Windows fog up in winter because warm, moist indoor air touches cold glass and cools until its water vapor turns to liquid. A little fog that clears by midday is normal. Water that pools on sills day after day can soak wood, paint and caulk, and that steady dampness is where mold starts.
The clocks fall back on Sunday, November 1, and for a lot of Ann Arbor households that is the week the furnace starts running from late afternoon until bedtime. Windows that were clear all fall suddenly have a film of water in the morning, especially in bedrooms, kitchens and bathrooms.
Whether you live in a newer build or a house with original sashes in Old West Side, Burns Park or Water Hill, the physics are the same. The good news is that window condensation is one of the easiest moisture problems to spot and one of the easiest to get under control.
Why do my windows sweat more once the heat comes on?
Warm air can hold more water vapor than cold air. When you heat your home, the air inside picks up moisture from everyday living, and the glass on the other side of the room stays cold because it faces the outdoors.
When that warm air drifts against the cold glass, it cools quickly. Once it cools past a certain point, it cannot hold all of its moisture, and the extra water lands on the glass as tiny droplets.
Shorter days add to it. After the time change, more of your evening happens indoors with the heat on, the windows closed and the lights on. More cooking, more showers and more people in the house all happen in the hours when the glass is coldest.
What is dew point, in plain terms?
Dew point is the temperature at which air has to cool before water starts to come out of it. Think of a glass of iced tea on a summer porch. The outside of the glass gets wet because the air touching it cools below its dew point.
Your window works the same way in reverse. The more moisture in your indoor air, the higher its dew point, and the sooner it turns to water on cold surfaces. The colder the glass, the more easily that happens.
That gives you two levers. You can lower the amount of moisture in the air, or you can keep the surface warmer. Most homeowners have more control over the first one, and it costs very little to start.
Where does all that indoor moisture come from?
Most of it comes from ordinary life. None of these are a problem on their own, but they add up in a closed-up house.
- Cooking. Boiling pasta, simmering soup and running the dishwasher all put steam into the air.
- Showers and baths. A long hot shower with the door closed and no fan running can fog every surface in the room.
- Drying laundry indoors. Clothes on a rack release their water into the room as they dry. A dryer that is not vented to the outside does the same thing.
- People. Everyone breathes out moisture. A full house for the holidays raises humidity fast.
- Houseplants. Plants give off water through their leaves, and watered soil evaporates into the room. A big collection near a window can make that window wetter.
- Aquariums and humidifiers. An open tank or a humidifier set too high adds steady moisture all day.
Moisture from below matters too. A damp basement or crawl space can feed humidity into the rest of the house, so if your upstairs windows sweat heavily, it is worth checking what is happening downstairs.
Where does mold start around a window?
Glass itself is not food for mold. The trouble is the water that runs off the glass and sits on materials that soak it up. Look in these spots first:
- Window sills. Water collects on the flat sill and soaks into painted or bare wood.
- Frames and sashes. Wood frames swell and stain when they stay wet. Dark spots in the corners of the frame are a common early sign.
- Caulk and glazing. Caulk joints and window glazing hold moisture in their seams, and you may see black or gray specks along the bead.
- The wall below the window. Water can run down behind trim and into the drywall or plaster under the sill. Peeling paint, bubbling or soft spots there deserve a closer look.
- Window treatments. Curtains and blinds pressed against wet glass trap moisture and block airflow, so the window stays wet longer.
A musty odor near a window, staining that keeps coming back after you wipe it, or wood that feels soft are all signs the moisture has been there for a while.
How do I lower humidity in my home this winter?
The EPA recommends keeping indoor humidity below 60 percent, and ideally between 30 and 50 percent. A low-cost hygrometer, available at most hardware stores, tells you where you stand. Put one in the room with the worst windows and check it in the morning and evening for a week.
If the reading runs high, these steps help:
- Run the bathroom fan. Turn it on during every shower and leave it running for a while afterward so the steam actually leaves the house.
- Use the kitchen exhaust fan. Run it while you cook and while the dishwasher runs. A range hood that vents outside does far more than one that only recirculates.
- Vent the dryer outside. Make sure your dryer vents to the exterior and that the line is clear. Skip drying racks in small rooms during the coldest stretch.
- Run a dehumidifier. A dehumidifier in the basement or in a problem room pulls water out of the air. Empty or drain it regularly.
- Move the air. Open interior doors, pull curtains back from the glass and let air circulate. Ceiling fans on a low setting help too.
- Ventilate when it makes sense. Cracking a window briefly after cooking or showering lets moist air out, as long as it is practical for the weather.
- Check your humidifier. If you run one, lower the setting until your windows stay mostly clear.
On the surface side, keeping the glass a little warmer also helps. Storm windows, good weatherstripping and opening blinds during the day so the furnace air reaches the glass can all cut down on the water you wipe up each morning.
What should I do about condensation that is already there?
Wipe it. A dry towel each morning on the worst windows keeps water from sitting on sills and soaking into wood. It sounds simple, but it makes a real difference during the first few weeks of heating season while you work on humidity.
The EPA advises drying wet materials within 24 to 48 hours to keep mold from growing. If a sill or a strip of drywall has been wet for longer than that, look closely for staining, softness or growth.
For a small patch of surface mold on a hard, painted sill, the EPA notes that areas under about 10 square feet can usually be handled by the homeowner. Absorbent materials that become moldy, like drywall or carpet near the window, may need to be removed and replaced rather than cleaned.
When does window condensation point to a bigger problem?
Some fog on cold mornings is part of living in Michigan. These signs suggest the moisture is going further than the glass:
- Water forms between the panes of a double-pane window, which means the seal has failed.
- Condensation shows up on walls, closet corners or ceilings, not just on glass.
- The sill, frame or wall below the window is soft, stained or swollen.
- Mold keeps coming back in the same spot after you clean it.
- Your hygrometer stays well above 50 percent even with fans and a dehumidifier running.
- Several windows on one side of the house are much wetter than the rest, which can point to a leak, a damp wall or air coming from a wet basement or crawl space.
When you see these, the window is usually telling you about the moisture in the whole house. Finding the source is the step that keeps the problem from returning.
Window condensation: myth vs. fact
Myth: Condensation means my windows are bad.
Fact: Even good windows fog up when indoor air is humid enough. Water between the panes is the exception, since that points to a failed seal.
Myth: Mold grows on the glass, so wiping the glass solves it.
Fact: Mold usually grows on what the water touches next, like wood sills, caulk, paint and the wall below. Wiping helps, but lowering humidity is what stops the cycle.
Myth: More humidity is always better in winter.
Fact: The EPA recommends keeping indoor humidity below 60 percent and ideally between 30 and 50 percent. If your windows are dripping, your humidifier is likely set too high.
Myth: Bleach on the sill takes care of the problem for good.
Fact: Cleaning removes surface growth, but it comes back as long as the material keeps getting wet. Moisture control is the key to mold control.
Myth: Closing the curtains at night keeps the window warmer and drier.
Fact: Heavy curtains can trap cold, moist air against the glass. Leaving a gap or opening them during the day helps the window dry out.
How Mold Medics Ann Arbor can help
If the water on your windows has already left stains, soft wood or growth behind, we start with a free visual mold inspection using a thermal imaging camera and a moisture meter to see where the moisture is going. From there, our 6 Steps to Victory process (Assess, Contain, Remove, Vacuum with HEPA equipment, Wipe Down and Anti-Microbial Application) handles the affected area, with HEPA air scrubbers running during containment. When your windows are telling you something, book a free visual mold inspection with Mold Medics Ann Arbor.
