Rockford Window Replacement by the Numbers

Last reviewed 2026-07-25. Journalists, researchers, and community groups are welcome to cite any figure on this page with attribution and a link.

Every figure on this page comes from a published government or program source, linked in full below, and nothing here is estimated or interpolated. It covers the four things that actually determine what a window replacement in the Rockford area has to be: the U-factor Illinois requires, the heating load this climate imposes, which municipal desk handles the permit, and the two age-triggered rules, historic district review and federal lead-safe work practices, that reach a large share of the housing stock here.

Two figures readers often want are deliberately absent. The 2024 IECC solar heat gain coefficient requirement for Climate Zone 5 is not listed, because two published summaries of that table returned different values and the discrepancy could not be resolved from a primary source. Winnebago County's ENERGY STAR climate zone assignment is not asserted either, because the program's zone finder could not be read directly. The criteria for all four ENERGY STAR zones are published below instead, and the county assignment should be confirmed on the program's own tool.

The code threshold: one U-factor, fixed statewide

Illinois adopted the 2024 International Energy Conservation Code as the Illinois Energy Conservation Code at 71 Ill. Adm. Code 600.400. The rule incorporates the 2024 IECC including published errata but excluding published supplements, published May 2024, and the Capital Development Board lists the effective date as November 30, 2025. Compliance keys off the permit application: projects applying for a permit on or after that date comply with the new code. For fenestration in Climate Zone 5, the Pacific Northwest National Laboratory residential cost-effectiveness analysis identifies change REPI-028, which lowered the maximum U-factor from 0.30 to 0.28 Btu per hour per square foot per degree Fahrenheit.

The statewide lock is statutory rather than a matter of local practice. Section 15 of the Energy Efficient Building Act directs the state to adopt the code as the minimum and maximum requirements for residential buildings. Section 45 states that no unit of local government, including any home rule unit, may regulate energy efficient building standards for residential buildings in a manner that is either less or more stringent than the standards established under the Act, excepting only a municipality with a population of 1,000,000 or more. The Department of Energy's Building America county guide assigns Winnebago, Boone, and Ogle counties to Cold Zone 5A.

Rockford's heating load in National Weather Service normals

The National Weather Service's Chicago forecast office publishes 1991-2020 climate normals for Chicago/Rockford International Airport, the first-order station for this metro. The figure that matters most to anyone specifying windows is the ratio between heating and cooling demand: 6,497 annual heating degree days against 838 cooling degree days, close to eight to one. That is the arithmetic reason a low U-factor matters far more here than solar control does, and why the ENERGY STAR criteria written for northern climates put a floor under solar heat gain coefficient rather than a ceiling over it. January is the design month by a wide margin.

Which desk handles the permit, town by town

Permit authority is where this metro genuinely diverges, and each divergence is published on the town's own site. Rockford requires a permit for construction, alteration, enlargement, repair, or moving of a structure larger than 120 square feet, and its building-codes page states that the city follows the 2021 ICC code cycle. Machesney Park lists siding and window replacements of the same size among the home improvements that do not require a permit. Loves Park requires permits for anything that requires inspections. Belvidere and Loves Park both publish 2015-edition adoptions. None of that changes the energy requirement, which is preempted. All of it changes your schedule.

Code minimum versus ENERGY STAR, side by side

ENERGY STAR's Version 7.0 specification for residential windows, doors and skylights took effect October 23, 2023 and sets criteria by climate zone. In the Northern zone the prescriptive criterion is a U-factor of 0.22 or lower paired with a solar heat gain coefficient of 0.17 or higher, a floor on SHGC rather than a cap, because winter sun through the glass is useful heat in a heating-dominated climate. The specification also publishes equivalent energy performance pathways for the Northern zone, allowing a higher U-factor when SHGC rises: 0.23 or 0.24 at an SHGC of 0.35 or above, and 0.25 or 0.26 at 0.40 or above.

One published savings figure is worth stating precisely, because it gets misquoted constantly. The Pacific Northwest National Laboratory cost-effectiveness analysis for the 2024 IECC reports, in Table 4.2, a national average annual energy cost saving of $133 per dwelling unit per year in climate zone 5. That figure is national in scope, applies to the entire code package rather than to windows alone, and models new construction rather than a retrofit of an existing house. It is not a projection of what any individual homeowner's gas bill will do after a window replacement, and no honest quote will present it as one.

The rules that key off the age of the house

Rockford's Historic Preservation Plan documents six locally designated districts: Haight Village, designated 1980; Garfield Avenue, 1982; Indian Terrace, 1984; Brown's Hills-Knightsville, 1984; Northeast State and Main, 1993; and Peacock Brewery, 2009. The framework comes from Chapter 113 of the Rockford Code of Ordinances, and any modification that materially alters the exterior of a landmark must first receive a Certificate of Appropriateness from the Historic Preservation Commission. The Commission meets at 5:30 p.m. on the second Tuesday of each month in the City Council chambers, which sets the practical cadence for any project that needs review.

