Condensation vs Humidity in Your Home
Wet windows are common.
They are also commonly misunderstood.
In most homes, wet or foggy windows are caused by condensation, not leaks and not rain getting inside. Condensation is a normal physical process that becomes visible when indoor moisture meets a cold surface.
This page explains what is happening and how to tell the difference between normal condensation and a real moisture problem.
What Window Condensation Is
Condensation happens when warm, moist indoor air touches a colder surface.
Windows are usually the coldest surface in a room. When the glass temperature drops below the air’s ability to hold moisture, water forms on the surface.
This is the same reason a cold drink “sweats” in summer.
Why It Happens More in Winter
Condensation is most noticeable in cold weather.
Common reasons include:
- Cold outdoor temperatures cooling the glass
- Warm indoor air holding more moisture
- Reduced ventilation when windows stay closed
- Everyday moisture from cooking, showers, laundry, and people
The colder the glass and the wetter the indoor air, the more condensation you see.
Why It Can Also Happen in Summer
Condensation can also appear during warm months.
This usually happens when:
- Indoor air is heavily air-conditioned
- Outdoor air is hot and humid
- Warm, humid air contacts cooler glass or frames
In summer, condensation often appears on the outside of windows. That is normal and usually harmless.
Condensation vs a Leak
It is important to separate condensation from actual water intrusion.
Signs of Condensation
- Water appears evenly across the glass
- Moisture comes and goes with weather or time of day
- No staining or damage around the window frame
- Wiping the glass removes the water
Signs of a Possible Leak
- Water appears after rain, not temperature changes
- Wetness is localized to one corner or edge
- Staining, bubbling paint, or soft wood is present
- Moisture persists even in dry weather
If water appears only on the glass and disappears when wiped, it is almost always condensation.
What Condensation Says About Indoor Humidity
Condensation is a symptom, not the problem itself.
It usually means:
- Indoor air contains more moisture than the cold window can handle
- Air movement near the window is limited
- The glass surface is significantly colder than the room
Condensation does not automatically mean your home is “too wet,” but repeated or heavy condensation is a sign that moisture levels deserve attention.
Why New Windows Can Still Condense
New or high-efficiency windows can still show condensation.
Reasons include:
- Better sealing traps indoor moisture
- Improved insulation elsewhere shifts cold spots to the glass
- Homes hold more moisture than older, drafty houses
Seeing condensation does not mean the windows failed.
When Window Condensation Becomes a Problem
Occasional condensation is normal.
Persistent condensation is not.
It becomes a concern when:
- Water pools on sills or frames
- Moisture is present daily for long periods
- Wood, paint, or drywall stays damp
- Mold or mildew appears nearby
At that point, the issue is no longer cosmetic. It is a moisture balance issue inside the home.
Summary
- Wet windows are usually caused by condensation
- Condensation happens when moist air meets cold glass
- It is common in winter and possible in summer
- Condensation is different from leaks
- Persistent condensation signals excess indoor moisture
Understanding the difference helps you avoid unnecessary repairs and focus on the real cause.