Note: The responses to the questions below are based on the Illinois Energy Conservation Code. SEDAC is the Energy Code Training Provider for Illinois EPA; however, SEDAC is not the code official nor the authority having jurisdiction. As such, the opinions expressed below are advisory and not a legal interpretation.

"The Code" or "the Energy Code" refers to the current Illinois Energy Conservation Code, which is based on the 2021 IECC with Illinois Amendments. Section numbers are from the 2021 IECC.

Links to referenced codes can be found here.

Administration/Legal Questions

Q: When did the updated Illinois Energy Code based on the 2021 IECC become effective? 
A: The Illinois Energy Code based on the 2021 IECC with Illinois Amendments became effective for permit applications beginning 1/1/2024. For the City of Chicago, the updated Energy Code based on the 2021 IECC became effective for permit applications beginning 11/1/2022.

Q: Is there a 6-month grace period for enforcement? 
A: No. According to the Illinois Energy Efficient Building Act, the Capital Development Board must adopt the latest IECC (International Energy Conservation Code) within 12 months of its publication and set the effective date no later than 6 months after adoption. The 2021 IECC was adopted by the Capital Development Board on November 14, 2023, with an effective date of January 1, 2024. The time between these two dates is less than 6 months and complies with the Illinois Energy Efficient Building Act.

Q: How do I know which version of the Illinois Energy Code my project is required to follow?
The code version is generally determined by the date of the permit application. For permits in Illinois, all permits submitted after 1/1/2024 must comply with the 2021 IECC version. For permits in Chicago, all permits submitted on or after 11/1/2022 must comply with the 2021 IECC version. Ultimately, the Authority Having Jurisdiction has the final say on which version to use, so check with your local Authority Having Jurisdiction to verify the version for your project if you are unsure.

Q: Can I use both ASHRAE 90.1 and IECC for Energy Code compliance?
Projects must choose one compliance path to demonstrate Energy Code compliance. Designers are not permitted to "cherry-pick" a customized path to compliance by combining provisions of both ASHRAE Standard 90.1 and IECC 2018 (for example 90.1 for HVAC and IECC for lighting would not be permitted). Either code must be used in its entirety with applicable Illinois Amendments to demonstrate compliance. Projects may choose to go beyond minimum code compliance, but code compliance must be demonstrated using a single compliance path.

Q: I am working on an elementary school. Who is the Authority Having Jurisdiction for code enforcement?
According to Title 23 of the Illinois Administrative Code Part 180 Health/Life Safety Code for Public Schools, the Authority Having Jurisdiction is the Regional Superintendent.

Q: I have a mixed-use building. How do I determine which provisions (residential or commercial) apply?  
A: [C101.4.1, R101.4.1] For mixed residential and commercial buildings 3 stories or less above grade (4 or less in the City of Chicago), each portion must separately consider and meet the applicable commercial or residential provisions. Mixed residential and commercial buildings larger than three stories (four in the City of Chicago) are considered commercial buildings, and residential portions of these larger buildings do not need to meet separate residential provisions unless referenced in the commercial provisions.

Q: Does an industrial building need to comply with the Energy Code? If so, how does the Code apply?
A: [C202, 600.310] The Code defines a commercial building as all buildings that are not included in the definition of “Residential Building”. Therefore, industrial buildings are classified as Commercial Buildings for the purposes of the Energy Code. Some industrial buildings may not need to comply with the Illinois Energy Conservation Code if they are determined by the local jurisdiction to have no comfort conditioning. See Illinois Administrative Code Title 71 Section 600.310.

Commercial Questions

Q: How might the Energy Code apply to a proposed building without heating and cooling?
A: [600.310]
 If the building does not "contain conditioned space," it does not need to comply with the Illinois Energy Conservation Code. Be aware that if the building undergoes a change of use and/or is conditioned in the future and no longer qualifies as a low-energy building, it may require substantial work to bring the envelope into compliance (See Title 71 Section 600.310 of the IL Administrative Code).

Q: We have a few questions relating to building envelopes in commercial buildings now that IECC 2021 is in effect. Is a blower door test required for commercial buildings? If so, can the contractor perform the test with the AOR/EOR/Cx agent as a witness?