The city's Design Guidelines for Historic Properties state the standard applied to windows: deteriorated historic features will be repaired rather than replaced, and where the severity of deterioration requires replacement of a distinctive feature, the new feature will match the old in design, color, texture, and, where possible, materials. Separately and federally, EPA's Renovation, Repair and Painting rule covers homes, childcare facilities and preschools built before 1978, requiring firms to be certified and employees trained in lead-safe work practices. Illinois is not among the states EPA has authorized to administer their own program, so EPA runs it here directly.

Winnebago County's health department publishes thirteen ZIP codes designated high risk for pediatric lead exposure: 61024, 61063, 61077, 61079, 61080, 61101, 61102, 61103, 61104, 61107, 61108, 61109, and 61111. Children living in them are to be tested at 12, 24, and 36 months, and effective January 1, 2025 a blood lead level above 3.5 micrograms per deciliter requires public health intervention. That list governs pediatric screening rather than contractor practice, while the federal rule above keys off the build year instead. Windows are a friction surface, so the two frequently apply to the same project.

Sources & Methodology

Every figure above was taken from the primary sources below (accessed 2026-07-25). Where a number is derived (arithmetic on published figures) or covers a broader period or area, the text says so.

1. Illinois Capital Development Board - Illinois Energy Conservation Code — Adoption of the 2024 IECC as the Illinois Energy Conservation Code and its November 30, 2025 effective date.
2. Illinois JCAR - 71 Ill. Adm. Code 600, Illinois Energy Conservation Code — Section 600.400 incorporating the 2024 IECC by reference, including published errata and excluding published supplements.
3. Illinois Compiled Statutes - 20 ILCS 3125, Energy Efficient Building Act — Section 15 minimum-and-maximum language and Section 45 home rule preemption with the 1,000,000 population exception.
4. NAHB - Significant Changes to the 2024 IECC — Climate Zones 5 and 6 fenestration U-factor of 0.28 under the 2024 IECC.
5. PNNL - Cost-Effectiveness of the 2024 IECC for Residential Buildings — Change REPI-028 lowering the Zone 5 fenestration U-factor from 0.30 to 0.28, and the Table 4.2 national average annual energy cost figure.
6. National Weather Service Chicago - Rockford Climate Normals — 1991-2020 heating and cooling degree days, annual and January mean temperatures, and annual snowfall.
7. ENERGY STAR Windows, Doors and Skylights Version 7.0 Specification — The October 23, 2023 effective date and the prescriptive U-factor and SHGC criteria for all four climate zones.
8. U.S. DOE Building America - Guide to Determining Climate Regions by County — Cold Zone 5A assignments for Winnebago, Boone and Ogle counties in Illinois.
9. University of Illinois EnergySense - Illinois Energy Conservation Code — The permit-application trigger and the statement that local governments may not adopt standards less or more stringent than the state code.
10. City of Rockford - Building Codes and Ordinances — The city's statement that it follows the 2021 ICC code cycle.
11. City of Rockford - Building Permits — The 120 square foot threshold for permitted construction, alteration, enlargement, repair or moving of a structure.
12. Village of Machesney Park - Permits and Fees — Same-size siding and window replacement listed among the home improvements that do not require a permit.
13. City of Loves Park - Building Codes — The 2015 code adoptions effective 5/1/2016, including the 2015 IECC and the 2015 IEBC.
14. City of Loves Park - Building Permits — The statement that permits are needed for anything requiring inspections, and the $25.00 per permit per trade extension fee.
15. City of Belvidere - Building Department — Adoption of the 2015 IBC, IRC, IFC, IMC, IFGC and IEBC, and the Windows and Doors remodeling topic.
16. City of South Beloit - Community Development — The applications and permits list whose first entry is a Winnebago County Building Permit.
17. Winnebago County - Permits and Forms — The statement that county permits apply to the unincorporated areas of Winnebago County.
18. Rockford Historic Preservation Plan — The six locally designated historic districts, their designation years, Chapter 113, and the Certificate of Appropriateness requirement.
19. City of Rockford - Design Guidelines for Historic Properties — The repair-before-replace standard applied to windows and the matching requirement for design, color, texture and materials.
20. EPA - Lead Renovation, Repair and Painting Program — Pre-1978 scope, firm certification and renovator training requirements, and the list of EPA-authorized states.
21. Winnebago County Health Department - Lead Poisoning High-Risk ZIP Codes — The thirteen designated Winnebago County ZIP codes, the 12, 24 and 36 month testing schedule, and the 3.5 microgram intervention level.
22. U.S. Census Bureau QuickFacts - Loves Park city, Illinois — Loves Park 2020 population, July 2025 estimate, owner-occupancy rate and median owner-occupied home value.
23. U.S. Census Bureau QuickFacts - Machesney Park village, Illinois — Machesney Park 2020 population, July 2025 estimate, owner-occupancy rate, median home value and median household income.
24. U.S. Census Bureau QuickFacts - Belvidere city, Illinois — Belvidere 2020 population, July 2025 estimate, owner-occupancy rate, median home value and median household income.
25. U.S. Census Bureau QuickFacts - South Beloit city, Illinois — South Beloit 2020 population, July 2025 estimate, owner-occupancy rate, median home value and median household income.