Summing it Up: R-Value Confusion

We now have identified several factors that may be very confusing to a building designer trying to determine the appropriate code-required insulation R-values. First, the thermal value requirements of the IECC or IgCC may differ from similar requirements in ASHRAE 90.1 or 189.1, which are intended to be functional equivalents. Next, determining the thermal value requirements of the IgCC is complicated and involves several steps to arrive at the appropriate insulation R-values. Finally, proper determination of roof and wall R-values requires additional understanding of the specific type of roof and wall assembly involved and as well as the correct climate zone for the project.

Cutting Through the Confusion: A New Design Guide

In an effort to improve understanding of these confusing and complicated prescriptive R-value requirements, two leading building organizations have teamed up to publish a simple and easy-to-use guide for building designers. This new guide, developed by the Center for Environmental Innovation in Roofing (CEIR) and the Polyisocyanurate Insulation Manufacturers Association (PIMA), provides a simple reference tool to help building designers make the best roof and wall insulation decisions for new and existing buildings. The CEIR / PIMA Roof and Wall Thermal Design Guide specifically addresses the complexity of different prescriptive roof and wall thermal values in the 2012 I-Codes by providing an organized step-by-step approach to determining the climate-appropriate R-value for common roof or wall assemblies.

In order to increase the guide’s usefulness for the code novice, color graphics are employed to illustrate key energy concepts embodied in the I-Codes. First, a map of North America with color-coded climate zones is provided to help the user locate the appropriate climate zone. Next, graphic illustrations of each major type of roof and wall assembly referenced in the code are used to guide the user to the appropriate R-value tables. These illustrations not only direct the user to the appropriate R-value table but they also identify the correct location of insulation within the assembly. Finally, the illustrations are placed directly above color-coded R-value tables showing a side-by-side comparison of the different ICC and ASHRAE R-values referenced within the code.

The guide also simplifies the new International Green Construction Code by providing pre-calculated 2012 IgCC R-values for all roof and wall assemblies by climate zone. As a supplement to the calculation, footnotes to the tables provide the specific reference in the IgCC for making the calculation as well as a description of the steps involved.

Using the Guide

Using the CEIR / PIMA design guide is fast and easy. All the designer needs to do is follow these simple steps:

1.)  Select the climate zone. In addition to an illustrated map of all North American climate zones, the guide provides a link to detailed county-by-county climate zone information maintained by Pacific Northwest National Laboratory of the U. S. Department of Energy.

2.)  Select the roof or wall assembly. The guide provides prescriptive R-value information for the following major roof and wall assembly types as referenced in the I-Codes:

• Roofs with insulation above deck
• Attic and other roofs
• Wood framed walls
• Metal framed walls
• Mass walls

3.)  Select the model code path. The guide provides separate R-value tables for the following I-Code and

• ASHRAE standards:
• 2012 IECC
•  2012 IgCC
• ASHRAE 90.1-2010
• ASHRAE 189.1-2011

4.)  Look up the R-value. After the appropriate R-value table is located, the correct R-value may be determined by cross-referencing the relevant I-Code /ASHRAE standard with the appropriate climate zone.

Limitations of the Guide

The CEIR / PIMA Roof and Wall Thermal Design Guide achieves its simplicity and ease-of-use by narrowly focusing on one aspect of the I-Codes while not addressing many other important requirements. The specific prescriptive thermal values for wall and roof assemblies provided by the guide are required only if the energy efficiency of the building is not determined using approved energy modeling software. Although many new buildings are designed using such software, the use of prescriptive standards is still used frequently for smaller buildings and for building retrofits, especially re-roofing projects. In addition to building thermal design requirements, building projects must comply with all other relevant code requirements, especially life safety requirements. As a result, many wall and roof assemblies must incorporated structural, fire, wind and seismic design requirements not shown in the simple illustrations of this guide. Finally, the guide does not cover other important thermal design requirements for roofs and walls, including the use of “cool” roof surfaces in the warmest climate zones and the use of roof and wall air barriers. However, many of these additional requirements are discussed in the introduction to the guide or footnoted below the R-value tables in the guide.

In addition to a narrow focus only on prescriptive R-values, the guide also focuses only on the most common types of roof and wall assemblies used in conventional commercial building design. As a result, roofs and walls for metal buildings as well as below-grade walls have not been included in order to maintain simplicity.

One final limitation of the guide may be the most important. Although the I-Codes and related ASHRAE standards discussed within the guide are recognized to be national models for building energy efficiency, these codes and standards must be adopted formally by state and local jurisdictions, typically by legislative statute or mandated code adoption process. As a consequence, some states may have adopted a version of the 2012 I-Codes, while other states may continue to recognize earlier versions. Although some jurisdictions may not have adopted the 2012 I-Codes, it is usually prudent and best to use the most up-to-date design information when designing new building or renovating existing buildings. In addition, each state and municipality adopting the I-Codes may include revisions or amendments to the code that may change the prescriptive R-value requirements as shown in this guide. Some states and municipalities also may adopt only the ASHRAE version of the code to be the prevailing standard. Finally, the new “green” above-the-code standards of the IgCC and ASHRAE 189.1 may be adopted by states and municipalities to apply only to certain building projects, such as public facilities rather than private buildings.

Drilling down to the specific requirements of any specific state or municipal code jurisdiction is obviously beyond the scope of such a basic guide. However, a very useful online reference for state and local code requirements is sponsored and maintained by the Building Codes Assistance Project, a Washington-based nonprofit. This reference is the Online Code Environment and Advocacy Network, which can be accessed at http://energycodesocean.org/code-status-commercial Here, the building designer can select any state from an interactive U. S. map and discover which model code has been adopted, what amendments (if any) have been enacted, and what procedures and timetable are followed to update the code.