Home

Why are my windows sweating in winter? (and how to stop it)

By Nazrul Islam. Updated October 11, 2026.

Why do windows get condensation on the inside in winter?

Every air mass has a dew point, the temperature at which its water vapor turns back into liquid. When warm indoor air touches glass colder than that, water beads up. Below freezing, you get frost. NDSU Extension says heavy condensation or frost may signal too much indoor moisture or a cold window.

Dew point of 70°F indoor air at different humidity levels:

These come from the Magnus formula: e_s(T) = 6.112 × exp(17.62 × T ÷ (243.12 + T)), with T in °C. On a very cold night, inner glass can drop below 45°F, so even 40% humidity can be too much.

What indoor humidity stops window condensation?

NDSU Extension recommends keeping winter indoor humidity under 40%. The Center for Energy and Environment (CEE) gives a sliding scale:

Treat it as a starting point; leaky or single-pane windows may need lower settings. Check with a digital hygrometer. See also what indoor humidity should be in winter.

How do I reduce moisture in my house?

The University of Minnesota Extension lists bathing, cooking, dishwashing, breathing, plants, humidifiers, stored firewood, plumbing leaks and unvented appliances as indoor moisture sources.

• Turn off the humidifier. NDSU says to shut humidifiers off.
• Use exhaust fans. Run outside-venting fans while cooking and during baths and showers (NDSU, University of Minnesota).
• Cut extra sources. Move firewood outside and thin out houseplants.
• Vent the dryer outdoors. Make sure its duct exits the house.
• Air out briefly. The University of Minnesota suggests opening windows just a little for a short time in winter.
• Fix leaks. Repair plumbing leaks and seal air gaps. See finding and sealing air leaks.

Don’t count on a dehumidifier. NDSU notes most only lower humidity to about 50%.

Safety: Never use an unvented kerosene or gas heater indoors. Combustion makes water vapor as well as carbon monoxide. Keep working CO alarms on every level.

How do I warm up the glass?

• Add plastic film or storm windows. NDSU suggests covering the window with plastic. Montana State University Extension says storm windows reduce moisture on the window.
• Air-seal the window. Montana State advises caulking and weatherstripping the main sash and frame.
• Let room air reach the glass. Heavy curtains and closed blinds keep warm air off the glass. Open them during the day.
• Replace failing windows. ENERGY STAR lists single-pane glass and degraded sills and sashes as reasons to consider replacement.

Budget options: insulating windows for winter.

Why is there condensation between the panes?

Fog trapped between the panes of a sealed double-pane window means a failed seal. ENERGY STAR explains that failed seals let moisture in and reduce insulating power. Lowering humidity won’t fix it; the glass unit or window needs replacing.

Moisture between a separate storm window and the main window is different. It usually means moist indoor air is leaking past the main sash, so seal that sash first.

Can window condensation cause mold?

Yes. NDSU Extension says water and ice on windows can damage the house and promote mold. Wipe up water each morning and check sills and frame corners for dark spots or soft wood. The University of Minnesota notes mold can cause allergies and respiratory problems.

Quick answers

Is some condensation normal? A little fog at the glass edge on the coldest mornings usually isn’t a concern. Water running onto the sill, or frost that lasts all day, means humidity is too high.

What humidity stops sweating windows? Under 40% in winter (NDSU), and lower as it gets colder, such as under 30% at 0°F to 10°F outside (CEE).

Can I fix fog between double-pane glass? Not by changing humidity. The seal has failed, so the glass unit needs replacement.