Condensation in Double Pane Windows

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Condensation between window panes occur when warm air pockets are far apart from one another and allow
the containment of a relatively large quantity of water vapor. As air cools, its molecules get closer together and
squeeze the tiny vapor droplets closer together as well. A critical temperature, known as dew point, exists
where these water droplets will be forced so close together that they merge into visible liquid in a process
called condensation.

Household air is humidified from high levels of water vapor in human and animal exhalation, plant
transpiration, and fixtures such as showers and dryers. This humidity can rise significantly higher than outside
air because of the insulative design of a house. Cold indoor surfaces can cool the surrounding air enough to
force vapor to condense. This often happens on single-pane windows because they lack the necessary
thermal insulation available to better windows. Double-pane windows have a layer of gas (usually argon or air)
trapped between two panes of glass and should be insulated enough to prevent the accumulation of
condensation. If this type of window appears misty or foggy, it means that its seal has failed and the window
needs to be replaced.

Silica Desiccant
A desiccant is an absorptive material designed to maintain dryness within its vicinity. A common type of
desiccant is silica gel, a porous plastic used to prevent spoilage in various food products. A tightly packed
assortment of silica pellets is contained inside the aluminum perimeter strip of a window to dehumidify
incoming household air that was not stopped by the window’s seal. If not for this substance, incoming air could
condense on the glass.

Silica gel has an immense surface area, approximately 800 m²/g, which allows it to absorb water vapor for
years. Eventually, the silica pellets will become saturated and will no longer be able to prevent condensation
from forming. A double-paned window that appears foggy has failed and needs to be repaired or replaced.

Why Double-Paned Windows Fail - Solar (Thermal) Pumping
Although double-paned windows appear to be stable, they actually experience a daily cycle of expansion and
contraction caused by “thermal pumping.” Sunlight heats the airspace between the panes and causes the gas
there to heat up and pressurize. Expanding gas cannot leave the chamber between the panes and causes the
glass to bulge outward during the day and contract at night to accommodate the changing pressures. This
motion acts like the bellows of a forge, pumping minute amounts of air in and out of the airspace between the
panes. Over time, the constant pressure fluctuations caused by thermal pumping will stress the seal and
challenge its ability to prevent the flow of gas in and out of the window chamber. Incoming humid air has the
potential to condense on the window surface, if it is cold enough.

Can Failed Windows be Repaired?
Inspectors should be aware that there are companies that claim to be able to repair misty windows through a
process known as “defogging.”

This repair method proceeds in the following order:

  1. A hole is drilled into the window, usually from the outside, and a cleaning solution is sprayed into the air
  2. The solution and any other moisture are sucked out through a vacuum.
  3. A defogger device is permanently inserted into the hole that will allow the release of moisture during
    thermal pumping.

Inspectors should know that there is currently a debate as to whether this process is a suitable repair for
windows that have failed or if it merely removes the symptom of this failure. Condensation appears between
double-paned windows when the seal is compromised and removal of this water will not fix the seal itself. A
window “repaired” in this manner, although absent of condensation, might not provide any additional
insulation. This method is still fairly new and opinions about its effectiveness range widely. Regardless,
“defogging” certainly allows for cosmetic improvement, which is of some value to homeowners. It also
removes any potential damage caused by condensation in the form of mold or rot.

Window condensation will inevitably lead to irreversible physical window damage. This damage can appear
in the following two ways:

  1. Riverbedding – Condensed vapor between the glass panes will form droplets that run down the length of
    the window. Water that descends in this fashion has the tendency to follow narrow paths and carve
    grooves into the glass surface. These grooves are formed in a process similar to canyon formation.
  2. Silica Haze – Once the silica gel has been saturated, it will be eroded by passing air currents and
    accumulate as white “snowflakes” on the window surface. It is believed that if this damage is present,
    the window must be replaced.

In summary, condensation in double-paned windows indicates that the window has failed and needs to be
replaced. Condensation, while it can damage windows, is itself a symptom of a lack of integrity of the window’
s seal. A failing seal will allow air to transfer in and out of the window even if it is firmly closed. Inspectors
should be aware of this process and know when to recommend that clients’ windows be replaced.