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What causes condensation on windows?
Whenever there is excess humidity in the home, it will manifest
itself on the coldest area of a wall, which is normally the
windows. The warmer the air, the more moisture it will retain,
so when it comes in contact with the colder glass surface
and the air is subsequently cooled, moisture is released
Do windows cause condensation?
No, condensation on windows is not the fault of the window,
unless the window seal has failed. However, by replacing drafty
windows or even installing a new roof, you are reducing air
flow in your home and making it tighter. Tighter homes retain
Impact of Temperature, Humidity
and Glass Choice on Center-of-Glass Condensation
The graph below shows condensation potential on the center
of glass area (the area at least 2.5" from the frame/glass
edge) at various outdoor temperature and indoor relative
humidity conditions. Condensation can occur at any points
that fall on or above the curves. As the U-factor of windows
improve, there is a much smaller range of conditions where
condensation will occur. These values are based on center-of-glass
temperatures. Condensation may occur at lower humidity
levels on the glass edge.
Less Condensation, Efficient
Windows Collaborative, University of Minnesota
Where on a window does condensation normally form and why?
Condensation often forms at the meeting rail and at the bottom
of the lower sash on the interior of the glass.
This is because when warm air cools, it falls down across
the interior surface of the window at the same time the air's
temperature is falling. The air contacts the horizontal surface
of the meeting rail which acts like a dam, slowing the air's
rate of fall and creating the perfect opportunity for the
trapped water vapor to escape and form on the meeting rails
surface. The air then rolls over the edge of the meeting rail
and again gains speed until it encounters the lower handle
of the sash. At this point, the water vapor again makes its
exit and lies at the bottom of the sash.
Can I reduce the condensation on my windows?
Yes. In order to reduce condensation, humidity must be controlled
and air movement must be generated. As the exterior temperature
drops, the humidity level needs to decrease if condensation
is to be controlled .
What steps can I take to reduce humidity in my home?
The two main things you can do are to control sources of
moisture and increase ventilation. To decrease or control
excess humidity and condensation:
- Use exhaust fans in your kitchen, laundry and bathrooms.
- Vent gas burners, clothes dryers, etc. to the outdoors.
- Shut off furnace humidifiers and other humidifying devices
in your home.
- Be sure that louvers in your attic or basement crawl
spaces are open and amply sized.
- Open fireplace dampers to allow an escape route for moisture-laden
- Air out your house a few minutes each day.
To reduce humidity levels in your home:
- Use ventilation fans ventilation fans in kitchens and
baths to control moisture.
- Your clothes dryer should be vented directly to the outside.
Inspect the vent duct. Make sure it is attached securely
to the dryer. Check that it is clear of obstructions (e.g.
lint). Check for holes that leak air. If vent duct is damaged
replace it with a metal duct. The vent duct should be cleaned
at least once a year. The Consumer Products Safety Commission
additional safety tips for dryer vents .
- If you have single pane windows, especially with metal
frames, install storm windows or consider replacing your
existing windows with ENERGY STAR labeled windows.
- If you can't afford to add storm windows or replace your
windows now purchase and install a shrink film or polyethylene
sheet, window insulation kit from a home center or hardware
- If you have a humidifier, check it regularly for proper
operation. It could be adding too much moisture to your
on Window, Energy Star
- Moisture control is the key to mold control, so when water
leaks or spills occur indoors — act quickly. If
wet or damp materials or areas are dried 24-48 hours after
a leak or spill happens, in most cases mold will not grow.
- Clean and repair roof gutters regularly.
- Make sure the ground slopes away from the building foundation,
so that water does not enter or collect around the foundation.
- Keep air conditioning drip pans clean and the drain lines
unobstructed and flowing properly
- Keep indoor humidity low. If possible, keep indoor
humidity below 60 percent (ideally between 30 and 50 percent)
relative humidity. Relative humidity can be measured
with a moisture or humidity meter, a small, inexpensive ($10-$50)
instrument available at many hardware stores.
- If you see condensation or moisture collecting on windows,
walls or pipes act quickly to dry the wet surface and reduce
the moisture/water source. Condensation can be a sign
of high humidity.
Moisture and Mold Prevention and Control Tips, A
Brief Guide to Mold, Moisture, and Your Home, U.S. EPA,
Office of Air and Radiation