Window condensation forms when moist indoor air touches a cold window surface and hits its dew point. The fastest fixes are lowering indoor humidity (turn down the humidifier and run bath and kitchen exhaust fans) and letting warm room air reach the glass (open blinds and drapes on cold nights). The ten fixes below solve the vast majority of cases. For the stubborn ones, keep reading. Mark Meshulam is an expert witness and consultant for window condensation, and he has been drying out windows for four decades.
First, Figure Out Where the Water Is
Not all window condensation is the same problem, and the location tells you which article you need.
- On the room-side surface of the glass or frame: classic interior condensation. This page is for you.
- Between the panes of glass, where the window looks permanently foggy and no amount of wiping helps: that is not condensation you can manage with humidity. It is insulated glass seal failure, and the glass unit needs replacement.
- On the outside of the glass on cool, humid mornings: congratulations. Exterior dew is usually a sign your glass insulates so well that the outer pane stays cold. It evaporates as the day warms and needs no fix at all.
Still not sure whether you are looking at condensation or a leak? Start with My Windows Leak! for the full diagnostic path.
The Day Chicago Froze (and Every Window Cried)
Here is what happened after one of Chicago’s typical cold snaps. Temperatures dropped from an average of 30°F to almost 0°F in a single day, and Windy City winds pushed the wind chill to minus 20°F.
Our bodies endured the new arctic reality with shock. The forgotten pastime known as shivering violently was revisited like a psychoanalytic catharsis. Heavy coats, hats, gloves and scarves came from nowhere to engulf our citizenry. A hopeful few sprayed cans of deodorant skyward, hoping to accelerate global warming.
Our buildings reacted to the first big chill with shock, too. Windows that had behaved well for months were unveiled that morning, looking like scenes from a winter wonderland, coated with frost, ice, and beads of window condensation. How did our world go topsy-turvy so quickly? Why was it raining and snowing inside our homes?
The answer is science, my friends. The result of a simple equation with only four variables:
- Exterior temperature
- Interior temperature
- Interior humidity
- Thermal performance of the windows
Push any of these variables beyond their designed limits, and sooner or later condensation, then frost, will appear on your windows. Interestingly, we control three of the four. Exterior temperature alone is in the hands of the Almighty.
What Is Condensation, Exactly?
Condensation occurs when water vapor (a gas) in the air undergoes a temperature drop, such as when moist air moves across a very cold window. As the vapor cools, it reaches its dew point, the temperature at which vapor turns back into liquid water. That liquid collects on the cold surface, and it keeps accumulating as more moist air circulates past the glass.
Interior temperature matters because room-side heat flowing across the window warms its surface above the dew point, and condensation stops forming (and what is there evaporates). This only works if warm air can actually reach the glass. Blinds and curtains block that heat, and in extreme cold they must be left open.
Interior relative humidity (RH) matters even more because the higher the humidity, the higher the dew point. Holding room temperature at 70°F: at 25% RH, condensation forms on a surface at 32°F. At 95% RH, it forms at 68°F. If you want drier windows, dry the air.
How Much Indoor Humidity Is Safe in Winter?
For year-round health and mold prevention, the U.S. EPA recommends keeping indoor relative humidity between 30 and 50 percent. In a deep cold, even 30 percent can be too much for your windows. Industry guidance at a 70°F room temperature:
| Outside temperature | Recommended maximum indoor RH |
|---|---|
| +20°F to +40°F | 40% |
| +10°F to +20°F | 35% |
| 0°F to +10°F | 30% |
| -10°F to 0°F | 25% |
| -20°F to -10°F | 20% |
| Below -20°F | 15% |
With that introduction, here are…
Top 10 Fixes for Window Condensation
Fix #1: Control the humidifier
As the outside temperature goes down, so should your humidifier setting. Use the table above as your dial. If your humidifier has no humidistat, get one that does, or add a standalone hygrometer so you are adjusting from data instead of vibes.
Fix #2: Use bathroom exhaust fans
Showers pump a lot of moisture into the air all at once. Run the fan during and for a half hour after the shower to expel the moisture before it reaches the windows. (At this point we are NOT recommending going unwashed.) A revelation on this point: if privacy permits, crack the door open to give the fan an air supply, and it becomes far more effective. If that is not practical, cut a louver into the door or trim 3/4 inch off the bottom. Door undercuts are a time-honored way to let air move between rooms.
Fix #3: Use kitchen exhaust fans
While we recognize the occasional need to eat, cooking dumps substantial humidity into the air in a sudden way. Run the exhaust fan during and shortly after cooking. If you have a “ventless” range hood, do not kid yourself: it will not help with this problem. It just returns the moisture to the living space.
Fix #4: Vent the laundry, and beware “ventless” dryers
Washers and dryers add moisture, so run exhaust fans if you have them, or crack a window. Always duct the dryer to the exterior. If you own a ventless dryer, make sure its condenser captures the water and sends it to a drain, not into your air.
Fix #5: Direct heat to the window
Best practice puts the heat source, a baseboard heater or supply register, beneath the window to warm it. Sadly, most buildings do not fully implement this idea, and even where they do, a deep window sill can block the warm convective airflow from reaching the coldest part of the window, the bottom.
A corollary to this postulation: Arrange window treatments so warm air can flow between the drapes and the glass. One of the grand ironies of physics is that draperies keep heat away from a window but not moisture. Curtains are insulators, not vapor barriers. A phenomenon called vapor drive keeps pushing moisture into that space, just so you can open your drapes in the morning and find penguins living there.
