What causes fog between window panes?

A failed perimeter seal on the insulated glass unit: the sealed cavity breathes in humid air as temperatures cycle, the desiccant saturates, and moisture condenses on the inner glass surfaces where nothing can wipe it.

Can a fogged window be fixed without replacing the whole window?

Usually: the insulated glass unit (the sealed double-pane sandwich) replaces within the existing frame and sash on most windows, at a fraction of full window replacement.

Is defogging service worth it?

Defogging drills and clears the visible moisture, improving appearance, but it cannot restore the lost gas fill or insulating performance. It is a cosmetic option, priced and chosen accordingly.

Do failed seals mean the whole window is bad?

No: seal failure is a glass-unit event, and frames, sashes, and hardware may be fine. Multiple units failing together, though, dates the originals and starts the whole-window conversation.

Failed Window Seals: Fog Between Panes

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Fog, haze, or dirty-looking film between the panes of a double-pane window, unwipeable from either side, is the signature of one specific failure: the insulated glass unit’s perimeter seal has let the outside in. Condensation between window panes is the window’s own defect, distinct from the surface condensation that reports house humidity, and it has its own mechanism, timeline, and repair menu. This reference covers all three.

The mechanism: a sealed sandwich breathing

An insulated glass unit (IGU) is two panes (or three) bonded to a perimeter spacer, the cavity between them sealed, filled with air or argon, and kept dry by desiccant packed in the spacer. The assembly insulates because the trapped gas layer resists heat flow, and it stays clear because the desiccant holds the cavity’s humidity at nothing. The failure is mechanical fatigue with a pumping engine: every day the cavity’s gas warms and expands, cools and contracts, flexing the seal, solar pumping, and over years, faster on hot sun-side exposures, at altitude changes, and where sashes flex or water sits against the seal edge, micro-paths open. The unit inhales humid air each cycle, the desiccant absorbs it until saturated, and then the moisture appears where it must: condensing on the cavity’s glass surfaces in cool weather, first as intermittent morning fog that burns off, then persistent haze, and finally the mineral tracks and white ghosting of years of condensation cycles, permanent etching no repair fully erases. The timeline is honest: units carry seal warranties of 10 to 20 years, failures cluster past year 15, and the skylight version runs the same script overhead with harder sun.

What is lost besides the view

The fog is the visible loss; the invisible one came first. Argon-filled low-e units lose their gas fill through the same breach, well before fogging, so a failed unit has already surrendered a real share of its insulating performance, the window running colder in winter, which circles back as more surface condensation on the room side and a colder seat by the glass. Failed units also mark their siblings: IGUs from the same production era on the same house share materials, exposure, and age, so the second and third failures usually follow the first within a few seasons, which is why the repair math below often gets computed house-wide rather than pane by pane.

IGU (glass-unit) replacement is the standard answer: the sealed sandwich replaces within the existing sash and frame on most window types, measured by a glass shop and swapped in under an hour per opening, at a fraction of full window replacement, with the low-e and argon specs re-ordered to match or better the originals. It is the right call where frames and sashes are sound, and the moment to check them: rot probed at sills and sash bottoms, hardware and weatherstripping serviced while the glass is out. Defogging services drill the unit, flush or evaporate the moisture, and install one-way valves: appearance improves, often dramatically, but the gas fill and full insulating value do not return, making it a legitimate cosmetic tier for view-critical or hard-to-source units, sold honestly or not at all. Full window replacement wins where the failure is one item on a longer list, rotted frames, failing hardware, single-pane siblings elsewhere, or a house-wide failure cluster on builder-grade units, at which point the storm-window-versus-replacement economics and the flashing-integration rules of the window installation reference take over the decision. Warranty first, always: seal failures are the exact subject of glass warranties, transferable on some brands, and a manufacture date etched in the spacer or frame sticker is worth reading before any invoice.

The bottom line

Between-pane fog is the seal’s obituary: the cavity breathed, the desiccant filled, and the argon left before the fog arrived. Check the warranty and the date, replace the glass unit where the frame deserves it, treat defogging as cosmetics with honest labeling, and let a cluster of failures reopen the whole-window question, because IGUs age as a generation, not as individuals.