Condensation on windows means different things inside, outside and between the panes. Learn the cause of each, how to stop it and when it signals a problem.
In this article
Beads of water on the inside of the bedroom window on a cold January morning. A fog on the outside of the glass on a sticky August morning. A cloudy haze trapped between the panes that no amount of wiping will touch. All three are condensation on windows, but they have different causes, and only one of them means the window has failed.
Where the water sits is the clue. Once you know which side of the glass it’s on, you’ll know whether you need a towel, a bathroom fan or a closer look at the window itself.
What causes condensation on windows
Air always carries some water vapor. When it touches a surface colder than its dew point (the temperature at which the air can’t hold all that moisture), the vapor turns back into water, just like on a glass of iced tea on the porch in July.
In winter, the glass is usually the coldest surface in a room, so indoor moisture shows up there first. In summer, things flip: the outside of your glass can end up cooler than the humid air outdoors.
| Where you see it | What it usually means | What to do |
|---|---|---|
| Inside (room side) | Indoor air is too humid for how cold the glass is | Lower humidity and improve ventilation |
| Outside | Humid outdoor air met glass that cooled overnight | Nothing; it clears as the day warms |
| Between the panes | The seal on the glass unit has failed | Replace the sealed glass or the window |
Condensation on the inside of windows
Room-side condensation, often called window sweating, is mostly a cold-weather problem. Our mild winters keep it from being a daily battle here, but on the coldest mornings it shows up where moisture builds: bathrooms, kitchens, laundry rooms, and bedrooms where people have been breathing all night with the door closed.
It usually starts as a strip of droplets along the bottom of the glass, because the edges of a window tend to be its coldest surfaces on the room side.
A light fog that’s gone by mid-morning is more of a signal than a problem. Water that pools on the sill morning after morning is different: over time it can lead to peeling paint, mildew and eventually rot in wood sills and trim. The EPA advises drying condensation quickly, because, in its words, “the key to mold control is moisture control.”
How to stop window condensation on the inside
- Check your humidity. A simple hygrometer tells you where you stand. The EPA’s guide to mold and moisture recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent.
- Use your fans. Run the bathroom fan while you shower and the range hood while you cook, and make sure the dryer vents outdoors.
- Go easy on the humidifier. If you run one in winter, turn it down when the windows start to sweat.
- Let warm air reach the glass. Open blinds and curtains during the day, and leave a small gap at the bottom when they’re closed at night.
- Clear the sill. Houseplants on the windowsill add moisture right where you don’t want it.
If your humidity sits in a normal range and the glass still streams with water on cold mornings, the glass itself is probably too cold. Single-pane glass in particular runs much colder on the room side than modern insulated Low-E glass. That calls for better windows, not a bigger dehumidifier.
Condensation on the outside of windows
On a still, muggy morning in late summer or early fall, you may find the outside of your windows fogged over so thickly you can’t see the yard. It can be a surprise if your windows are new, but it’s usually a good sign.
Two things cool the outer pane. Overnight, glass gives off heat to a clear sky, which is why your lawn and car are wet too. On single-pane glass, air conditioning can also chill the glass from the inside. Once the outer surface drops below the dew point of the humid outdoor air, water condenses on it.
Energy-efficient glass can show this more often than old glass. Low-E coatings, argon and low U-factors slow the flow of heat from your rooms to the outer pane, so on a cool, clear night that pane can end up colder than the outdoor air, as glass maker Pilkington explains in a technical bulletin on exterior condensation. Our muggy summers add plenty of moisture, too. Atlanta building scientist Allison Bailes has written about condensation on the storm windows of his own house during a muggy week with dew points in the low to mid 70s.
Outside condensation is harmless. It isn’t a defect, it doesn’t mean anything is leaking, and it clears on its own as the day warms the glass.
Condensation between window panes
Fog or droplets between the panes is the one kind you can’t wipe away, and the one that means something has failed. Double- and triple-pane windows are built around a sealed insulating glass unit: panes held apart by a spacer and sealed around the edge, often with argon gas in between. When that edge seal fails, humid air works its way inside.
