Condensation on the interior surface means indoor humidity is too high for the glass temperature. Condensation on the exterior surface means the window is working well. Moisture between the panes means the insulated glass seal has failed and the unit needs replacing.
Window condensation is three completely different problems that look similar, and the fix for each is unrelated to the others. Where the moisture sits tells you which one you have, and that takes about five seconds to determine.
Wipe the interior surface. If it clears, the moisture was inside. Go outside and wipe the exterior. If it clears, it was outside. If neither wipe clears it, it is between the panes.
The three kinds
| Location | Cause | Fix | Urgency |
|---|---|---|---|
| Interior surface | Indoor humidity above the glass dew point | Lower humidity, improve glass | Address it, can cause mold |
| Exterior surface | Glass well insulated, colder than outdoor dew point | Nothing, this is good | None |
| Between the panes | Insulated glass seal failed | Replace the glass unit | Not urgent, but performance is lost |
| Only at the frame edges | Cold bridging or a perimeter air leak | Check insulation and weatherstripping | Address it |
Interior condensation: a humidity problem, not a window problem
This is the most common and the most misunderstood. Warm air holds more moisture than cold air, and when warm humid indoor air touches cold glass, the air right at the surface cools below its dew point and the moisture condenses out. The glass is not defective. It is doing what cold glass does. In a Capital Region winter, indoor relative humidity above roughly forty percent will condense on most double pane glass, and above thirty percent will condense on single pane. The fix is on the humidity side: run bathroom and kitchen exhaust fans and actually leave them on for twenty minutes after use, stop using a whole-house humidifier so aggressively, cover the sump or crawlspace, and make sure the dryer vents outside. Better glass raises the surface temperature and helps, which is why interior condensation often disappears after a window replacement, but the root cause is the moisture in the air.
Why interior condensation matters beyond the nuisance
- Water running off the glass collects at the sill, and a wood sill that stays wet through a winter rots.
- Persistent moisture at the sash and the trim supports mold growth, which is the more serious concern.
- Paint fails at the interior trim, which is usually the first visible sign that this has been going on.
- The same humidity condensing on cold glass is also condensing inside wall cavities at cold sheathing, which you cannot see.
- If condensation is only happening in one room, that room has a specific moisture source worth finding.
Target indoor humidity by outdoor temperature
A practical rule for a Capital Region winter: the colder it gets outside, the lower your indoor humidity has to be to avoid condensation. Around thirty degrees outside, forty percent indoor relative humidity is usually fine. Around ten degrees, thirty percent is closer to the limit. Below zero, you may need to go to twenty-five percent. This is why condensation appears suddenly during a cold snap on windows that were fine the week before. Nothing changed about the window. The dew point at the glass surface dropped. An inexpensive hygrometer tells you where you actually are.
Ranges reflect typical Capital Region projects and are for planning only. Every Sundog quote is written, itemized, and free.
Frequently asked questions
Why do my windows sweat on the inside?
Indoor humidity is higher than the glass surface temperature can tolerate. Warm humid air meets cold glass and the moisture condenses out. It is a humidity issue more than a window issue, though better insulated glass raises the surface temperature and reduces it. Bathroom and kitchen exhaust fans, and easing off a whole-house humidifier, are the first things to try.
Is condensation on the outside of my windows a problem?
No, the opposite. It means the glass is well insulated enough that the exterior surface stays cold, drops below the outdoor dew point on a cool morning, and collects dew the same way your car windshield does. It is a sign of good performance and it clears as the sun comes up.
Can foggy glass between the panes be fixed?
The seal cannot be repaired, but the insulated glass unit can be replaced while keeping your frame, sash and hardware, typically for $150 to $600 per sash. Defogging services that drill and vent the unit clear the visible fog but do not restore the gas fill or the seal, so the insulating performance stays degraded.
What indoor humidity should I run in winter?
It scales with outdoor temperature. Around forty percent is generally fine when it is thirty degrees out, thirty percent when it is around ten degrees, and lower still below zero. Get an inexpensive hygrometer, watch it during a cold snap, and adjust. Condensation appearing suddenly during cold weather is the signal that you are above the line.
Will new windows stop my condensation?
They will reduce it significantly, because better insulated glass with a warm-edge spacer runs at a higher surface temperature, which means the air has to be more humid before it condenses. They will not eliminate it if the underlying indoor humidity is genuinely too high, and in that case the moisture simply condenses somewhere else you cannot see, like inside a wall cavity.
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