Compliance paths: Prescriptive
versus U-factor
Once the preferred code or standard is identified, the next challenge is selecting the correct compliance path through the applicable building energy code. This is not an inconsequential decision, as energy codes are truly flexible, offering multiple compliance paths to suit all types of designers. For roofing, selecting the appropriate path can lead to material and labor savings.
There are four basic paths to comply and they have the same process for either ASHRAE or IECC. Each has prescriptive, performance, component trade-off, or whole building analysis path options. A whole-building analysis typically utilizes a sophisticated software package including the building envelope and all other forms of energy consumption to determine compliance and often requires the assistance of a specialty consultant.
The prescriptive assembly path in IECC typically has just one assembly listed per envelope element type, per climate zone. The U-factor performance path allows builders to use any assembly with a U-factor at or below the stated U-factor for the particular climate zone or assembly, so long as they can provide a test or supportive data for the assembly. For the purpose of this article, the focus will be mainly on the prescriptive versus a tested U-factor assembly.
The prescriptive path in ASHRAE and IECC prescribes insulation requirements according to the building’s climate zone. While the prescriptive assemblies path was previously a common choice because of its ease and affordability, builders may find more cost savings today by opting for the associated U-factor performance path.
The prescriptive assemblies are often very generic in nature and may be conservative to the actual assembly. Over the years, the performance values assigned to these prescriptive assemblies have been revised and often reduced. Previous prescriptive assemblies that may have met a particular standard could fall short in meeting those of today. This exemplifies the importance of being aware of the U-factor path so builders can provide the most cost-effective solution for the building owner. Some manufacturers offer a variety of tested assemblies that will meet the performance U-factor at a more competitive cost than the prescriptive assembly.
