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The Illinois Energy Conservation Code, Public Act 093-0936 (Illinois Energy Conservation Code [IECC] for Commercial Buildings) was signed into law in August, 2004. The Illinois Energy Conservation Code for Commercial Buildings became effective April 8, 2006. On October 9, 2007 the Law was revised to mandate the latest published edition, excluding supplements, of the International Energy Conservation Code. As of August 18, 2009 the Illinois Energy Conservation Code for Commercial Buildings is the 2009 International Energy Conservation Code. On August 28, 2009, Public Act 096-0778 requiring an energy code for residential buildings was signed into law. Public Act 096-0778 was signed into law on August 28, 2009 amending the Energy Efficient Commercial Building Act by including residential buildings and amending the name of the act to the Energy Efficient Building Act. The new requirements for residential buildings became effective on January 29, 2010. Effective July 23rd, 2013 the 2012 International Energy Conservation Code was State Law.

The 2015 IECC became effective in Illinois on January 1, 2016. On July 1, 2017, Illinois Executive Order 17-03 transferred responsibility for the Illinois Energy Conservation Code from DCEO to Illinois Environmental Protection Agency. The 2018 IECC is currently effective in the State of Illinois. Work has begun to adopt the 2021 IECC and is expected to be adopted fall of 2023.

When Does This Code Apply?

  • Any new construction (additions, alterations, renovations and repairs) including window replacement
  • Additions can comply alone or in combination with existing buildings
  • Any building that is provided with an electrical service in excess of 100 amps (This includes garages, pole barns, etc)
  • Any non-conditioned space that is altered to become conditioned space shall be required to be brought into full compliance with this code
  • Exemptions:
    • Unaltered portions
    • Storm windows over existing fenestration
    • Glass only replacements
    • Existing ceiling, wall or floor cavities exposed during construction if already filled with insulation
    • Where existing roof, wall or floor cavity isn't exposed
    • Reroofing for roofs where neither sheathing nor insulation is exposed

There are two big terms to learn with this code:

  • Prescriptive - required but can be lessened or eliminated in trade for compensating improvements elsewhere
  • Mandatory - required and cannot be traded down, even in the simulated performance path.

Building envelope

  • Ceilings
    • For our area, R-49 becomes the required insulation (table R402.1.1).
    • The code provides tables in R-values and assumes standard truss systems. The code does encourage raised-heel trusses (also known as an energy truss).
    • There are accommodations if rafters/trusses are not large enough for standard insulation.
    • Access hatches and doors between conditioned and unconditioned spaces must be insulated and weather-stripped.
    • If you have loose fill (blow-in insulation) you must provide access to all equipment that prevents damaging or compressing the insulation.
  • Walls
    • For our area, R-20 becomes the required insulation. This can also be met with R-13 plus R-5 sheathing (table R402.1.1).
    • Walls must be insulated including those next to unconditioned spaces
    • Rim joists must be insulated
    • Table R402.1.1 does address different types of wall construction (wood framed, mass, steel, etc.)
  • Basement Walls must be insulated to R-15 continuous insulation or R-19 cavity insulation.
  • Floors over unconditioned space require R-30 insulation and it must contain permanent contact with the underside of the subfloor.
  • Slabs must be insulated to R-10 to a depth of 2 feet.This includes areas where there are walkout walls.
  • Crawlspace walls must be insulated to R-15 continuous insulation or R-19 cavity insulation.
  • Fenestration U-factors (windows, skylights and doors)
    • Products must be NFRC-rated and -certified
    • Skylights must meet a U-factor of 0.55
    • Windows must meet a U-factor of 0.32
  • Infiltration control measures-sealing to prevent air leaks (Table R402.4.1.1)
    • Air barrier and thermal barrier - Exterior thermal envelope insulation for framed walls is installed in substantial contact and continous alignment with building envelope air barrier. Breaks and joints in the air barrier are filled or repaired. Air-permeable insulation is not used as a sealing material. Air-permeable insulation is inside of an air barrier.
    • Celing/attic - Air barrier in any dropped ceiling/soffit is substantially aligned with insulation and any gas are sealed. Attic access, knee wall or dropdown is sealed.
    • Walls - Corners and headers are insulated.
    • Windows and doors - Space between window/door jambs and framing are sealed.
    • Rim Joists - are insulated and include an air barrier.
    • Floors (including above-garage and cantilevered floors) - Insulation is installed to maintain permanent contact with the underside of subfloor decking. Air barrier is installed at any exposed edge of insulation.
    • Crawls space walls - Insulation is permanently attached to walls. Exposed earth in unvented crawl spaces is covered with a Class I vapor retarder with overlapping joints taped.
    • Shafts, penetrations - Duct shafts, utility penetrations, knee walls and flue shafts opening to an exterior or unconditioned space are sealed.
    • Narrow cavities - Batts in narrow cavities are cut to fit, or narrow cavities are filled by sprayed/blown insulation.
    • Garage separation - Air sealing is provided between the garage and conditioned space.
    • Recessed lighting - Recessed light fixtures are air tight, IC rated, and sealed to drywall.
    • Plumbing and wiring - Insulation is placed between outside and pipes. Batt insulation is cut to fit around wiring and plumbing, or sprayed/blown insulation extends behind piping and wiring.
    • Shower/tub on exterior wall - Shower adn tubs on exterior walls have insulation and an air barrier separating them from the exterior wall.
    • Electrical/phone box on exterior walls - Air barrier extends behind boxes or air sealed-type boxes are installed.
    • Common wall - Air barrier is installed in common wall between dwelling units.
    • HVAC reister boots - HVAC register boots that penetrate building envelope are sealed to subfloor or drywall.
    • Fireplace - Fireplace walls include an air barrier.
  • Air Leakage (R402.4.1.2) - The building or dwelling unit shal be tested or verified as having an air leakage rate not exceeding 5 air changes per hour. Testing shall be done with a blower door test.

