Top 10 changes in the 2015 IL Energy Code (IECC)

Blower DoorRecently, the revised 2015 IECC (a.k.a. Illinois' newly adopted energy code) quietly took effect for any permits issued after Jan 1, 2016. Before our "Top 10 Residential Changes," here is a quick list of three major changes in the new statewide commercial energy code.


  1. Commissioning: Commissioning does not need to be completed to get a COO, but reports must be generated and given to the inspectors. While envelope commissioning isn't required, the HVAC, ventilation, and lighting systems must be verified as meeting the specified performance criteria.
  2. HVAC & Lighting: Increased use of controls and automation to earn efficiencies. Plus there are now six Efficiency Option Packages detailed in C406 to choose from (up from the current three).
  3. Ventilation: Demand-controlled ventilation is now required and is no longer an option for most spaces (larger than 500 sq ft). Perhaps a CERV would be appropriate for light commercial applications?


Now for our top 10 major changes to the IL Energy Code residential chapter:

  1. Prescriptive levels - following the exact specifications in the code. There were few changes to the prescriptive requirements.
    1. Insulation levels remain the same as 2012 energy code.
    2. Window performance remains the same as 2012 energy code.
    3. Infiltration rate (i.e. blower door test) remains the same as 2012 energy code.
  2. Performance UA Tradeoff- allows for some tradeoffs in an assembly, for example better windows than specified prescriptively can account for less stringent. This can be done via a UA tradeoff approach (for example, R40 insulation with better-than-code windows).
  3. Simulated Performance Alternative (R.405) - This option can be thought of as the whole-house tradeoff analysis showing the benefits of improved air-tightness (lower infiltration) instead of a straight UA calculation. Achieving this usually requires an energy rater such as Eco Achievers to use energy modeling software.
  4. New Energy Rating Index (ERI) pathway - this is similar to a HERS 55 or less in northern Illinois (which is Climate Zone 5).
  5. Actually for a HERS rating organization like Eco Achievers, that last item for the new ERI Pathway is important enough that it deserves mentioning again - since we can now use a HERS rating to show compliance with the Illinois energy code. (Sound the trumpets, this is great news!)

Other notable changes in the Residential chapter include:

  1. Ventilation airflow measurements - here's a new one. While mechanical ventilation has been required since the 2012 energy code, now it must be field measured, per R403.6.6.2. In other words, your exhaust fan ventilation, supply-side system, or ERV system that is required per R403.6.3 now must be measured to show flow rates are meeting the required ventilation rate.
      1. How much outdoor air ventilation do I need? If you're wondering about the required ventilation rate, the calculation is: CFM Rate =  0.01 (CFA) + 7.5(# of Bedrooms + 1), where CFA is the conditioned floor area including basements.
      2. Example: For a 3-bedroom home at 3,450 square feet (including a semi-finished basement), the required ventilation rate is: 0.01(3,450) + (7.5 x 4) = 64.5 CFM of continuous ventilation.

    03 unit 215, b7b db set up

  2. Duct Leakage Testing - Total leakage testing is now required on every duct system per  R403.3.3, but there is a target leakage rate ONLY if following the Prescriptive pathway in the energy code.  The target duct leakage rates remain the same as before:
    1. 3% total duct leakage if measured at rough construction (our preference)
    2. 4% total duct leakage if measured at final completion
  3. Insulation levels in the prescriptive table have stayed the same, but duct insulation requirements have increased to R-8 (which we've been seeing as standard practice in the field anyway).
  4. Remodeling applications in the energy code now have more flexibility. If adding or enclosing a previously unused space (think attic, porch, basement conversions), there are a few compliance pathways available if space usage patterns remain the same.  Also, duct testing is now excluded if altering an existing system but not fully replacing it (which makes sense).
  5. Bonus: For projects meeting the R.405 Simulated Performance Alternative or R.406 ERI Pathway, there is a strong possibility that the builder / developer will qualify for a $2,000 45L tax credit.

Energy codes can be (very) confusing, so we hope that this has shed some light on the recent changes in Illinois. If you have additional questions or follow ups, please don't hesitate to give us a holler. For a full list of Illinois amendments to the 2015 IECC with modifications shown, please see this redlined document from the DCEO.

7 Responses

  1. Jason, thanks for this excellent summary. I'm glad to see ventilation airflow measurements.
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