Glass packages, U-factor and SHGC for a Chicago window

Chicago caps a replacement window at a U-factor of 0.30 and a solar heat gain coefficient of 0.40 for a residential building up to four stories above grade, read in the ordinance adopting the 2022 Chicago Energy Transformation Code, 2026-08-11. Those numbers are written against the whole assembly, frame and glass together. The code names no number of panes, no coating and no gas fill, so double against triple glazing is a comfort, noise and money decision rather than a compliance one. The only attributable price for the upgrade is an extra $150 to $350 per window (Modernize Illinois, Isabella Andersen, reviewed by Catie Bonner, March 18, 2026). This page publishes no U-factor for any particular glass package, because no source we read measures one. What it does instead is explain what the two numbers mean, what is inside the unit, and exactly what to make a quote state in writing.

This page sits under the materials and glass hub. The code itself, quoted at length with its scope and its exceptions, is on the energy code page. The standard every figure here is held to is on our methodology page.

The two numbers the city already decided for you

Almost every glass conversation a homeowner has starts in the wrong place, with panes and gas and coatings, which are inputs. Chicago starts with outputs, and outputs are checkable. Three values govern a residential building up to four stories above grade.

What is measured	Chicago maximum	Where it is written
Window (fenestration) U-factor	0.30	Table R402.1.2, repeated in Table R402.1.3
Glazed fenestration SHGC	0.40	Same two tables. Footnote b: "The SHGC column applies to all glazed fenestration"
Skylight U-factor	0.55	Same two tables. The same footnote excludes skylights from the window column

Read in SO2022-2008.pdf, the ordinance adopting the 2022 Chicago Energy Transformation Code, hosted on chicago.gov, 2026-08-11. Chicago is climate zone 5A (Section C301.1). The scope sentence reads: "The residential provisions of the Chicago Energy Transformation Code apply to Group R-2, R-3, R-4, and R-5 occupancies with no more than four stories above grade plane."

What each number is actually telling you

• U-factor is the rate heat escapes. Lower is better. In a city with a long heating season this is the number that governs your winter, your draught complaints and the cold surface you feel when you sit near a window. It is a rate, not a total, so it says nothing on its own about a fuel bill.
• SHGC is the share of the sun's heat the window admits. Lower means less solar heat indoors. That helps a west-facing room in August and costs you free warmth in January, which is the trade nobody selling you a window tends to describe as a trade.
• Both are written against the assembly, not the glass. Table R402.1.2 is titled "MAXIMUM ASSEMBLY U-FACTORS AND FENESTRATION REQUIREMENTS". An assembly is the frame and the glass together. A number quoted for the glass alone does not answer the question the code asks.

The solar cap is the part that surprises people

Plenty of national advice for cold climates says to buy higher solar gain on south-facing glass and let the winter sun do some of the heating. Chicago's prescriptive table does not offer that option. Footnote b states that the SHGC column applies to all glazed fenestration, which means the 0.40 cap sits on every glazed opening in the house, north elevation included. That reading is ours, drawn from the footnote, and we label it as ours rather than as a quotation.

What is actually inside a modern window

Everything below is general building science, described so you can follow a quote. None of it is a Chicago measurement and none of it is a figure from any source we read. Where a number would be needed to make an argument, this page does not have one and does not invent one.

Part	What it is	Why it matters in a cold climate
Panes	Two or three sheets of glass held apart in a sealed unit	Each added pane adds another still cavity and another surface that can carry a coating. It also adds weight, which the sash and the hardware have to carry
Cavity	The sealed gap between panes	Still air or gas insulates. The gap is engineered to a width that limits circulation inside it, which is why a wider gap is not automatically a better one
Gas fill	Air, or a denser inert gas such as argon or krypton	A denser gas conducts heat less readily and moves less inside the cavity. It is a refinement of the cavity, not a different mechanism
Low-E coating	A microscopically thin metallic layer on a surface facing into the cavity	Passes visible light, reflects long-wave infrared. Which surface carries it decides whether it favours keeping room heat in or solar heat out
Edge spacer	The strip holding the panes apart at the perimeter, carrying the seal	It bridges inside to outside all the way round the glass, so what it is made of is a real heat path. It is also where the seal lives, and the seal is what fails

General building science, labelled as general. No source read this session measures any of these by product, so no performance figure is attached to any row.