A: [C402.5.1] C402.5.1 requires air barriers to be installed in accordance with several provisions including C402.5.1.2, which requires buildings to be tested for air leakage (typically using a blower door) unless they are using one of the exceptions which are only allowed in select climate zones. Climate Zones 4A and 5A are not on the list of exceptions, therefore all new construction commercial buildings will be required to be tested in Illinois. This testing must be conducted by an approved person, which means one that is acceptable to the code official. Typically, these are 3rd party contractors, i.e., commissioning agents or other licensed/certified professionals. Some jurisdictions have been observed allowing the construction contractor to perform the testing, with the test being witnessed by the code inspector. This testing is important as the mechanical design is based on a target air leakage rate. If the envelope leaks more than the target, the right-sized mechanical equipment may not be able to keep up due to the increased load from air leakage. The testing needs to be completed prior to final inspection. We typically recommend that testing be done as early as practical once the air barrier is complete and all penetrations of the air barrier have been made and sealed. This makes remedying any deficiencies easier, before they are all covered up by interior finishes.

Q: What are the requirements for air barrier testing for alterations or additions?
A: [C502.1, C503.2]
Additions and alterations, which include new building envelope assemblies require those assemblies to be tested in accordance with C402.5.2. Part of the design process should include a plan for this testing and consider where the test equipment is located and how it may be performed. Note that this section should not be confused with R503 requirements, which exclude the existing residential building alterations from air leakage testing.

Q: Where can I find someone to perform envelope testing?
A: [C402.5.1]
Many buildings may be tested using the same equipment that has been used for a long time in the residential space. Building envelopes of around 12-13,000 square feet of envelope area can be tested with a single residential fan set up at the commercial test pressure of 75 pascals. Equipment manufacturers design setups that fit 3 fans in a single door enabling 36-40,000 square feet of envelope area to be tested using a single door. Organizations that train testers include RESNET (Residential Energy Services Network,, BPI (Building Performance Institute,, ABAA (Air Barrier Association of America,, among others.

Q: We are working on a 45,000 square foot unoccupied tenant space in IL, to be turned over as a "cold dark shell" ready for tenant build-out. As part of the work, we are installing a new 230 square foot loading dock. Is the new loading dock required to be insulated?
A: [C402.1.1, C402.5.6] 
The loading dock may qualify as a low-energy building (or portion thereof) (see C402.1.1), which would exempt it from needing to be insulated. The loading dock would need to be thermally separated from the remainder of the building unless the whole building qualifies as a low-energy building. This may impact future use or make build-out more expensive if a future client desires heat. It would then no longer qualify as a low-energy building and require insulation.  If the loading dock does not qualify as a low-energy building (or portion thereof), it is required to comply with C402 of the IECC including C402.5.8 (Loading dock weather seals).

Q: Where do I find design conditions for my project?
A: [C403.1.1] 
You may ask your building department if they already have design conditions they want you to use. If they do not, ASHRAE has a Climatic Design Conditions Tool, which may be found at Select the most recent data at the top of the page and the nearest site to your project location for the most accurate data, including heating and cooling design conditions. The best practice for equipment sizing is the use of the 99% heating dry bulb and the 1% cooling dry bulb and coincident wet bulb temperatures for outdoor conditions. This is the recommendation in the ACCA Manual N Commercial load calculation methodology.

Q: Are you permitted to install occupancy sensors in a stairway? 
A: Maybe. This question is not addressed in the 2021 IECC; instead, please check the safety requirements of other local and national codes that have jurisdiction over the project. Some common safety codes include the Chicago Building Code, the International Building Code (IBC), and the Life Safety Code (NFPA 101). We reviewed these codes and found that generally you can have energy saving motion sensors for lighting in designated egress zones, as long as they meet certain safety requirements. For instance, you are not required to illuminate paths of egress in normally unoccupied areas of the building or service support areas. Therefore, support areas may be great places to take advantage of motion sensitive lighting controls. Your local AHJ or planning office may have additional guidance.

Q: Is daylight harvesting and daylight zone control required by the Energy Code? 
A: [C405.2.4] Yes, if there are more than 150 watts of general lighting in the primary sidelit or toplit zone or more than 300 watts of general lighting in the primary and secondary sidelit zone. There are some exceptions provided for certain applications. For instance, areas are exempt from meeting the daylight zone control requirements if the area weighted lighting power density is less than 40% of normal LPD for the space type (Exemption #3, see also C406.3).