At first the fog may come and go, appearing for a few hours after a quick temperature swing. Over time it comes back more often, and minerals left behind can make the glass look hazy all the time. The window still opens and closes, but the view won’t clear up and the seal won’t repair itself.
Your options for foggy windows
- Replace the sealed glass unit. A glass company can usually build a new insulating glass unit for your existing sash. That’s a sensible fix for one or two foggy windows when the frames, sashes and hardware are in good shape. We don’t do glass-only replacement, and we’ll tell you when a glass shop is the better call.
- Replace the window. If several windows are fogging, or the fog comes with drafts, sticky sashes or soft wood, new windows are usually the better long-term answer. Our guide to the signs it’s time to replace your windows can help you decide.
Check your warranty first
A failed seal on newer windows may be a warranty matter, so dig out your paperwork before paying for anything. If we installed your windows, call us first and we’ll help you with the claim, whether you have Vytex or VinylMax. Both manufacturers back their windows with limited lifetime warranties, summarized on our warranty page.
How new windows help prevent condensation
New windows won’t stop condensation on windows in a house that’s too humid, but they keep the room side of the glass warmer, so it takes more moisture in the air before water forms. Two details make a difference.
Warm-edge spacers
The spacer between the panes sits at the edge of the glass, where room-side condensation usually starts. Warm-edge spacers are designed to carry less heat across that edge, so it stays warmer:
- Super Spacer® is an all-foam, metal-free spacer that reduces heat loss and condensation at the edge of the glass. It comes in our Potomac-hp windows and is one of the spacer options in Fortis.
- Duralite®, used in Georgetown windows, reduces heat transfer at the glass edge by up to 45% and helps prevent condensation, according to Vytex.
- VinylMax lists warm-edge spacer systems for its 4700 Series windows.
The condensation rating on the NFRC label
ENERGY STAR® certified windows carry a label from the National Fenestration Rating Council (NFRC). As ENERGY STAR explains, the label can include a Condensation Resistance rating from 0 to 100, where higher means less condensation build-up. NFRC is replacing it with a newer Condensation Index, scored from 1 to 100, where higher is again better.
Two cautions. ENERGY STAR certification is based on U-factor and SHGC only, so the condensation rating is a separate number to look for. And NFRC says there is “no assurance” a window won’t experience condensation, so use the ratings to compare windows, not as a promise.
Frequently asked questions
Is condensation on the inside of windows bad?
An occasional light fog that clears by mid-morning isn’t a problem. Water that pools on the sill day after day is worth fixing, because ongoing moisture damages paint and wood and can lead to mildew.
Why are my new windows sweating on the outside?
Energy-efficient glass insulates the outer pane from your indoor air, so after a clear night it can cool below the dew point of the humid air outside. It’s harmless and burns off as the day warms up.
Can foggy windows be fixed?
Fog between the panes means the seal has failed, and it can’t be cleaned from either side. A glass company can usually replace the sealed glass unit in a sound window. If several windows are fogging or the frames are worn out, replacing the windows is often the better investment.
What humidity should my house be in winter?
The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. If your windows still sweat at the low end of that range, the glass is likely too cold.
Will new windows stop condensation?
They’ll cut down on room-side condensation by keeping the inside of the glass warmer, especially with Low-E glass and warm-edge spacers. They can’t make up for very high indoor humidity, and they may still fog on the outside on humid mornings, which is normal.
Get your free estimate
Crescent Windows and Doors serves homeowners in Johns Creek, Duluth, Alpharetta, Roswell, Milton, Cumming, Suwanee, Buford and Lawrenceville. If condensation on windows is a regular problem in your home, or your glass is fogged between the panes, we’ll take a look, tell you honestly whether a glass shop or new windows make more sense, and leave a written quote if replacement is the answer. Call 470-533-0777 or book your free estimate online.