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Suddenly... Condensation!
Every year, usually in January, we (Builders Architectural - a
top Chicago-based window contractor) receive phone calls from
customers who complain of sudden condensation on their windows.
This condensation can be surprising because of its sudden onset
and large volume. Condensation is widely misunderstood but
actually it is quite simple.
Think of a glass of cold beer on a muggy summer day when you are
outdoors. The glass in your hand is dripping water on its outer
surface to such extent you might almost think the liquid is
actually leaking through the glass.
Unless you are drinking from a practical joker's dribble glass,
leaking glass is probably not the cause. The real cause is
airborne water vapor(a gas) converting to liquid when it cools
while contacting the cold glass. The water vapor has cooled to
its "dew point" and condensed into a liquid.
This happens on your windows during a winter cold snap in the
exactly same way.
Lets say you live in Chicago and so far the winter has been
moderate. Your humidifier is set at 35-40% relative
humidity(RH). One day the outside temperature drops from +30F to
+10F.
Suddenly your windows are fogged. In extreme cases water drips
down the face of the windows. Left unattended, water can damage
window frames and finishes.
Then you call your favorite window contractor and complain that
your windows are defective. Chances are they are not. The
elevated humidity in your home has come in contact with very
cold glass surfaces. The water vapor within the air has changed
to a liquid form and is now visible on your windows.
Generally, most windows which are "thermal"- meaning they have
insulated glass and either wood frames or aluminum frames with
thermal breaks - are designed to function well to a range of
+10F exterior and +70F interior, with an interior relative
humidity (RH) of 25-30%. If any of these variables change,
condensation may result.
Window treatment should help, shouldn't they?
One of the most surprising aspects of condensation can be that
it frequently take place behind fixed, insulated, or tightly
fitting window treatments such as heavy drapes or
honeycomb-style window treatments.
This is not what one would expect. We correctly think of these
treatments as adding insulating value to the house. Why would
condensation take place here?
The answer is simple. While it is true that window treatments
can reduce the flow of heat through a window opening, there is
an unfortunate by-product:
Window treatment prevents the flow of roomside heat from warming
the glass surface. Glass surface temperature will cool. An
environment may be created for condensation.
While window treatments retard the flow of heat, they don't do
much for retarding the flow of water vapor. You need a fully
taped foil or plastic vapor barrier for that.
Remember osmosis and diffusion from high school biology? Of
course you do. These forces of nature are playing themselves out
on your windows and draperies. To refresh your memory:
Osmosis: The force in nature wherein water naturally migrates
from a place of greater concentration to a place of lesser
concentration. This is your roomside humidity equally dispersing
itself throughout your house.
Diffusion: Osmosis through a semi-permeable membrane. The
membrane is your window treatment. Diffusion is also known as
"vapor drive".
What can be done in my home? # Wipe off the condensation. By
doing this, you are physically reducing roomside humidity.
# Open the windows an inch or so. The dry outside air will mix
with humid inside air and reduce roomside humidity.
# Lower your humidifier settings to 25% or below.
# Open up your blinds and drapes to a level which will allow the
roomside heat to warm the glass surfaces.
# Use exhaust fans during and after showering and cooking.
# If you are painting or drywall taping open the windows to
allow moisture to dissipate.
# Look at your clothes dryer. Make sure it vents to the
exterior. "Vent-less" clothing dryers can bring large amounts of
moisture into living spaces.
It could be the windows too...
After excluding non-window sources, look for these possibilities
on the windows themselves:
# Weather-stripping not tightly compressed allowing air
infiltration into the space. Although we know that air
infiltration can actually reduce interior humidity, it can also
focus a cold spot on a metal window or glass edge.If there is a
cold spot you could see condensation or even frost which could
be window related.
# There could be a similar cold spot where caulk is missing at
the perimeter of the window.
# Your windows could be single-glazed, where only one layer of
glass, rather than two, separates you from the exterior.
Deal with humidity first
Good strategy would be to deal with the living space issues
first. If you attack them as we describe above, condensation can
disappear in 24-48 hours.
If condensation persists, look to the windows and caulking.
About the author:
Mark Meshulam is Executive Vice President and Director of
Engineering for Builders Architectural, a top Chicago-based
window contractor. http://www.buildersarch.com
Mark Meshulam
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