Q: I am renovating a commercial building. Do I need to bring my building's envelope up to full compliance with the 2021 IECC? 
A: [C502, C503 & C505.1]. If a space changes occupancy or use that would increase the demand for fossil fuel or electrical energy, the space must meet the 2021 IECC envelope requirements. Some indications of increasing demand for fossil fuel or electrical energy include needing a service upgrade or larger HVAC equipment. For example, if a warehouse storage area is converted into an office space or a warehouse is converted into residential lofts, these spaces must be insulated to the current energy code. If part of a building is altered or new construction is added to the building, the altered or added parts of the building must comply with envelope requirements as detailed in Chapter 5 of the Energy Code. The code requirements do not apply to the unaltered parts of the building if not changing the occupancy or use of the building.

Q: When I'm doing roof work, when do I need to add insulation?
A: [C503.3.1, C504]. There are several kinds of roof work. Some of them trigger compliance with the IECC envelope requirements, some do not.

  • Roof replacementis defined as the process of "removing the existing roof covering, repairing any damaged substrate and installing a new roof covering." During roof replacements, section C503.2.1 requires the roof deck to be brought into compliance with the IECC thermal envelope requirements if "the existing roof assembly is part of the building thermal envelope and contains insulation entirely above the roof deck." Roof replacement presents a unique opportunity to add insulation. We strongly recommend that you meet or exceed the minimum insulation requirements, if feasible. Unlike mechanical and electrical systems that can fail, insulation works from the time it is installed until it is damaged or removed; therefore, the benefits of insulation improvements last the life of the building. See below if there is not adequate room to provide the insulation required.
  • Roof recoveris defined as "the process of installing a new roof covering over an existing roof covering without removing the existing roof covering." Section C503.1 indicates that roof recovers are not required to meet the IECC insulation and envelope requirements.
  • Roof repairsare defined as "reconstruction or renewal of any part of an existing roof for the purposes of its maintenance." These are also exempt from meeting the IECC insulation and envelope requirements (C504).

Q: For a low slope roof replacement, what should be done if there is not enough room to provide the full R-30 insulation and install flashings properly?
A: [C503.2.1] 
Illinois has provided an exception, which allows for a registered design professional or an approved source to detail the limiting conditions and provide a design that minimizes the deviation from the level of insulation called for by the Code. The level of insulation after the roof replacement may not be less than what it was before the roof replacement.

Q: Can you provide clarification on recirculating pumps for lavatories on a small commercial project?
A: [C503.5]
 C503.5 explains that new service hot water systems that are part of the alteration must comply with C404 (service water heating). C404.5.1 identifies the maximum allowable piping length from the nearest source of heated water. The objective is to allow heated water to be available in a reasonable period of time and avoid running the water while waiting for heated water, particularly as efficient fixtures become the norm. New lavatory faucets with 0.5 gpm flow rates are only flowing about 1 ounce per second.  Meeting the maximum piping length may be achieved in several different ways, such as through a circulated loop or a point-of-use heater. If a circulated loop is used, C404.6 sets requirements for the heated water circulating or temperature maintenance system. This Smart Tip provides additional information on circulating systems.

Residential Questions

Q: My builder is telling me we can’t use 2x4 construction anymore. Is this true?
A: [Table R402.1.3]
While the insulation levels for residential buildings have increased, this statement is not accurate. Your builder is referring to building using the same methods and practices as in the past such as using cavity only insulation. 2x4 construction is feasible, and can be code compliant, but will require the use of exterior continuous insulation in addition to or in lieu of cavity insulation.

Q: Is the code cost effective?
Multiple studies including from the Pacific Northwest National Labs, the National Association for Home Builders, and ICF International have found the improved energy code to be cost effective. Over 30 years, PNNL found the life cycle cost savings to be above $1,000, accounting for increased mortgage costs and decreased utility costs.

Q: Where do I find design conditions for my project?
A: [R403.7] 
ASHRAE has a Climatic Design Conditions Tool, which may be found here. Selecting the most recent data at the top of the page and the nearest site to your project location will provide the most accurate data including heating and cooling design conditions.

