The first time I had to tell a building owner his approved insulation package was three quarters of an inch short of code, he asked me why nobody mentioned it during the bid walk. The answer was the bid walk happened in 2023, the project broke ground in 2026, and the jurisdiction had adopted IECC 2024 in between. That gap is now the most common conversation I have on retrofit jobs across Wisconsin, Illinois, Minnesota, and Michigan. The code moved. The polyiso math moved with it. The bid number that won the work eighteen months ago does not buy a code-compliant roof today.
I run the Upper Midwest division at MetalMaster-RoofMaster. Almost every commercial roof we touch sits in climate zone 5 or 6, which is exactly where IECC 2024 made the biggest jump. If you are specifying, estimating, or owning commercial roofs in this region, the following is what I tell owners and what I make sure my own team knows cold before we go to contract.
TL;DR: IECC 2024 raised commercial roof insulation to R-25 in climate zones 3 to 5 and R-30 in zones 6 to 8, per the code’s reference to ASHRAE 90.1-2022 (datadrivenaec.com). Wisconsin and Illinois split that line, so most retrofits scoped under the 2018 code now miss current minimums. The owner conversation has to start with the polyiso upgrade math, not the bid number.
What changed in IECC 2024 for commercial roofs
IECC 2024 lifted continuous-insulation R-values across the board, with the largest jump in northern zones: R-20 in zones 1 and 2, R-25 in zones 3 through 5, and R-30 in zones 6 through 8, according to a 2024 code summary from DataDrivenAEC. The C402.2 prescriptive table is where this lives. Most jurisdictions in our region are either on IECC 2024 directly or on an ASHRAE 90.1-2022 path that lands on the same number.
The jump from R-20 above-deck to R-30 above-deck in zone 6 is not small. On a polyiso buy that is roughly 5.5 inches of insulation instead of 3.6 inches, before tapered. On a 200,000 square foot roof that single change moves the insulation line item by a six-figure dollar amount, and it shifts the parapet height conversation, the curb extension conversation, and the door-pan flashing detail. None of that fits inside a number a GC pulled off a budget spreadsheet two years ago.
Where does the Upper Midwest fall in the climate zones
The IECC climate zone map cuts the Upper Midwest in half. Most of Wisconsin and Illinois sit in zone 5. The northern third of Wisconsin, almost all of Minnesota, and the Upper Peninsula of Michigan sit in zone 6. A building in Madison is on the R-25 side. A building in Wausau is on the R-30 side. A building in Eau Claire might be either, depending on which county line it sits on and which version of the code that county adopted. I keep a county-by-county adoption list on my phone because owners ask, and they do not want a guess.
This matters for portfolio owners with sites across both zones. The same prototype building, repeated in Rockford and in Duluth, has different insulation packages now. The Rockford store is R-25. The Duluth store is R-30. If your spec writer is still issuing one drawing set for both, the Duluth permit gets kicked back and the schedule slips. I have watched that exact thing happen twice in the last six months.
Why ASHRAE 90.1-2022 is the real document you need
IECC 2024 references ASHRAE 90.1-2022 as the energy-performance pathway, and most commercial design teams are working off the ASHRAE document directly rather than the IECC table. ASHRAE 90.1-2022 Addendum r, published May 2024, is the addendum that locked in the new assembly performance values. If you are running a COMcheck or a whole-building energy model, the inputs come from this document.
The plain-language version is worth reading too. The National Insulation Association’s codes-and-standards update in Insulation Outlook walks through the ASHRAE 90.1 cycle and flags that the next update, scheduled for 2025, will push R-values higher again. That is a warning shot for anyone signing a five-year-old spec without checking the date stamp on the energy code section.
How the polyiso buy actually shifts
Polyiso boards in our region run roughly R-5.6 per inch at design temperatures. To hit R-25 you need a minimum 4.5 inches of continuous polyiso. To hit R-30 you need 5.4 inches. In practice we spec two layers staggered to avoid thermal-bridge gaps, which means a typical R-30 stack is one 3-inch board and one 2.5-inch board, joints offset, mechanically attached or adhered. Add tapered on top of that for slope, and you are easily looking at 7 to 9 inches of insulation at the high side of a tapered system.
What I tell owners:
- The bid you got under 2018 code assumed 3 to 4 inches of polyiso. Current code is 4.5 to 5.5 inches base, before tapered. That is a real material delta on the order of 30 to 50 percent more polyiso, not a rounding error.
The other thing that changes is what happens at every penetration and every parapet. A roof that is 2 inches taller at the field needs taller cant strips, taller curbs, longer fasteners, and revised flashing details at the wall transition. The mechanical contractor needs to know because his curbs are no longer the right height. The general needs to know because his door thresholds may need redesign. None of this is hard once you know it is coming. All of it is painful if it hits you mid-installation.
