Repairing glass in a thermopane window is different than repairing single-glaze windows. A thermopane window is made of insulated units that typically have two panes of glass.
Instead of the puttylike glazing compound used with a single-pane window, a thermopane window uses a sealing component and a mechanical glazing stop, or glazing bead.
Two Reasons for Repair
A thermopane window is defined by the insulated glass unit. This insulated glass, or IG unit, is typically made up of two panes of glass that are sealed to a spacer, which is sometimes called double-pane glass.
The spacer is placed between the panes of glass and is continuous around the edges. When the unit is sealed, no air or moisture moves into or out of the enclosed airspace.
Broken glass will let air and moisture move into the space between the panes. This also can occur if the seal between the spacer and the glass is broken.
Outside air and moisture in the airspace will cause condensation; the moisture will fog on the glass due to the different temperatures inside the house and outside the window. In severe climates, this condensation may bead up and freeze.
Evaluating the IG Unit
If the glass is broken, the obvious repair is to replace the IG unit. However, if the glass is fogged, it is not so obvious that the IG unit should be replaced.
There are at least two methods of putting a hole in the spacer to allow moisture out of the airspace between the glass panes. Although these methods may improve the situation in the short term, if the seal is broken on the IG unit the moisture and fog will return.
Replacing the IG unit is the best policy.
Removing and Replacing the IG Unit
Typically an IG unit is "glazed" into the sash frame with a sealant, adhesive tape or a gasket that wraps around the edge. The glass is held in the frame by a glazing stop, which is a thin removable strip that is nailed to the frame or snapped into a groove in the frame.
Remove this strip with extreme care--it is required for installing the replacement IG unit.
Before removing the faulty IG unit, measure the glass width, height and thickness so the local glass shop can build a replacement unit. Also note the sealing feature--tape, silicon or gasket--so replacements can be obtained with the IG unit.
When the replacement unit is obtained, make sure the measurements match the frame, then remove the failed IG unit. Check to see that the unit is sealed into the frame continuously on all four sides.
Carefully replace the glazing stop. Clean up and repaint if necessary.