Moisture accumulation between window panes causing fog and condensation is the result of solar pumping.
Most believe that moisture in their windows is a sign that the “seal is broken”.
THIS IS FALSE!
The truth is, the absorbing material (desiccant) built into the window`s seal is saturated. Accumulation of moisture in the desiccant is due to the constant heating and cooling of the airspace between the panes of glass (Solar Pumping). If not repaired quickly, moisture will continue to accumulate and permanent glass damage will occur (Stages of Window Seal Failure).
99% of the time moisture occurs inside a window, not because of a failed seal, but solely due to the cycle of “solar pumping”.
The inner airspace of a window gets heated by the sun or ambient air during the day. The glass flexes outward minutely, pushing some of the hot air through the semi-permeable seal to the outdoors.
In the evening, the airspace cools. The glass returns to its normal position, drawing small amounts of air back into the cavity. The air drawn into the window from the atmosphere naturally holds moisture. The moisture is absorbed by the window spacer’s desiccant material.
When the desiccant is saturated and can no longer absorb, fog will show on the glass. The moisture continues to accumulate turning the fog into condensation. If left for too long, minerals in the moisture can get baked onto the glass causing staining. These stains get thicker and eventually etch into the glass causing permanent damage.