Seal failure occurs when the sealant between the two lites of glass breaks, subjecting the airspace to the surrounding environment. Since the sealant acts as the moisture barrier, if the seal breaks, condensation can develop in the airspace of the insulating glass window.

Its external environment such as temperature and air pressure can affect the environment inside an insulating glass unit. Changes in temperature and air pressure put additional stress on the insulating glass unit's seals. The use of less resilient sealant in the construction of the insulating glass unit can also cause the seals to fail.

Most manufacturers, however, design and test their products to withstand significant changes in air pressure and temperature, thereby reducing the chance of failures. Nevertheless, seal failure does and will occur simply because of the way in which insulated units are manufactured, shipped and installed. In fact, the insulating glass manufacturers association (SIGMA) has found normal seal failure to be at a rate of 1.5% of the installed units.

Premature seal failure is usually caused by one of three reasons:

Sealed unit manufacturers, window manufacturers, and builders have often indicated to consumers that the installation of any window film on a sealed units will cause or contribute to seal failure. While it is true that some window films are not suitable for sealed units, there are many films on the market today that are designed to be installed on these units. Some window manufacturers will void the consumer's warranty for the unit if any window film is installed on the glass. A written request to the major sealed unit manufacturers for the test results used to void these warranties found that no test data existed to substantiate these actions. The Window Film Committee of the Association of Industrial Metallizers, Coaters, and Laminators (AIMCAL) commissioned A. William Lingnell, PE, a technical consultant to SIGMA, to conduct the standard ASTM (E773 and e774) on standard sealed units with and without window film.

These tests were conducted by A. William Lingnell (BS Civil Engineering, MS Civil Engineering, MS Mechanical Engineering, MS Engineering Science) of Lingnell Consulting Services, the technical consultant to the Sealed Insulating Glass Manufacturers Association (SIGMA). He has over 30 years of experience in the field. He is a member of American Architectural Manufacturers Association; American Association for Wind Engineering; American Society of Civil Engineers; American Society of Heating, Refrigerating, and Air Conditioning Engineers; American Society for Testing and Materials; American Welding Society; Association of Construction Inspectors, Building Officials and Code Inspectors; Construction Specifications Institute; International Conference of Building Officials; National Fenestration Rating Council; National Fire Protection Association; National Glass Association; National Society of Professional Engineers, and Southern Building Code Congress International.

1/4" clear glass with a 1/2" airspace. Specified size: 14" x 20"
Primary seal - polyisobutylene
Secondary seal - silicone
Film used: Reflective 20% Aluminum

3/16" clear glass with a 5/8" airspace. Specified size: 14" x 20"
Single seal - butyl strip
Film used: Neutral 35% sputtered metal

Testing on commercial units: Results confirm all units with applied film and without applied film passed the CBA (highest rating) requirements of the ASTM E773 test method and E774 specification for sealed insulating glass. The indications show there were no observable differences between the insulating glass units with the film and without the film for the tests run on these samples.

Testing on residential units: Results confirm that 5 of the 6 units with applied film would have passed the CBA (highest rating) requirement of the ASTM E773 test method and E774 specification for sealed insulating glass. The one unit which passed the CB requirement but did not reach the CBA level was examined by the laboratory and concluded that the reason for this unit only achieving 2 of 3 levels of testing was not the fault of the film or the sealant but of the assembly of the unit itself.