IECC Compliance: Prescriptive vs. Performance

When it comes to the International Energy Conservation Code (IECC), there are two distinct approaches to making your project compliant: the prescriptive or the performance methods. Both are radically different in how they handle IECC compliance in general. 

“Prescriptive” method as I use it here refers to treating IECC compliance as following a set of rigid rules. Should you know what you want, (which is rare indeed), then this method often ends up being quicker, but less flexible for design.

If you have general time to kill and a passion for insulation, then pick the tougher, but more design-friendly performance method. Should you go through this route, you will have to prove that your assembly truly meets IECC compliance, by doing something called REScheck or COMcheck.

I’ve outlined a general procedure for both options for the budding architectural designer!

Prescriptive Method

If you’re starting out with the prescriptive method, you’ve come to the right place for quick answers. When cash is time and you don’t have time, or you’ve made a mistake and forgot to check for energy compliance this has you covered. Start by checking which climate zone you are designing in. The map in section C301.1 of the IECC (pictured below) will tell you the climate zone. Take that information and look at Table C402.1.3 in the IECC for the general requirements.

To make sense of this chart (which can seem like gibberish) you will most likely need to understand R-Values and U-Values (definitions in the glossary at the end!). If you’re interested, look deeper into these factors, as they will be invaluable if you try to brave the com check. Look at each part on the left side and make sure that you note on your drawings that you will comply with the prescriptive outline and then make sure to state the minimums and items within your assemblies.

It’s that easy! Let the builder do the rest, and let the owner pay for it.

(As a side note, these sections are for commercial purposes. If you’re looking for residential, then it’s the same section numbers, but in the R sections in the same IECC book.)

A black and white map of the United States. County lines are shown in each state, with each county given a different shade of greyscale to indicate the climate zone.
Figure C301.1 Climate Zones provided by IECC 2021

Performance Method

If you’re here for the performance method, and you’ve properly notified your loved ones that they won’t see you for a couple of days, then you are ready to begin. The basic principle is that you will have to provide thorough analysis of your assemblies and prove that what you have selected complies with the minimum as outlined in the prescriptive method.

The basic requirements are:

  1. Energy code.
  2. Builder and project locations.
  3. Area take-offs for envelope assemblies, which is the square footage of the facades divided between fenestration and solid walls.
  4. Insulation R-Values, and fenestration performance data (For windows, double check the frame as it’s always a lower value).
  5. Lighting fixtures and details (if you have them selected. Otherwise try to generalize or find an equal alternate).
  6. Heating and cooling systems and details.
  7. Service water heating details.
  8. Be attentive to thermal breaks.

Once you have those items, you’ll need to use an approved COMcheck method. (Generally, architects supply information for items one through four, and engineers provide items six and seven. Item five can come from either.) Once you’ve followed all instructions and input the needed info, the program determines if you have passed or failed the check. If your design passes, you get a neat little certificate to put into your drawings. Make sure to note the assemblies you used in your calculations!

If you failed, then your first step is to contemplate life, as step two is re-evaluating your assemblies and redoing the calculations from the start. As a note, if during design the building form or fenestration sizes change, you will need to recalculate. This has to be accurate and up to date for true IECC compliance.

There you have it! The two, and only, “easy” methods to show energy conservation compliance. I would suggest perusing the most up-to-date IECC sections for riveting tales of compliance checks, and to get accustomed to the information required. Happy designing!


U-Value is a measure of the flow of heat through an insulating or building material: the lower the U-value, the better the insulating ability.

R-Value is a measure of the resistance of an insulating or building material to heat flow, expressed as R-11, R-20, and so on; the higher the number, the greater the resistance to heat flow.

Typical R-Values:

  •       Batt Insulation: ~R-2.9 to ~R-3.8 per inch. Refer to spec sheets to make sure that what you have specified meets the minimum requirements.
  •       Rigid Insulation: ~R-6.5 to ~R-6.8 per inch. Refer to spec sheets to make sure that what you have specified meets the minimum requirements.
  •       Spray Foam Insulation: ~R-3.6 to ~R-3.9 per inch. Refer to spec sheets to make sure that what you have specified meets the minimum requirements.


2021 IECC
COMcheck for commercial projects
REScheck for residential projects


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