Cutting through I-Code Confusion

A new R-value guide for roofs and walls
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Sponsored by Duro-Last®, Inc., SIKA, and CEIR
Dr. James Hoff

Learning Objectives:

  1. Understand the features and benefits behind the new prescriptive thermal value requirements in the 2012 I-Codes and how these requirements support increased building sustainability.
  2. Identify the basic types of roof and wall assemblies covered by the I-Codes and how prescriptive thermal requirements are calculated for each assembly.
  3. Understand how the CEIR / PIMA Roof and Wall Thermal Design Guide is organized and how building professionals can use the guide to better understand the thermal design standards of the 2012 I-Codes.
  4. Gain proficiency in using the guide to determine prescriptive roof and wall thermal values for typical roof and wall constructions.


1 AIA LU/Elective
1 GBCI CE Hour
This test is no longer available for credit

I-Codes and Thermal Values

Since 2000, the I-Codes have served as models for almost all state and local building codes in the United States. The 2012 edition of the I-Codes includes several landmark advances in building energy efficiency and sustainability. Not only does the 2012 International Energy Conservation Code (IECC) include new and higher thermal standards for almost every roof and wall assembly, but these standards are further increased in the new International Green Construction Code (IgCC), which is intended to serve as an overlay code, or “above the code” standard for sustainable buildings. (For an overview of the I-Codes, see “Model Codes 101”.)

Why It Matters

The advancement of the levels of insulation matches the awareness and activities taking place, not just in the US, but across North America and the world. Energy efficiency and the subsequent reduction of energy use is paramount in the building industry.

According to the US Department of Energy’s Energy Efficiency and Renewable Energy website, “We spend more than $400 billion each year to power our homes and commercial buildings, consuming more than 70 percent of all electricity used in the United States, about 40 percent of our nation's total energy bill, and contributing to almost 40 percent of the nation's carbon dioxide emissions. And much of this energy and money is wasted - 20 percent or more on average. If we cut the energy use of U.S. buildings by 20 percent, we could save approximately $80 billion annually on energy bills, reduce greenhouse gas emissions, and create jobs.” (

Additionally, reduction of energy use by upgrading insulation amounts in our commercial and residential buildings also has significant co-benefits.

  • Comfort and saving money are important to building occupants and owners. Added comfort in a commercial building means increased productivity and happier occupants, and that can mean higher occupancy rates.
  • Upgrading our existing building stock (e.g., improving the energy efficiency of roofs and walls) means numerous jobs are produced, and many of those jobs are local. Improving your building improves the local economy.
  • There is less stress and more security of our energy grid. Using less energy to do the same amount of work (e.g., heating and cooling our buildings) means less need for additional utility infrastructure, reduced peak loads, and extends our use of precious natural resources.
  • Using less energy reduces our pollutant output which is better for humans and the environment. Buildings account for 40 percent of the US’s carbon dioxide emissions, 18 percent of nitrogen oxide emissions, and 55 percent of sulfur dioxide emissions. Human health is affected by pollution; energy efficient buildings help reduce pollution and therefore improve the health of the human population.
  • It’s important to understand why it is so important to build better building envelopes - whether for new construction or renovations - so our focus is not lost. And, it’s also important to understand that improvements in our building envelopes can provide very significant secondary benefits for the occupants and owners of buildings.

Although these advancements in energy efficiency are critical to the design of the next generation of sustainable buildings and renovation of our current stock, the number of new options may be confusing to the non-expert. For starters, the I-Codes now embrace two separate levels of thermal performance: a minimum code level in the IECC and an above-the-code level in the IgCC. In addition, the IECC and the IgCC offer two distinct paths to determine roof and wall R-value, one based on International Code Council (ICC) standards and one based on American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) standards. In past code editions, the R-values resulting from the ICC and ASHRAE paths frequently were identical, but recent editions of I-Codes and ASHRAE standards contain a number of significant differences in R-value requirements. An important change to be aware of is that the 2012 IgCC no longer contains the traditional roof and wall R-value tables used in previous codes. As a result, it may be difficult to determine exactly what is the correct roof and wall R-value for a particular new building or renovation project.

