Insulated glass, sometimes called by the old LOF trade name Thermopane, has been sold in the United States since the 1950s. Since then, the fundamental concept hasn’t really changed.

Insulated glass still consists of two or more pieces of glass that are bonded to, yet separated from one another through the use of a spacer on all sides. The space between them insulates as “dead air space”. This sandwich is glued together with sealant designed to keep the glass together and moisture out. The spacer contains a dessicant that removes moisture, which might remain from manufacturing.

Thermal Improvement with Insulated Glass
The resulting assembly reduces heat loss by about one half over a single pane of glass. For you R-value fanatics, by insulating the glass, you improve the R1 of single glass to a whopping R2 for insulated glass. When you add a Low-E coating, you can get about R3. If you then add argon gas in the airspace, R4 is possible.

Seal failure
This occurs when moisture penetrates the spacer seal and wets the airspace.

Seal Failure Conditions
Despite a good track record for insulated glass in general, you can eventually expect failures. A moderate failure will look like a cloud of moisture that has condensed on one of the inner surfaces of the glass in the airspace. It might come and go depending on the outside temperature. A serious failure will involve the permanent presence of moisture in the airspace.

Fixing Insulated Glass Failures
When seal failures occur, the glass must be replaced. There are some people out there who attempt to fix the failed unit by drilling holes in the spacer, forcing dry air into the airspace, and resealing the hole.  However, future failures are very likely, and typically the glass cannot be cleaned of the stains from the condensation.