I’m expanding the amount of reference material in the Library. I just wrote this and thought I’d share it…
Double-pane insulated glass (IG) windows consist of two hermetically sealed panes of glass separated by an aluminum or stainless steel spacer that contains silica beads that act as desiccants; they absorb moisture. The panes are sealed around this spacer with sealants like silicone or polyisobutylene. Some manufacturers use both to form primary and secondary seals that help extend the lifespan of the sealed unit.
Air in the space between the window panes expands when sun shines on the window, increasing air pressure between the panes. At night the window cools and air pressure between panes drops. If air pressure between the panes is higher or lower than the ambient air pressure (air pressure outside the panes) pressure will be created on the seals. During the day, high air pressure will try to push air out from between the panes and at night low pressure will try to pull air into this space. This is called “thermal cycling” or “thermal pumping”.
Seals have to resist thermal cycling, UV radiation, moisture ingress, and must retain any inert gas (like argon) installed between the panes.
As windows age, seals will develop tiny leaks that will grow over time. Once leaks have developed and grow, increasing amounts of air will be pulled into the space between panes. This air will carry moisture vapor with it that will be absorbed by the silica desiccant beads. This prevents condensation from forming on the sides of the panes facing each other.
Eventually, the silica beads will become saturated and can no longer absorb moisture. When this happens, under direct sunlight, condensation will develop on the sides of the panes facing each other. At first, this condensation will only be visible when the window is in direct sunlight. As leaks increase and moisture continues to enter the space between the panes, permanent stains will develop on the glass that will be visible even when the window is in shade. Over time, the glass will actually suffer permanent etching.
Repair of Windows with Failed Seals
Companies exist that offer to repair failed seals by various means. I recommend that you not make a recommendation other than that the client contact a qualified contractor or window manufacturer to discuss options and costs for repair or replacement, although I typically mention that replacement is more common.