Thermal Values in ICC and ASHRAE Standards

Although differences in prescriptive thermal values can be found between ICC and ASHRAE standards, it is important to recognize that ICC and ASHRAE are working together closely to make energy codes as consistent as possible. In fact, ICC and ASHRAE jointly signed a Memorandum of Understanding in 2006 formally recognizing their mutual contribution to advancing building safety and energy efficiency and committing to explore ways to optimize codes and standards development. As a result of this mutual agreement, recent energy-related I-Codes formally reference their corresponding ASHRAE standards as equivalent paths to code compliance. In the case of the 2012 IECC, ASHRAE 2010 Energy Standard for Buildings Except Low-rise Residential Buildings (ASHRAE 90.1-2010) is identified as an equivalent code and design approach, while the 2012 IgCC identifies ASHRAE 2011 Standard for the Design of High-Performance Green Buildings (ASHRAE 189.1-2011) as an equivalent approach. A recap of the I-Code and ASHRAE energy standards, their intended function and their relationship to each other is provided in Table A.

Table A: I-Code and ASHRAE Energy-Related Standards

I-Code Intended Function* Referenced ASHRAE Standard
2012 International Energy Conservation Code (2012 IECC) Establish minimum building energy requirements 2010 Energy Standard for Buildings Except Low-rise Residential Buildings (ASHRAE 90.1-2010)
2012 International Green Construction Code (2012 IgCC) Establish above-the-code building energy requirements 2011 Standard for the Design of High-Performance Green Buildings (ASHRAE 189.1-2011)

Although ICC and ASHRAE are working closely to harmonize and support building energy standards, minor differences may occur simply because the two organizations utilize separate development processes. Although ICC and ASHRAE incorporate many similar approaches to achieve consensus, the very fact that their development processes are convened at different times and locations virtually assures that some differences will occur. An example of how these differences may affect roof and wall prescriptive thermal values is illustrated in Table B. This table identifies the minimum R-value as identified by the four relevant ICC and ASHRAE approaches for one of the most common commercial roofing assemblies (roofs with insulation above deck) located in Climate Zone 6. (Milwaukee, Wis., would be a typical city in this climate zone.)

Table B: Minimum Prescriptive R-Values for Roofs with Insulation above Deck (Climate Zone 6)

Code / Standard R-Value
2012 International Energy Conservation Code (IECC) R-30
ASHRAE 90.1-2010 R-20
2012 International Green Construction Code (IgCC) Not Listed
ASHRAE 189.1-2011 R-35

As illustrated in Table B, minimum R-values for this climate zone vary considerably, even between codes intended to be functionally equivalent. Although it would be reasonable to assume that the R-35 required by an above-the-code standard such as ASHRAE 189.1 would be higher than the R-20 or R-30 required by minimum code standards such as the 2012 IECC (R-30) or ASHRAE 90.1 (R-20), it is difficult to understand why the R-values in the two minimum code standards differ so significantly. Unfortunately, the R-value differences between the 2012 IECC and ASHRAE 90.1-2010 appear to be related to procedural problems and timing differences occurring during the development of these standards. In the case of ASHRAE 90.1, a successful appeal by the glazing industry involving prescriptive thermal values for windows effectively delayed the inclusion of prescriptive thermal value tables in the 2010 edition. As a result, ASHRAE 90.1-2010 was published with a reference to the thermal value tables in the previous edition, so the R-values remained low (R-20). Although it appears likely that ASHRAE will publish revised thermal value tables this spring with roof and wall R-values much closer to the current 2012 IECC, the current discrepancy serves as a good example of the differences that may occur due to separate development processes.

The IgCC Approach to Thermal Values

Table B also illustrates how the development process for both ICC and ASHRAE may result in not only minor variations in table values but also significant differences in the basic approach to determining R-value. While the previously-mentioned glazing industry appeal was working its way through the ASHRAE development process, the ICC was conducting final hearings for the 2012 IgCC. Although it would be difficult to conclude that the IgCC deliberations were directly affected by the ongoing dispute at ASHRAE, it appears that the ICC decided to take a new path in regard to prescriptive thermal values (and possibly avoid conflicts among industry stakeholders). Instead of including tables of thermal values for roofs and walls, the 2012 IgCC incorporates specific instructions for calculating these above-the-code values. To accomplish this, Section 605.1.1 of the 2012 IgCC provides the following instructions:

“The building thermal envelope shall exceed the requirements of Tables C402.2 and C402.1.2 of the International Energy Conservation Code by not less than 10 percent. Specifically, for purposes of compliance with this code, each U-factor, C-factor, F-factor and SHGC in the specified tables shall be reduced by 10 percent to determine the prescriptive criteria for this code.” (2012 IGCC Section 605.1.1)"

