Have you ever looked up the minimum R-value requirements in a referenced energy code or standard for high-performance building and thought the requirements seemed a bit low? Take ASHRAE Standard 90.1-2007 as an example. This is the referenced energy standard for LEED-NC v2009. For a project in Indianapolis, which falls in Climate Zone 5A, a nonresidential, low-sloped roof assembly exhibiting insulation entirely above deck is only required to have a continuous insulation value of R-20. If extruded polystyrene is specified, a four-inch minimum thickness would meet the standard’s prescriptive requirement. By comparison, the 2009 International Energy Conservation Code (IECC) calls for a similar minimum of R-20.8 (U-0.048).
In this era of heightened energy consciousness and high-profile projects with “superinsulation,” an R-20 roof in Indianapolis might seem a bit lax. However, there is actually a very good reason why ASHRAE, the International Code Council and others have set the mark at these specific levels. The reason has to do with the diminishing return on investment for increasing R-values.