Solar pumping is a natural condition in windows caused by the heat of the sun and cooling of the night. Dual paned or thermopane windows are constructed with two panels of glass separated by a spacer bar (normally silver, the bar that runs around the perimeter of the window) and then sealed together with a sealant. Think of the spacer bar as a square hollow bar that is filled with a desiccant. The desiccant is very similar to the little white beads in the packet, when you buy new shoes. In general the cavity of the window has only ambient (outside) air when it is manufactured, and the desiccant is used in the spacer bar to soak up any moisture that is in the air when the glass panels are sealed together.
Harmful Effects of the Sun
During warm seasons, sunshine has the greatest effect on the longevity of the windows seal. As the sun penetrates the glass, it heats the air in the windows cavity. When air is heated it expands and pushes the two panels of glass outward. It may not be noticeable to the homeowner but the glass does expand in warm weather. When the sun goes down and the outside temperatures cool, so does the air within the cavity of the window. Cool air naturally contracts and when this air in the window cavity contracts it draws the panels of glass inward. This heating and cooling, expanding and contracting cycle, on the window happens repeatedly, until eventually the seal weakens causing a hole to develop.