Low-E coatings, without the invented numbers

A low-emissivity coating is the single biggest change to window glass in the last few decades and it is also the part most often sold with numbers nobody can trace. Here is what can be said without one. The coating reflects long-wave infrared while staying largely transparent to visible light, so it sends heat back toward the side it came from. Nearer the inner pane it works mostly on room heat trying to leave. Nearer the outer pane it works mostly on solar heat trying to enter. Manufacturers sell several coating recipes tuned to different balances of those two jobs, often marketed by name.

What we will not do is rank those recipes for a Chicago winter, because doing so honestly needs measured U-factor and SHGC values by coating and we read none. What you can do instead is make the specification concrete. Ask which coating is on the unit, ask which surface it is on, and then ask for the rated whole-unit U-factor and SHGC, which is the pair Chicago actually regulates.

Gas fill and the spacer, in plain terms

Argon is denser than air and conducts heat less readily, which is the general reason it is used in a sealed cavity. Krypton is denser still and costs more, which is why it tends to appear in narrow cavities where there is no room for a wider argon gap. Those are gas properties, not Chicago measurements. We found no attributable source for what either is worth in U-factor, so no such figure appears here, and none for how quickly a sealed unit loses its fill, so no leak rate appears either.

The spacer gets less attention than it deserves. It runs the full perimeter of the glass and connects the inside face to the outside face, so it is a continuous heat path around the edge of every unit in the house. Less conductive spacer materials exist and are usually sold as warm-edge. We publish no performance figure for any of them. The spacer also carries the seal, and when that seal fails you get fogging or a haze between the panes that cleaning cannot touch. That is the classic end-of-life signal for a sealed unit, and it is a single-unit problem before it is a whole-house one. What to do about one failed unit is a cost question, and it is worked through on repair against replace.

Double against triple, decided honestly

This is the decision the page exists for, so here is the whole of what we can and cannot tell you, in one table.

Question	What is settled	What we cannot tell you
Does the code force the third pane?	No. The requirement is 0.30 on the assembly and it names no pane count (Tables R402.1.2 and R402.1.3, read 2026-08-11)	Nothing. This one is fully answered
What does the upgrade cost?	An extra $150 to $350 per window (Modernize Illinois, Andersen, reviewed by Bonner, March 18, 2026, read 2026-08-11)	Whether your quote follows that band. It is a market observation for Illinois, not a schedule and not a Chicago-only figure
What does it save?	Nothing we can publish	Any saving, payback period or bill figure. No source we read publishes one for Chicago
Is it quieter?	Nothing we can publish	Any sound rating by pane count. Noise performance depends on pane thickness, spacing and asymmetry, and we read no measured figures
Does it change the window itself?	Yes. A third pane adds weight and depth, which the sash, the hardware and sometimes the opening have to accommodate	How much, for your units. That is a manufacturer specification question for the exact product

Two things follow from that table. First, if a salesperson tells you the third pane is needed to meet Chicago code, that is checkable and it is wrong: the code sets 0.30 and 0.40 on the assembly and says nothing about panes. Second, if you want the third pane anyway, that is a legitimate purchase and you should price it as an option rather than accept it bundled. Across 12 openings the sourced band works out at $1,800 to $4,200 on top of the job, which is arithmetic on that range rather than a separate figure. Against the same source's Chicago vinyl bands of $650 to $1,200 for a standard window and $700 to $1,250 for an energy-efficient one, that is a meaningful share of the bill and it deserves its own line.

All four price bands from Modernize, "Window Replacement Cost and Pricing in Illinois (2026)", Isabella Andersen, reviewed by Catie Bonner, updated March 18, 2026, read 2026-08-11. Market observations, not published fees. Full attributed price tables are on cost by frame material.

