Why it shows up in cold weather
The EPA's guide to controlling moisture in your home explains that cooler air can hold less moisture, which is why drops of water form on cold surfaces such as the inside of a window in cold weather. It lists everyday sources of extra moisture: long showers, boiling or steaming food, houseplants, drying clothes indoors and running a humidifier too high. The same page notes that moisture problems and their fixes differ by climate, and it describes the Northeast, which includes Philadelphia and Bucks County, as cold and wet.
Find where the water is before deciding what it means
On interior glass you can reach safely, wipe a small area with a dry cloth. If the moisture clears, it was on the room side. Fog that stays between the two layers of a sealed double-pane unit is a different problem: the Building America Solution Center's window sash retrofit guide says condensation or fogging in the space between the layers of glass identifies a failed insulated-glass seal.
Water on the frame, the trim or the wall below the window needs separate attention. The Solution Center's window and frame replacement guide says water staining and peeling paint or wallpaper below a window can indicate water getting in, condensation or both, and that condensation on frames can stain interior finishes on its own. It recommends monitoring the area to avoid a false diagnosis, so note the weather and the exact wet spot, especially if it appears during rain. If the glass itself needs work, our guide on what to ask before replacing a broken window pane covers the next questions.
Measure the humidity and keep a short log
A household humidity meter tells you more than judging by feel. The EPA's Mold Course, Chapter 2 says indoor relative humidity should be kept below 60 percent, ideally between 30 and 50 percent if possible. That range doesn't guarantee dry windows: the EPA moisture guide reprints recommended maximum indoor humidity levels that fall as it gets colder outside, from 35 percent at 20°F to 25 percent at 0°F.
Write down the humidity reading, the room temperature, the outdoor weather, which windows are affected and whether anyone recently showered or cooked. A pattern, such as droplets after cooking or on one window only during cold nights, is useful to share with anyone you ask to look at the problem. Our checklist of photos to take before a repair can help you document it.
Reduce the moisture or warm the glass
The EPA's Mold Course, Chapter 9 describes two ways to prevent condensation: lower the moisture in the air or raise the temperature of the cold surface. Run bathroom and kitchen exhaust fans while showering and cooking; the moisture guide says they should remove moisture to the outside, not into the attic. If a fan runs without clearing the moisture, see our guide on why a bathroom fan might not clear condensation. If you use a humidifier, the EPA suggests turning it off when you notice moisture on windows.
Chapter 9 says bringing in outside air helps when that air is cold and dry, while dehumidifying suits warm, humid weather. To keep the glass warmer, the moisture guide points to storm windows and caulking around windows, along with better air circulation, such as opening doors between rooms and moving furniture away from wall corners so heat reaches cold surfaces. If you aren't sure where an exhaust fan actually vents, or the fix would involve changing ductwork or the windows themselves, have a qualified professional check it.
When repeated wetness needs more than wiping
The EPA's Brief Guide to Mold, Moisture and Your Home says to act quickly when condensation collects on windows, walls or pipes: dry the wet surface and reduce the source of the moisture. It also says water-damaged areas should be dried within 24 to 48 hours to prevent mold growth. If a room smells musty and you can't see why, the guide warns that disturbing hidden mold can release a large amount of spores and suggests considering an experienced professional, so don't open up walls yourself to look.
For a problem that keeps coming back, ask whoever inspects it to tell apart room-side condensation, a failed glass seal and rainwater getting in before recommending repairs. A useful question is: “What evidence shows where the water is coming from, and how will this work address it?”