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Windows are where a building meets the weather, and in Chicago the weather votes twice a year. They are also where a lot of showing-day worry lives: fogged glass, painted-shut sashes, drafts, and the quiet question of who pays when a condo tower needs four hundred new ones. This page gives you the vocabulary, a fair fight between old windows and new ones, and the truth about broken seals, which are almost never the crisis they look like.
Window people talk fast. Here is the whole dictionary in one place, and once you have it, every inspection report and every contractor quote reads clearly.
Sash. The moving part: the frame of wood or vinyl that holds the glass and slides or cranks. A double-hung window has two sashes, upper and lower, and both are supposed to move.
Frame, jamb, sill, casing. The frame is the fixed surround the sash rides in; the jambs are its sides, the sill is the sloped shelf at the bottom that sheds water outside, and the casing is the trim that dresses the whole thing, inside and out.
Glazing. The glass itself. Single-glazed means one pane, the vintage standard. Double-glazed means two panes sealed around a spacer with an insulating gap, sold as an insulated glass unit, or IGU. Triple exists, and matters more in Minnesota than here.
Low-e and argon. The two quiet upgrades inside a modern IGU: a microscopic metal coating that bounces heat back where it came from (in during winter, out during summer), and argon gas between the panes because it insulates better than air. You cannot see either one. Both are standard in decent replacement windows now.
Muntins and lites. The grid. Each pane of a divided window is a lite; the little bars dividing them are muntins. In vintage sashes the grid is real wood holding real separate panes, called true divided lites. In most modern windows it is a decorative grille between or on the glass. Neither is wrong, but they restore very differently.
Egress. The code word for get-out-in-a-fire. Bedroom windows must open big enough and low enough to climb through, and basement bedrooms usually need a dedicated egress window with a well. It is the first question on any garden-unit or attic-bedroom listing.
Storms and screens. A storm window is a second, removable layer outside (or inside) an old single-glazed window. A good storm turns a vintage single-pane window into a respectable performer, which is the whole argument for keeping original sashes.
Tap any part of either drawing; the panel explains it.
The city has a window named after it, and once you know it you will see it everywhere. The Chicago window is a wide opening with one big fixed pane in the middle, for light, flanked by two narrow double-hungs, for air. It was born downtown in the 1890s on the first steel-frame office buildings, when the new skeleton construction let architects open up walls that masonry had always kept small, and it spread from the Loop into courtyard buildings and storefronts across the neighborhoods.
Vintage buildings with grand window openings feel brighter than their square footage, and that is exactly what the Chicago window was invented to do. When you tour a greystone or courtyard vintage unit at midday and the rooms glow, thank an 1890s engineer.
Original wood windows in a Chicago two-flat or greystone can be a century old, and that is not automatically a strike against them. They were built from dense old-growth lumber that outlasts most modern wood, every part is repairable with a knife and a putty gun, and the classic rope-and-pulley weights hidden in the jambs are a simple machine that a handy person can service forever. A vintage sash that rattles is not dying; it is asking for weatherstripping.
The restoration path: free the sashes if paint has sealed them shut, replace frayed cords, re-glaze loose panes, add weatherstripping, and hang good storms. Done well, that package gets an old window surprisingly close to replacement-window comfort while keeping the wavy glass and deep sills that make vintage rooms feel vintage. In landmark districts it may also be the required path, since visible replacements need approval.
Paint on windows in anything built before 1978 should be assumed to contain lead, and windows are the worst offender in the house because sashes rub on every open and close, making dust. Repainting and restoration are done wet and careful, or by a certified crew. Our hazards guide covers it properly.
Replacement windows come down to two decisions: the material and the installation style. On material, vinyl is the budget default and varies wildly in quality, from flimsy builder-grade to solid; fiberglass costs more and moves less with temperature swings, which seals last longer around; wood-interior windows clad outside in aluminum are the premium look for vintage homes; bare aluminum, common in mid-century buildings, is strong but cold to the touch and sweats in January.
On installation, an insert (or pocket) replacement slips a new window inside the old frame: cheaper, faster, and it shrinks the glass a little while trusting that the old frame is sound. A full-frame replacement strips to the rough opening and starts over: more money, and the right call when sills are rotten or the old frames are out of square.
Every legitimate new window wears an NFRC sticker with two numbers worth reading. U-factor is how fast heat escapes, lower is better, and around here you want it comfortably under 0.30. SHGC is how much sun-heat comes through, and on big west-facing glass a lower number earns its keep every July afternoon.
Here is the one that spooks buyers at showings. A modern double-pane window is a sealed sandwich; the perimeter seal keeps the argon in and the moisture out. When that seal fails, and every seal eventually does, the insulating gas seeps away and humid air takes its place. The tell: fog, droplets, or a milky haze between the two panes, where no cloth can reach. In long-failed units the trapped moisture slowly etches the glass into permanent rainbow-tinted streaks.
Now the size of the problem. A dead seal is not structural, does not leak water into the wall, and does not mean the window unit is junk. It means that one glass sandwich lost its insulation and its looks. In many windows a glazier can replace just the IGU, the glass sandwich alone, inside the existing sash for a fraction of a full window, and no, you do not have to match your neighbors and replace all thirty.
Where the moisture sits tells you everything. On the room side of the glass: that is your home’s humidity meeting cold glass, a ventilation question, not a window failure. Between the panes: dead seal, per the above. On the outside on a mild morning: relax, that is a sign of a well-insulated window doing its job.
At a showing, scan for between-pane fog on the south and west faces first, since sun cycles age seals fastest. On an inspection report, count the failed units and treat them as a line-item credit conversation, priced per opening, not as a reason to fall out of love with the place.
Open every window you can reach. You are testing four things in ten seconds each: does it move without a fight, does it stay up on its own, do the locks and tilt latches work, and was it painted shut by the last decorator. Press a thumbnail into the outside sill wood where you can; soft means water has been winning. Look across every pane at an angle for between-pane haze. In vintage buildings, check whether the sash cords are intact or cut, and whether the storms are present or stacked in the basement.
In many Chicago condo buildings the windows are common elements or limited common elements, which means the association, not the unit owner, decides when they are replaced and how the bill lands. Before you write on a condo, have your attorney confirm who owns the windows in the declarations, and ask the 22.1 disclosure whether a window project or its special assessment is anywhere on the horizon. A tower-wide window job is exactly the kind of project that shows up in reserve studies and lending decisions; our glossary entry on non-warrantable buildings explains why lenders care.
Ranges, not promises, because size, material, and access move every number: freeing up and weatherstripping a vintage sash is a modest handyman line; a glass-only IGU swap for one fogged unit typically runs a few hundred dollars; quality insert replacements commonly land in the several-hundreds-to-low-four-figures per opening installed, with wood-clad, oversized, and masonry-opening work above that; and full-frame replacement of a whole vintage two-flat is a real project worth three quotes. The pattern that repeats: the cheapest vinyl bid ages the worst, and the fogged-glass fix is almost always cheaper than the showing-day panic assumed.
Keep reading the building: our building types guide tells you what windows a home was born with, buying something older covers the whole-house version of the restore-or-replace question, and the spotter’s guide puts fogged glass and painted-shut sashes on your walk-through checklist. Questions on a specific building, just ask us.