The requirement applies even when nobody checks

This is the part of the glass decision most homeowners never hear, and it changes how the contract should be written. Chicago publishes that a building permit is not required for "Repairing or replacing (in-kind) an exterior window or skylight in the existing opening" in a residential building up to four stories above grade. On the same page it publishes the sentence that settles what that does not mean: work performed without a permit "must still comply with all applicable requirements of the Chicago Construction Codes", and the section allowing it "does not waive any applicable requirements of the Chicago Construction Codes or of the Municipal Code other than the requirement to obtain a building permit".

Where a permit does apply, the city says it a second way. The Express Permit Program terms for exterior window and door replacement read: "Replacement windows and doors installed under this permit must meet the U-value and solar heat gain requirements of the Chicago Energy Transformation Code."

Our reading, labelled as ours. On a permit-exempt job no inspector arrives, no plan review happens, and nobody from the city ever looks at the glass that went in. The requirement is real and the enforcement is absent. That means the rated numbers written into your contract are where compliance actually gets settled, and they are the only record that will exist. Get them in writing before anything is ordered, not after.

City of Chicago, work not requiring a building permit, exterior, and Express Permit Program terms and conditions for exterior window and door replacement. Both read 2026-08-11. The permit rules in full are on the permit page, and landmark property is a separate question worked through on the landmark districts page.

You may not need a new glass package at all

This site is paid a referral fee when a homeowner asks to be matched with a window contractor, which is set out in full on how we make money. So it is worth being blunt about the cases where the glass is not your problem and a new glass package is not the fix.

• The code never requires you to touch a window you have left alone. Chicago's revised R501.1.1 reads: "Except as specified in this chapter, this code shall not be used to require the removal, alteration or abandonment of, nor prevent the continued use and maintenance of, an existing building or building system lawfully in existence at the time of adoption of this code. Unaltered portions of the existing building or building system shall not be required to comply with this code." A single-glazed 1920s sash you leave in place is lawful. Nobody can cite the energy code at you as a reason you have to act.
• Storm windows are outside both rulebooks. The code's exception list names "Storm windows installed over existing fenestration" as an alteration that does not have to meet new-construction requirements, and the city separately lists installing, maintaining or removing a storm window as work needing no building permit in any building at any height. That makes a storm the one improvement here that sits outside the permit rule and outside the performance numbers. It is worked through on the storm windows page.
• Surface-applied window film is exempt by name too, where it is applied to reduce solar heat gain and the code does not otherwise require the assembly to be replaced. Both exemptions carry the same condition, "provided that the energy use of the building is not increased".
• A draught you can feel is often not the glass. Air moving past a sash, a failed weatherstrip, an unsealed perimeter or an open weight pocket are all frame and installation problems. A better glass package does not close any of them, and a new window installed badly can reproduce every one. If the complaint is moving air rather than a cold surface, chase the leak before you chase the U-factor.
• One fogged unit is one unit. A failed seal in a single insulated glass unit is evidence about that unit. Repairing damage to an existing window and replacing glass in an existing sash are both on the city's no-permit list in any building. Neither is proof the house needs replacing.

R501.1.1 and the exception list read in SO2022-2008; the permit exemptions read at chicago.gov. All 2026-08-11.

Turning all of this into a purchase specification

The reason the numbers matter is that they make two quotes comparable. Two prices for two different performance levels are not a price comparison at all, and that is the most common way a window bid gets misread. Take this list to the sales visit.

Ask this	Why it matters	What a usable answer looks like
What is the rated U-factor and SHGC of the exact unit you are quoting?	These are the two values Chicago caps, at 0.30 and 0.40, for a residential building up to four stories	Two specific numbers, per product, written on the quote. Not a range and not a brochure
Is that the whole-unit number or the glass alone?	Table R402.1.2 is titled "MAXIMUM ASSEMBLY U-FACTORS AND FENESTRATION REQUIREMENTS", so the assembly is what is regulated	An explicit statement that the figure is for the complete window, frame included
Does every opening get the same unit?	Elevations, sizes and shapes often get different products in the same job, and a bay or a shaped head may differ	A schedule by opening, or a plain statement that one product covers all of them
Which coating, on which surface, and what gas?	It pins the specification so the unit delivered matches the unit priced	A named coating and fill on the quote, not "energy-efficient glass"
Is triple glazing priced as a separate line?	The sourced upgrade band is $150 to $350 per window, so bundling it hides a real sum	An itemised option you can accept or decline without renegotiating the job
What does the written warranty cover on the seal?	The seal is the part that fails, and the remedy is what you actually own later	A written document naming the term and what happens on a failed unit

The 0.30 and 0.40 caps and the table title read in SO2022-2008 on chicago.gov, 2026-08-11. The upgrade band from Modernize Illinois, Andersen, reviewed by Bonner, March 18, 2026.

