What causes a window to get condensation, mist or fog between the panes of glass?
To understand this issue and how it occurs you need to first understand how a double pane window is constructed. A double pane window is commonly referred to as an insulated glass unit (IGU). It is two pieces of glass with an air gap. The space between the two pieces of glass is the air gap. The air gap is created by the spacer material. The two pieces of glass and spacer material are held in place by butyl adhesive. 
There are primarily two types of spacer materials that our team uses. The most common is an aluminum tube filled with desiccant beads and the second is a foam core spacer with a desiccant embedded with the foam. The desiccant material helps absorb any moisture that builds up between the panes of glass. Desiccant materials have a life span and once it is exceeded the window will failure. The moisture adsorption capacity of the desiccant along with weather conditions determines the life span. The foam core spacer provides a thermal break between the panes of glass and therefore is a better insulator. The foam core spacer provides a higher insulating value. The aluminum spacer material conducts temperature through the glass. Here at Glass Doctor® of Virginia Beach we use a Super Spacer.
Seal Failure
There are a couple of areas that cause a seal to fail. • House Settlings • Butyl Adhesion Failure • Desiccant Failure • Extreme Heat / Sunshine When a house settles over time it can put pressure on the frame of the window. This downward pressure can cause a pane of the glass to shift a small amount and cause a gap to open between the spacer and pane of glass. If the glass Is not cleaned properly, the butyl is not brought up to the correct working temperature while it is being applied or the IGU is not allowed to cure properly it can result in the butyl not adhering to the glass. Glass Doctor of Virginia Beach will help you determine which windows have experience seal failure. We believe in just replacing the glass and saving the frame. Call us today or schedule an appointment online with our glass specialists!