If there is no heat source at the base of the window, add a baseboard or space heater below it. Keep flammable materials well away from any heat source.
Fix #6: Be aware of construction moisture
Paint, drywall joint compound, fresh lumber, and new concrete keep releasing moisture into living spaces for a long time. A building can take a year or more to reach its service equilibrium, so do not panic when windows frost up right after an expensive renovation.
A related twist: retrofit shiny new energy-efficient windows into an old building, and they sometimes fog up immediately. The old leaky windows were exhausting moisture straight through their gaps, keeping indoor humidity artificially low. The new windows tightened the envelope and trapped the humidity. Education and HVAC adjustment usually fix this.
Fix #7: Improve window U value
Non-thermally-improved windows with single glazing just will not cut it anymore. Beyond paying to heat your local suburb, poorly insulated glass tolerates very little cold or humidity before sweating. Improve insulating value by replacing the windows or by adding a tightly fitted interior or exterior storm window. Interior secondary glazing (an interior storm window) works extremely well for improving U factor and cutting cold air infiltration, and is particularly suited to schools, hospitals, offices, historic buildings and curtain wall buildings.
Fix #8: Tighten window air infiltration
With all the talk of R values and U factors, poor little air infiltration gets neglected. A window that leaks excessive air wastes most of your insulating efforts, and that same cold breeze chills the interior surfaces and creates concentrated spots of condensation and frost. On a cold, windy day, run your hand around the window perimeter and hunt for moving air. Seal breezy joints with caulk. At operable portions, replace gaskets or add foam blocks. For the full treatment, see our guide to fixing window air infiltration in cold, drafty windows. Or call us and we will do it.
Fix #9: Fix the perimeter caulk
Breaches in the exterior perimeter caulk deliver cold air to the interior plane of your window, exactly where you do not want it. Make sure the outside caulk is doing its job before winter does its worst.
Fix #10: Understand the wall cavity
If you were Superman and had X-ray vision, you could get rich and hire someone to read this article for you. You could also look inside the walls around your window and discover a startling fact: there may be empty space in there. Masonry walls almost always have a cavity behind the brick, and other wall designs have cavities too.
Huge amounts of cold air can circulate in that cavity, chilling your window frame or blowing around it into the room, bringing condensation and energy loss with it. Remedies we have implemented include injectable foam, retrofitting compartmentalization into walls, and sealing interior joints. If this sounds like your building, give me a call.
Two Bonus Fixes for Stubborn Cases
- Run a dehumidifier: When source control is not enough (hello, aquariums, plant collections, and families of six), a properly sized dehumidifier in the problem zone drops RH directly. Empty it or plumb it to a drain.
- Balance your ventilation: Chronically humid, tight modern buildings often need mechanical fresh air. An ERV or HRV (energy or heat recovery ventilator) exhausts stale moist air and brings in dry outdoor air while recovering the heat, which is the grown-up version of cracking a window in January.
Is Window Condensation Bad?
Occasional light misting on a brutal morning is a nuisance. Persistent condensation is a warning. Water that sits on sashes and sills peels paint, rots wood, corrodes hardware, and feeds mold in the surrounding wall. Aluminum frames often show heavier condensation than wood because the frames run colder. If you see frost buildup, water pooling on sills, black staining, or a musty smell, stop wiping and start fixing, because leaks and mold in walls are far more expensive than any item on this list. And if the fog lives between the panes, that is seal failure, not humidity.
Condensation in High-Rises and Commercial Buildings
In large buildings, condensation is rarely a humidifier knob problem. We have been retained to investigate condensation complaints in luxury high-rise condominiums where the causes ran from ventilation code compliance and HVAC air balance to value-engineered window and curtain wall details and thermal bridging, all documented with instrument measurements and infrared thermography.
If your building has widespread or recurring condensation, unit-by-unit tips will not save it. That calls for a forensic investigation to identify the defects and often field testing to prove the mechanism. To go deeper on the diagnostic method, read Diagnosing Window Condensation Using Absolute Humidity.
FAQs
Why do my windows have condensation only in the morning?
Overnight, outdoor temperatures bottom out, and your glass reaches its coldest point while indoor humidity from evening cooking, showers, and breathing peaks. The moist air hits the cold glass, crosses the dew point, and you wake to wet windows. It often evaporates as the day warms. Persistent morning wetness means your humidity is too high for your glass.
Does condensation mean my windows are bad?
Not necessarily. Even excellent windows sweat if indoor humidity is high enough. The exceptions: fog between the panes always means the insulated glass seal has failed, and single-pane or non-thermal frames will condense at humidity levels that modern windows shrug off.
What indoor humidity should I keep in winter?
Between 30 and 50 percent for general health, stepping down as it gets colder outside, all the way to 15 to 20 percent in subzero weather. See the table above.
Will replacing my windows stop condensation?
Usually it helps, because better glass stays warmer. But a tighter new window can also trap humidity that used to escape through the leaky old one, which is why some buildings fog up right after an upgrade. Manage the humidity, and the new windows will perform.
Does condensation ever correlate with weather?
Absolutely. When Chicago temperatures crashed 48 degrees in two days, traffic to this very article rose roughly 500 percent as wet-windowed readers searched for answers. Sudden cold snaps are condensation’s favorite party trick; we charted it in this analysis.
Windows still weeping after all ten fixes, or is your whole building doing it? Book a free consultation with Façade Consultants or call 847-878-8922.