One more line worth adding to the contract, and it costs nothing: a sentence stating that the units supplied will meet the U-factor and solar heat gain requirements of the Chicago Energy Transformation Code. The city puts exactly that obligation on permitted work in its own terms. On an exempt job, writing it in yourself is the only version of it that will exist.

What we could not confirm, and will not guess

• Any U-factor or SHGC by glass package. No source we read measures performance by pane count, coating, gas fill or spacer, so this page attaches no number to any of them. The only performance figures here are the code maximums.
• What argon or krypton is worth, and how fast it leaves. Both circulate as numbers and neither traced to a source we could read.
• Any saving or payback for a better glass package. Nothing attributable was read for Chicago, so nothing is published, including the arithmetic that would make one look precise.
• How long a sealed unit lasts. Service-life figures for insulated glass are everywhere and none we found named a source. The written warranty is the number you can actually hold someone to.
• Whether a glass-only replacement has to meet 0.30 and 0.40 in Chicago. The exception list names storm windows and window film. Glass-only work is not named, and absence from a list is not an answer. Ask the Department of Buildings.
• Who certifies a window's rated numbers under Chicago's code. We read the maximums. We did not read a Chicago provision naming a certifying body, so we state none and you should ask whose rating is on the label.
• The values for a building over four stories. Those live in commercial Table C402.4, which did not survive text extraction from the ordinance PDF. No number for tall buildings appears anywhere on this site. Your association declaration is a separate question, and often the binding one.
• The suburban numbers. Illinois preempts local residential energy standards and carves out Chicago by population, so Oak Park, Evanston and Berwyn are generally on the state standard. We have not read the state values at source and do not publish them. Ask your village or city building department.

On federal tax treatment, the programme is the Energy Efficient Home Improvement Credit, administered by the Internal Revenue Service. This site publishes no amounts, caps, percentages or eligibility rules for it, because those belong to the agency and they change. Read it at the agency: the IRS page for this programme, and see financing and rebates for what we did and did not confirm about programmes generally.

Where to go next

• The Chicago energy code page, which quotes the alterations rule, the exception list and the scope provisions in full, and explains why a replacement window is subject to them in four published steps.
• The materials and glass hub, which sets the frame decision beside this one and links every page in the pillar.
• Storm windows, the one improvement here that Chicago exempts by name from both the permit requirement and the performance numbers.
• What is actually in a window quote, for reading two bids against each other once you have the rated numbers on both.

Sources

All read 2026-08-11: the adopting ordinance SO2022-2008, for Tables R402.1.2 and R402.1.3 and their footnote b, the R402.1.2 table title, the residential scope sentence, Section C301.1, 14N-R5-R503 with its exception list, and 14N-R5-R501; the Chicago Energy Conservation Code page; the Express Permit Program terms for window and door replacement; work not requiring a building permit, exterior; and 20 ILCS 3125 at the Illinois General Assembly. Price bands from Modernize, "Window Replacement Cost and Pricing in Illinois (2026)", Isabella Andersen, reviewed by Catie Bonner, updated March 18, 2026, named with its author and date rather than linked. No competitor page, contractor blog, manufacturer brochure or search summary informed any statement above.

If you want quotes you can hold against the two numbers on this page, the form below passes your details to an independent licensed window contractor working in your part of the city. What that relationship is, and is not, is set out on the about page.

Straight answers

Questions Chicago homeowners ask

Does Chicago require triple pane windows?