Although this instruction is reasonably straightforward, it is important to note that R-value (or R-factor) is not mentioned. Instead, the mathematical reciprocal of R-value (U-value or U-factor) is referenced. This means that in order to identify the minimum R-value for a roof or wall assembly under the IgCC, the designer must first identify the minimum U-value in the 2012 IECC for the roof or wall assembly in question, reduce it by 10 percent, and then calculate the reciprocal R-value. As an example, in the case of the Climate Zone 6 example in Table 1, the minimum U-value for a roof with insulation above deck in Table 402.1.2 of the 2012 IECC is 0.0320. Next, applying the 10 percent prescribed reduction results in a new IgCC U-value of 0.0288. Finally, converting the IgCC U-value of 0.0288 into its reciprocal R-value results in an IgCC R-value of 35.7. A detail of the calculation is illustrated in Table C.

Table C: Calculating IgCC R-value for Roofs with Insulation above Deck (Climate Zone 6)

Step Source / Calculation Result
Determine U-value 2012 IECC Table 402.1.2 U = 0.0320
Reduce U-value by 10 percent (0.0320) – (0.0320 X 10 percent) U = 0.0288
Convert to R-value 1 ÷ (0.0288) R = 35.7

For a wall or roof assembly with insulation located only in one place within the assembly, calculating the appropriate IgCC R-values is relatively simple, even if a little confusing initially. However, for walls or roofs with insulation located in more than one place within the assembly, the calculation may become more difficult. As an example, wood and metal framed walls in almost all climate zones now require two separate locations for insulation: (1) insulation installed within the wall cavity (i.e., between the wood and metal studs), and (2) continuous insulation (referenced as “ci” within the 2012 I-Codes) installed on the exterior side of the wall cavity. Examples of cavity insulation and continuous insulation in wood and metal framed walls are illustrated in Figure 1.

A major problem in calculating the IgCC R-values for insulation in framed wall assemblies involves the physical limitation of the cavity portion of the wall. Because the depth of this cavity is determined by the size of the standard framing members (typically 3 ½-inch or 5 ½-inch wood or metal studs) and because current minimum IECC standards for cavity insulation effectively reach the maximum value possible for the established cavity depth, the required IgCC increase in R-value usually must be applied only to the continuous insulation portion of the wall. As an example, a wall framed with two-by-sixes incorporates a 5 ½-inch cavity typically insulated with R-20 glass fiber or similar material. Because the IECC minimum R-value for a wood-framed two-by-six wall in Climate Zones 1 through 5 is R-20, the increase in R-value (or decrease in U-value) required by the IgCC can only be applied as continuous insulation on the exterior side of the wall without increasing the width of the studs. In this case, the necessary additional R-value is approximately R-2, which can be achieved by installing a layer of continuous insulation board to the exterior side of the wall framing.

Finally, the calculation of IgCC R-values involves an additional confounding factor. The R-value tables in the IECC are not derived as exact mathematical reciprocals of the corresponding U-value tables. To illustrate using the previous example of a roof with insulation above deck in Climate Zone 6, the IECC R-value table lists a value of R-30, which compares to an exact mathematical U-value reciprocal of U-0.033. However, the corresponding U-value table in the IECC lists a value of U-0.032, which is slightly lower than the mathematical reciprocal. This difference can be attributed to assumptions regarding effective thermal values of other elements of the assembly such as air films and framing members which can increase or reduce overall U-value. Another way to explain this is that the IECC and ASHRAE U-value tables provide values for the complete assembly, while the IECC and ASHRAE R-value tables provide values for the insulation within the assembly. This means that although a simple mathematical conversion of reducing IECC U-value by 10 percent will yield the correct IgCC U-value, a similar mathematical conversion of the new U-value to the new R-value will not necessarily yield the correct result.

Fortunately, this difference between U-value and R-value is addressed in Normative Appendix A of ASHRAE 90.1, which provides adjusted equivalent insulation R-values for assembly U-values. As a result, the actual determination of insulation R-value for the IgCC requires an additional step of looking up the wall or roof assembly in ASHRAE Normative Appendix A and extrapolating the required insulation R-value from the listing of equivalent R and U values listed in the appendix. Again, drawing on our roof example in Climate Zone 6, the mathematically calculated R-value reciprocal of the IgCC U-value is R-37.7. However, an interpolation from ASHRAE 90.1 Normative Appendix A yields a slightly lower R-value of R-37.5. In this case the minor difference of R-0.2 can be attributed to air films above and below the roof assembly.


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Originally published in Architectural Roofing and Waterproofing