Why Is There Mold Growing on My Window Sills in Winter?

During winter, the air inside your home holds more moisture than the cold air outside. When that warm, damp air touches the cold surface of a window, it cools quickly and releases water as condensation. You see this as droplets on the glass and pooling on the sill.

The colder the outside temperature, the more condensation forms. If your windows are older or single-paned, they lose more heat, making the surface even colder and the condensation worse. Even double-paned windows can sweat if humidity indoors is high.

Lowering the humidity in your home is the most direct way to stop window mold. Simple steps include using exhaust fans when cooking or showering, covering pots while boiling, and hanging laundry outside or in a vented dryer. A portable dehumidifier can help in rooms that stay damp.

Aim for indoor humidity around 30 to 50 percent in winter. You can measure it with a hygrometer, which is inexpensive and easy to find. Keeping the heat steady, rather than letting the house cool down overnight, also reduces condensation on cold surfaces.

If you call in the pros, they usually start by inspecting the window and surrounding wall to find the moisture source. They might remove damaged drywall or trim to check for hidden mold and water damage. Then they dry the area with dehumidifiers and air movers.

The work often involves repairing or replacing affected materials, like the sill or drywall, and addressing the leak itself. If the window is old and inefficient, a window replacement might be part of the solution. The exact steps depend on what the inspection finds.