HVAC / Systems

  • Equipment Efficiency
    • Federal law establishes minimum efficiencies through Department of Energy rulemaking. IECC simply points to federal minimums.
  • Ducts
    • Ducts must be insulated (prescriptive) Supply ducts in attic require an R-8 rating. All other ducts require R-6. (R403.2)
    • Sealing (mandatory) - Building framing cavities shall not be used as ducts or plenums (R403.2.3). Joints and seams must comply with IRC Section M1601.4.1.
    • Duct tightness shall be verified by either post construction test or a rough-in test. Duct tightness test not required if air handler and all ducts are located within conditioned space.
  • Thermostats
    • If primary heating system is a forced-air furnace there must be at least one programmable thermostat per dwelling unit. Must be initially programmed with heating point no higher than 70 degrees and cooling point no lower than 78 degrees.
  • Heat pump controls are required to prevent supplementary electric-resistance heat when pump can meet the heating load.
  • Service hot water system
    • Circulating hot water systems shall be provided switch to turn off the system when not in use. (R403.4.1)
    • Insulation for hot water pipe shall be a minimum of R-3 as required in table R403.4.2.
  • Mechanical Ventilation - The building shall be provided with mechanical ventilation (R403.5).Outdoor air intakes and exhausts shall have automatic or gravity dampers that close when the ventilation system is not operating.
  • Equipment sizing
    • Load calculations determine the proper capacity (size) of equipment. The goal is to make sure system is big enough to ensure comfort but not bigger.
    • Heating and cooling equipment shall be sized in accordance with ACCA Manual S on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling methodologies.


  • 75% of lamps in permanent fixtures must be high-efficiency. This is a simple count of bulbs, not fixtures or wattages. Examples would be compact fluorescent (CFL), light emitting diodes (LED), etc. (R404.1)
  • Recessed lighting fixtures must be sealed with a gasket or caulk between the housing and interior wall or ceiling covering. They must also be Type IC rated and labeled as meeting ASTM E 283 (R402.4.4).

Other Systems

  • Vapor-retardant pool covers are required on heated pools and inground permanently installed spas when heated to more than 90 degrees Fahrenheit, unless over 70% of the energy for heating is from site-recovered energy (R403.9.3).
  • Pool heaters must have a readily accessible on-off switch and can not have a continuously burning pilor light if fired by natural gas (R403.9.1).
  • Snow melt systems must have an automatic or manual shutoff when the outdoor temperature is greater than 40 degrees Fahrenheit.
  • New wood-burning fireplaces shall have gasketed doors and outdoor combustion air.


  • Appliances are not addressed by this code


Most of these actions will be inspected during the normal permit process. There may be a few new inspections or time you need to have the inspector present. Upon completion of a project Section 401.3 requires a permanent certificate to be posted on or in the electrical distribution panel. The certificate is to clearly depict information for energy code compliance assessment by the field inspector.