meet any prescriptive IECC performance requirements provided my energy model passes.



Is this accurate?

 

[adekom]

Even when using the total building performance compliance path, 2018 IECC C401.2.3 still requires that a number of other sections within the energy code be met in addition to section C407. These mandatory sections do not include section C402.4 which contains the thermal requirements for glazing, so when doing total building performance, you do not need to meet the prescriptive SHGC and U-value requirements of C402.4. That said, the actual U-values and SHGCs of the underperforming skylight(s) will have to be included in the energy model and be offset by other higher performing elements.