What about recover systems and value engineering
The single most common pushback I hear from owners is some version of: my consultant said we can recover, and recover does not have to meet the new R-value. That is partly true and frequently misunderstood. Under IECC 2024 a roof recover over an existing roof system can use the existing assembly’s R-value, but only if the existing assembly already met the code in effect at the time it was installed, and only if the recover meets specific conditions in C503.3. Most roofs I look at do not meet those conditions. The existing insulation is wet, or compressed, or it was installed under a code that no longer counts as a basis.
The value-engineering version of this conversation is owners asking if we can quote both options: code-minimum and quote a strip-and-replace at the old R-value, then let the AHJ tell them which they have to do. I will not do that. The AHJ will tell them strip and replace at the current minimum, and now they have a bid that is technically non-compliant on file, and the rebid has to happen against the right number anyway. Bid the right number the first time and use the rebate path to soften the cost.
Where the rebate money is hiding
Most utilities in our footprint run prescriptive rebates on commercial roof insulation upgrades. Focus on Energy in Wisconsin, ComEd and Ameren in Illinois, Xcel in Minnesota, and Consumers and DTE in Michigan all have current programs. The numbers move, and the rebate is usually per square foot above a baseline R-value, which means a code-required upgrade does not always qualify, but a voluntary upgrade above code does. The play is to walk an owner past the R-30 minimum to R-35 or R-38 and capture the rebate on the marginal inches. On large roofs that math works. On smaller jobs it usually does not.
I have one owner in southern Wisconsin who routinely buys to R-38 on every reroof, takes the Focus on Energy rebate, gets a federal tax incentive on the energy improvement, and ends up at a net cost roughly even with code-minimum after incentives. That is not a magic trick. That is a regional GC who learned to read the rebate language and built it into his ROI model. Most owners do not have that machinery internally. If your contractor does not bring it to you, ask why.
What to tell your owners this quarter
If you specify, install, or own commercial roofs in zone 5 or 6, the conversation right now is short. The code moved. The polyiso buy is bigger. The recover loophole is narrower than it sounds. The rebate path is real but specific. Any quote that does not show the IECC 2024 R-value on the drawing and the polyiso thickness in the material schedule is a quote you cannot evaluate. Send it back and ask for both.
I am happy to walk a peer through the math on a specific project. The numbers are not complicated once you have them in front of you. The cost of not having them in front of you is finding out at permit submittal that the bid you signed eight months ago does not buy you a roof you can occupy.
About Khary Penebaker
Khary Penebaker is Division President at MetalMaster-RoofMaster, the Upper Midwest division of Wolkow Braker Roofing Corp. He previously built Roofed Right America from startup to $35M+ in revenue with 180 employees (2014-2025) and founded Penebaker Enterprises, growing it from $1.5M to $15M. A gun violence prevention advocate and former Everytown for Gun Safety Fellow, Khary brings two decades of leadership in commercial roofing, architectural sheet metal, and civic engagement.
Common questions
Which climate zone is my building in
Most of Wisconsin and Illinois sit in IECC climate zone 5. Northern Wisconsin, almost all of Minnesota, and the Upper Peninsula of Michigan sit in zone 6. The line cuts roughly across central Wisconsin. The exact county-by-county assignment is set by the IECC climate zone map referenced in your local energy code adoption. Owners with portfolios across both zones now have different insulation packages on the same prototype building, which matters at permit submittal.
What polyiso thickness gets me to R-30 in climate zone 6
Polyiso boards run roughly R-5.6 per inch at design temperatures in our region. To hit R-30 continuous you need 5.4 inches minimum. In practice we spec two staggered layers, typically a 3-inch board and a 2.5-inch board with joints offset to prevent thermal bridging. Add tapered insulation on top for slope. A tapered R-30 system can run 7 to 9 inches at the high side, which changes parapet heights, curb dimensions, and door-pan details.
Are utility rebates available for commercial roof insulation upgrades
Yes, but the rebate is typically paid on insulation installed above code minimum, not for meeting code. Focus on Energy in Wisconsin, ComEd and Ameren in Illinois, Xcel in Minnesota, and Consumers and DTE in Michigan all run prescriptive commercial roof programs. The play is to spec R-35 or R-38 and capture the rebate on the marginal inches above R-30. On large roofs that math works. On smaller jobs the rebate often does not cover the upgrade cost.
Does a roof recover have to meet the new R-value
Sometimes. IECC 2024 C503.3 allows recover over an existing roof using the existing assembly's R-value, but only if that assembly met the code in effect when it was installed and only if specific conditions are met. Most existing assemblies I evaluate do not qualify because the insulation is wet, compressed, or was installed under a code basis that no longer counts. Plan for a full strip-and-replace to current minimums unless the existing assembly is documented and intact.
When does ASHRAE 90.1 update again
ASHRAE 90.1-2022 is the current document referenced by IECC 2024. The next ASHRAE 90.1 update is scheduled for 2025 and is expected to push R-values higher again, per Insulation Outlook. That means anyone signing a multi-year specification today should write the energy code section against the current published standard and review it before each project breaks ground. A five-year-old spec sheet is no longer a safe document to issue without a fresh code check.