Q: What are the air barrier and insulation installation requirements for commercial buildings?
A: [Table R402.4.1.1]. 
Table R402.4.1.1 contains the requirements for both air barriers and insulation installation requirements broken down by component area. The general requirements are for the air barrier to be continuous, breaks or joints in the air barrier to be sealed, and not using air-permeable insulation as a sealing material apply to all locations. Components of the building envelope should be installed according to the manufacturer's instruction. Footnote b of the table exempting rim joists from air sealing also applies to rim joists in ventilated crawlspaces since the air barrier would not include the rim joist area.

Q: What is the maximum air leakage rate allowed per Code?
A: [R402.4.1.2] 
Code requires buildings to be tested and verified as having an air leakage rate not exceeding 3 air changes per hour (ACH). The building should be provided with a whole-house mechanical ventilation system as designed in accordance with Section R403.6. Air leakage testing should be conducted in accordance with ASTM E779 or ASTM E1827 and reported at a pressure of 50 pascals. Smaller or Low-rise multi-family buildings may use the exception to demonstrate an air leakage rate of no greater than 0.3 cfm per square foot of enclosure area at a pressure of 50 pascals when tested in accordance with ASTM E779 or ASTM E1827.

Q: Is diagnostic testing required for assessing compliance with the Energy Code?
A: [R402.4.1.2] 
Yes. The air leakage testing called for in Section R402.4.1.2 will require diagnostic testing equipment such as a blower door to be used for compliance verification. This is a mandatory provision of the code and, therefore, is required regardless of the compliance path chosen.

Q: Are permits required for window replacements? 
A: Permit requirements vary by jurisdiction. Check with your local jurisdiction to determine their permit requirements.

Q: If I open an exterior wall, what do I do if there isn't enough depth to insulate to current code? 
A: [R503.1.1] Building envelope assemblies that are part of the alteration are required to follow certain envelope requirements, including insulation requirements, but there are multiple exceptions. Exception 2 would likely apply to this situation: "Existing ceiling wall or floor cavities exposed during construction" do not need to comply with these requirements "provided that the energy use of the building is not increased" and "provided that these cavities are filled with insulation." Thus, the wall would not need to be expanded to install R-30 insulation, but it would require insulation to fill the cavity. R-13 or R-15 insulation would likely be typical for a 2X4 wall.

Note that for masonry walls, an air gap should be left between the masonry and the wall/insulation because masonry absorbs water and needs to be able to dry or it will degrade. Leaving no air gap would violate the requirement that "alterations shall not create an unsafe or hazardous condition or overload existing building systems" (R503.1). Leaving this air gap will still qualify as filling the cavity to the best of one’s ability.

Q: I am renovating a house but not changing space conditioning requirements; what envelope requirements must I meet?
A: [R503.1.1]. If your renovation is not changing occupancy that increases the energy use of the space or converting additional space into a dwelling unit, you are often not required to bring the existing portions of the building into compliance with the 2021 IECC envelope requirements in section R402. When a floor, wall or ceiling cavity is opened in the process of a renovation, that portion of the envelope is not required to be brought up to the new construction insulation level as long as the cavity is filled with insulation, and the renovation is not changing space conditioning requirements. If the cavity is not filled with insulation, the 2021 IECC only requires that you fill it, but does not require a specific R-value.

Q: For a low slope roof replacement, what should be done if there is not enough room to provide the full R-49 insulation and install flashings properly?
A: [R503.1.1.2] 
Utilizing the U-factor table, particularly for low-slope roofs, will be easier to comply with rather than using the R-value table. Illinois has provided an exception, which allows for a registered design professional or an approved source to detail the limiting conditions and provide a design that minimizes the deviation from the level called for by the Code. The level of insulation after the roof replacement may not be less than what it was before the roof replacement.

Q: Does a basement with interior blanket insulation still meet the requirements for continuous insulation if the electrical panel penetrates the insulation to mount to the wall or the blanket is interrupted for intersecting walls?
For insulation to qualify as continuous, it must be a layer of insulation uninterrupted by framing members or penetrations except for service openings and fasteners. The electrical panel or intersecting walls do not qualify as a service opening and would require insulation between them and the exterior wall for the insulation to qualify as continuous.