Every January we get the same worried calls: "My windows are sweating — do I need new ones?" Sometimes the answer is genuinely no. Condensation is warm, humid indoor air touching a surface cold enough to wring the moisture out of it, and in a Minnesota cold snap even good glass gets cold.
The diagnostic question is simple: where is the moisture? Inside surface, between the panes, or on the frame itself — each location tells a different story.
Condensation on the inside surface: usually humidity
If you can wipe the moisture off with a towel, it's surface condensation. During a -20°F stretch, some fogging at the edges of even brand-new glass is normal physics — the edge of the glass unit is its coldest point.
But if your windows drip all winter at normal temperatures, your indoor humidity is too high for the glass temperature. Aim for roughly 30–40% relative humidity in winter, lower during deep cold snaps:
- Run bath and kitchen exhaust fans during and after use — and confirm they vent outside, not into the attic.
- Turn down your furnace humidifier as outdoor temperatures fall.
- Keep blinds and curtains cracked open so air circulates against the glass.
- Store firewood outside; a cord of wood releases gallons of water as it dries.
Fog between the panes: the seal has failed
Insulated glass units are sealed sandwiches — two panes, a spacer, and (in good windows) argon gas. When the perimeter seal fails, humid air gets pumped in and out with temperature swings, and eventually you see fog, droplets, or white mineral staining between the panes where no towel can reach.
This is permanent, and it means the unit's insulating gas is long gone too. The fix is replacing the glass unit or the window, depending on age and condition — we cover the decision in detail in our guide to foggy windows and failed seals.
Frost on frames and sashes: air leakage
Moisture that forms as frost or ice on the frame, sash edges, or locks — not the glass — is a different problem. That pattern means cold outdoor air is leaking through or around the window and freezing the indoor humidity it meets.
Common culprits, roughly from cheapest to most involved:
- 1Worn or compressed weatherstripping that no longer seals the sash.
- 2A sash that isn't latching fully — locking a window pulls it tight; an unlatched window leaks.
- 3Failed exterior caulking letting wind into the frame cavity.
- 4Missing insulation around the rough opening — common in older installs where the gap was stuffed with a little fiberglass or nothing at all.
When condensation means it's time to replace
A window has reached the end of its useful life when the fixes stop making sense: failed seals in multiple units, frames that leak air no matter how the humidity is managed, rot in sills or sashes, or single-pane glass that turns into a frost sheet every winter.
Modern replacement windows with low-E glass, warm-edge spacers, and proper foam-insulated installation run far warmer interior surfaces — which means dramatically less condensation at the same indoor humidity, plus the energy savings that come with it.
Not sure which category your windows fall into? We'll tell you straight at a free in-home assessment — including when a $40 humidity fix beats a window order. JMB Builders serves the entire Twin Cities metro: 763-204-2099.
Frequently asked questions
Is window condensation in winter normal?
Some surface condensation during extreme cold is normal, especially at the glass edges. Condensation that appears at ordinary winter temperatures, drips onto sills, or shows up between the panes is not — those point to high indoor humidity, air leakage, or a failed glass seal respectively.
What indoor humidity should I keep in a Minnesota winter?
A practical target is 30–40% relative humidity, dropping toward 25–30% when outdoor temperatures fall below zero. If your windows sweat at those levels, the glass surface is running too cold — typically a sign of older, poorly insulating windows.
Can a failed window seal be repaired?
The seal itself can't be repaired, but the insulated glass unit can often be replaced without replacing the whole window if the frame and sash are in good shape. On older or builder-grade windows, full replacement usually makes more financial sense than chasing glass units one by one.
Why do only my north-facing windows get condensation?
North walls get no solar warming, so that glass runs coldest all day and condenses first. It's the same physics as the rest of the house — those windows are just the canary. If they're also older units, they're the natural place to start a phased replacement.
