Q: I have customers who complain about condensation
forming on insulated windows and skylights. What should we look for
in a window or skylight to prevent this problem?
A:Steve Easley responds: Windows do not cause
condensation; it's high humidity that's the culprit. Understanding
how condensation forms on windows will help you determine what
should be done to correct the problem.
Moisture vapor in the air condenses on cold surfaces in a home
where the air reaches its dew point. This is common when there is a
large difference between inside and outside temperatures and when
the relative humidity — the amount of moisture in the air
relative to its temperature — of the warm side is high.
Typically, the coldest surfaces in a home are on the windows
— most often at the edges, where conduction is greatest. In
extreme cases, when indoor humidity conditions are very high,
chronic condensation at the edges of the glass can create a
significant moisture problem that leads first to peeling paint,
then to mildew and mold, and eventually to rot (Figure 1).
Figure 1. High indoor humidity levels,
aggravated by a window blind that reduced the drying effect of air
circulation, has created a condensation problem that has led to
mold growth. In this case, the homeowner's lifestyle lies at the
root of the problem, but it's not helped by an ordinary insulated
window without warm-edge low-conductive glass.
Window condensation is more than a simple inconvenience to the
person who is looking out the glass; it's a red flag that there
could be serious moisture problems in the home as well. If
condensation is forming frequently on the glass, it's likely to be
forming inside walls where there are pockets of poor insulation and
where air leaks are bringing warm, moist air into contact with cold
Condensation can form in both very hot and very cold weather. On a
cold wintertime night when the indoor temperature may be 50 degrees
or more than the outside temperature, condensation on the inside
edges of an insulated glass window is possible, depending on indoor
humidity conditions. Similarly, on a hot Florida summer day,
condensation may form on the outside surfaces of the windows of a
heavily air conditioned home.
Solutions to the Problem
You can reduce or eliminate condensation by changing the dew point
— the point at which the water vapor condenses — either
by reducing the indoor relative humidity or by increasing the
thermal performance of the window. In the wintertime, you can do
something about high indoor humidity — ventilate. In the
summer, there's no way to control the high outdoor humidity, so the
only solution is found in using high-performance windows.
The dew-point chart (Figure 2) helps to explain when indoor
condensation will occur, depending upon the type of window. Look at
the axis labeled "Indoor Relative Humidity (%)." The scale starts
out from zero at the bottom, and goes all the way up to 100%
relative humidity at the top. If the indoor relative humidity is
greater than 50%, condensation may be inevitable, depending on the
type of window. Above 65% RH, even the very best window available
is at risk. And this high a humidity level indoors will likely
cause other problems besides dripping windows. So, the first line
of attack should always be to examine the humidity conditions in
Figure 2. Dew-Point Comparison of
The colored lines on the graph above show when condensation will
appear under different conditions on the window glass of four
different types of windows.
But although keeping humidity levels as low as possible in very
cold weather is often the best strategy, it is not the only cure.
Sometimes selecting the right window can do the trick. Single-pane
glass (represented by the red curve on the bottom of the graph)
combined with an average winter outdoor temperature of 30°F
will produce condensation on the window when the inside RH is just
over 30%. That's a pretty low indoor humidity level. In this case,
simply switching to double-pane windows will solve the problem.
Here's a quick look at window options:
If windows need to be replaced. Switch from an
existing single-pane to a double-pane window. It's not uncommon to
see condensation on old single-pane windows, even in normal
humidity conditions. Switching to an insulated glass unit will
often solve the problem.
In new construction. Invest in a window that uses
warm-edge technologies to reduce conduction at the edges of
insulated glass units. This can keep the window edges warm and
reduce the chances for condensation to form (Figure 3). However,
because condensation forming on the edges of an insulated glass
unit indicates excessive moisture levels, it is usually better to
solve the humidity problem before investing in window
Figure 3. In the photos shown at left, the
window on the far left (in both the photo and the thermograph) has
a standard edge spacer — a highly conductive piece of
extruded aluminum that quickly lowers the temperature at the edge
of the glass and allows the indoor humidity to reach its dew point.
The window on the right of each image has edge spacers often
referred to as "warm-edge technology." These spacers conduct less
heat, lowering the window's potential for edge
When upgrading. Choose an insulated glass unit
with an argon or krypton gas fill, which provides a little better
insulation value and reduces convection between the panels. This
may be the best option for a window in a kitchen or bath, where
even exhausting the humidity may not be enough to avoid
condensation forming at the edges of the glass.
I recommend that contractors carry a digital hygrometer to measure
and record indoor relative humidity while in customers' homes
(Figure 4). These relatively low-priced tools can go a long way to
communicating clearly with clients about the indoor environment and
what the options are for fixing a problem.
Figure 4. Prices for a digital hygrometer
range from about $30 to $300, but a low-cost model such as this one
is sufficient for most remodelers. This tool is primarily an aid
for talking to clients about temperature and humidity and
explaining the causes of condensation on glass even when it's not
Of course, there are callbacks associated with condensation that
can be blamed on the window. The most obvious failure is
condensation between the panes of glass in an insulated glass unit
(IGU) caused by a broken seal. When the seal breaks, moisture-laden
air leaks in and condenses on the coldest surface inside the IGU.
The only cure is to replace the IGU (or usually the whole sash
Sometimes the seal may break and the environmental conditions do
not cause condensation inside the unit. Slowly, the low-E coating,
which is typically put on one of the inside surfaces, will oxidize.
This appears like a permanent smudge or fog that can't be wiped
off. This, too, warrants a replacement of the unit.
Steve Easley is president of BMI, a company that consults with
builders on field issues and provides training on building