IECC Archives - Hunter Panels

Complying with the International Energy Conservation Code (IECC)

PRESCRIPTIVE R-VALUE VS. PRESCRIPTIVE U-VALUE FOR ABOVE-GRADE EXTERIOR WALLS

To comply with the International Energy Conservation Code (IECC) requirements for insulating above-grade exterior walls, building professionals can follow one of two commonly used prescriptive methods: the R-value path or the U-value path, as set forth in the opaque thermal envelope requirements of the IECC.

R-VALUE PRESCRIPTIVE PATH

Complying with the R-value method is straightforward – simply use products with R-values that meet or exceed the values shown in the IECC for the appropriate climate zone and wall type. For example, the prescriptive R-value for metal framed walls in most climate zones (Zone 3 and above) is R-13 + 7.5 ci. This means the wall must incorporate insulation of R-13 or greater within the stud cavity, and insulation of R-7.5 or greater as continuous insulation, as shown in the following illustration:

U-VALUE PRESCRIPTIVE PATH

The U-value method is different. It takes the thermal resistance of all the components of the wall assembly into consideration, not just the insulation. The R-values of the wall assembly components are added together. Since U-value is the reciprocal of R-value, the U-value of the assembly is determined by dividing 1 by the total R-value. There are tables within the code documents that assign R-values to certain components of wall assemblies such as cavity air spaces and interior air film.

The U-value calculation will be slightly different for framed walls depending on whether the framing members are steel or wood. Since steel conducts heat, there is a reduction in effectiveness of the stud cavity insulation within a steel framed wall assembly — in other words, the effective R-value of the wall assembly is less than the insulation’s stated R-value. This is not any fault of the insulation, rather it is because the steel framing is a thermal short circuit in the wall. Therefore, the energy efficiency of the stud cavity insulation is reduced by a “framing factor” percentage found in the IECC. In the below example, the reduction is 54% for steel framed walls 16″ oc, meaning the R-value of the insulation is multiplied by .46 (1 – 0.54).

Wood framing provides some measure of insulating value, so it is treated differently than steel framing when calculating U-value for the wall assembly. The most common way to calculate wood framing U-value is the “parallel path” method, whereby the U-values for the framing path and the cavity path are calculated and then added. So, if the wood studs are 16″ oc, the code calls for using a framing factor of 0.25 (meaning the studs take up 25% of the wall area). The U-value of the framing path is then multiplied by 0.25 and the U-value of the cavity path is multiplied by 0.75 to account for the remainder of the wall assembly. The U-value of the total wall is the sum of the framing path U-value and the cavity path U-value.

How Much R-Value Do You Need?

It can be confusing to keep track of your climate zone code requirements due to the fact that the minimum insulation requirements have increased 66 – 100% over the last 9 years. It is important to remember that a building owner only gets one chance to increase the insulation in the roof system approximately every 20 years. When determining the desired R-Value, you should consider future energy requirements and associated energy costs over the entire lifespan of the installed roof system. The current codes reflect the MINIMUM requirements, we recommend when planning for the future to exceed the code by an additional R-10. (Paybacks typically can be seen in 6 – 10 years*)

This guide provides assistance when designing the minimum R-Value in commercial buildings for energy efficiency for thermal insulation above the deck. The respective R-Values are based on the climate for each stated region and only a suggestion. State codes may differ and should be considered. Current code information for each state is available on the websites for the Building Codes Assistance Project and the US Department of Energy Building Energy Codes Program.

Building Codes Assistance Project: http://bcapcodes.org/code-status/state/
US DOE: https://www.energycodes.gov/status-state-energy-code-adoption

The chart below shows the progression of the R-Value changes by climate zones since ASHRAE 90.1 2004.

Determining Minimum R-Values on Tapered Insulation Systems

According to the 2021 International Energy Conservation Code (IECC), under Insulation Requirements, the building thermal envelope shall meet the requirements of Tables C 402.2 based on the climate zones shown below:

Question:

How does this affect R-Values on a Tapered Insulation System?

Answer:

The R-Values shown above are the minimum R-Values required to meet Code requirements. Since a Tapered Insulation system will increase thickness away from the low points, the overall system R-Value will exceed the Prescriptive R-Value requirements.

Question:

Does an Average R-Value that meets the Prescriptive requirement satisfy Code standards?

Answer:

No. The terminology “Average R-Value” for tapered systems is an outdated term and not recognized by the IECC.

Question:

Does building code allow for a change in R-value when using sumps?

Answer:

There is an exception provided that allows the minimum thickness to be up to 1″ less than the prescriptive R-Value (at ¼” per foot slope) as long as the system R-Value meets or exceeds that Prescriptive R-Value.

Example:

Zone 6 requires an R=30 minimum, which is 5.2″ thick. This exception will allow the minimum thickness to be 4.2″ at the low point, or 5.2″, 4′ away from the drain on a ¼” per foot tapered system.

IECC 2021, SEC. C402.2 ROOF ASSEMBLY
Exception 2 Allows A 1-Inch Insulation Thickness Variation