No. For a residential building up to four stories above grade, Chicago caps a replacement window at a U-factor of 0.30 and a solar heat gain coefficient of 0.40, and it allows a skylight a maximum U-factor of 0.55. Those three values are read from Table R402.1.2 in the ordinance adopting the 2022 Chicago Energy Transformation Code, and the same three repeat in Table R402.1.3, which is the cross-check on the reading. Nothing in those tables names a number of panes, a coating, a gas fill or a spacer. The requirement is a performance target on the finished assembly, so any unit that reaches it complies whether it has two panes or three. Read in SO2022-2008.pdf on chicago.gov, 2026-08-11.

What is the difference between U-factor and SHGC?

U-factor measures how fast heat escapes through the window. Lower is better, and in a heating climate it is the number that governs your winter. Solar heat gain coefficient measures the share of the sun's heat the window lets through. Lower means less solar heat indoors, which cuts summer overheating and also cuts the free winter warmth a south-facing window collects. Chicago caps both, at 0.30 and 0.40, and the cap on solar gain applies to every glazed opening in the house. Footnote b to Table R402.1.3 reads: "The fenestration U-factor column excludes skylights. The SHGC column applies to all glazed fenestration." Read in the adopting ordinance on chicago.gov, 2026-08-11.

How much more does triple glazing cost in Chicago?

The only attributable figure we found puts the upgrade at an extra $150 to $350 per window (Modernize, "Window Replacement Cost and Pricing in Illinois (2026)", Isabella Andersen, reviewed by Catie Bonner, updated March 18, 2026, read 2026-08-11). That is an Illinois figure rather than a Chicago-only one, and it is a market observation rather than a published fee. Multiply it out before you decide: across 12 openings it is $1,800 to $4,200 on top of the job, which is arithmetic on that sourced range and not a separate figure. For scale, the same source puts a standard vinyl window in Chicago at $650 to $1,200 installed and an energy-efficient one at $700 to $1,250. What no source we read publishes is the saving on the other side, so we publish no payback period.

Is argon in the sealed cavity worth paying for?

We cannot price it for you, and we will not pretend otherwise. Argon is denser and conducts heat less readily than air, which is the general reason it is used in a sealed glazing cavity. That is gas physics, not a Chicago measurement. What we did not find at any source we could read is what argon is worth in U-factor, how fast it leaves a unit, or what it costs as a line item. So the honest move is to stop arguing about the fill and ask for the rated U-factor and SHGC of the exact unit being quoted, with and without the option. Chicago writes its requirement against the finished assembly, so the assembly number is the only one that answers the question.

What does a low-E coating actually do?

A low-emissivity coating is a microscopically thin metallic layer applied to a glass surface inside the sealed cavity. It stays largely transparent to visible light while reflecting long-wave infrared, so it pushes heat back toward the side it came from. Which surface it sits on changes what it favours: nearer the inner pane it works on keeping room heat in, nearer the outer pane it works on keeping solar heat out. That is general optics and it is not a measurement we read for any product. This page publishes no U-factor or SHGC by coating type because no source we read measures one. Ask which coating is specified, then ask for the whole-unit rated numbers in writing.

If I only replace the glass and keep the sash, do the code numbers apply?

We do not know, and this is a genuine gap rather than a hedge. Chicago lists "Replacing glass in an existing window sash or door frame" as work needing no building permit in any building, read at chicago.gov 2026-08-11. Separately, the code's exception list names two alterations that do not have to meet new-construction requirements, "Storm windows installed over existing fenestration" and surface-applied window film. Glass-only replacement is not named there. So the permit answer is published and the performance answer is not. Ask the Department of Buildings for your scope and get the answer in writing. For contrast only, the Illinois statewide provision at 20 ILCS 3125/20(c) does exempt "glass-only replacements in an existing sash and frame", but that is the state rule, and Chicago sets its own.

Will a lower U-factor than 0.30 pay for itself?

No source we read publishes a Chicago payback period, a heating-bill saving or a comfort measurement for any glass package, so this site publishes none. What we can say is what the number is and is not. It is a compliance floor at 0.30 for a residential building up to four stories, so a lower number is you buying something the city does not require. That can be a perfectly good purchase for comfort at the glass, for noise, or for condensation behaviour, and none of those is an energy calculation. Buy it as the thing it is, get the rated number written into the contract, and treat any quoted saving without a source the way you would treat any other unsourced number.