A detailed explanation of insulated glass unit components, the manufacturing process, and the standards that ensure long-term seal integrity.
The Core Components of an Insulated Glass Unit
An insulating unit consists of two (or more) glass panes separated by a spacer bar, typically aluminum or a low-thermal-conductivity polymer material, containing a moisture-absorbing material, with the gap filled with dry air or an inert gas like argon to improve thermal performance. The perimeter seal around the spacer is responsible for preventing moisture or gas leakage, and its quality determines the entire unit's service life.
Primary and Secondary Seals: Why Two Layers?
The primary seal, usually made of butyl, provides the core barrier against water vapor and gas permeation, while the secondary seal, typically silicone or polysulfide, provides the mechanical strength bonding the two panes together and resisting wind loads and varying atmospheric pressure. Adopting a dual-seal system instead of a single layer significantly increases the unit's durability and resistance to repeated annual thermal expansion and contraction cycles.
The Role of the Desiccant and the Dew Point
The desiccant material inside the spacer absorbs any residual moisture within the air gap during manufacturing and continues absorbing any small amount that seeps in through the seal over the years. This material's quality is tested by measuring the unit's internal "dew point" — the temperature at which water vapor begins condensing on the glass's inner surface; the lower this point, the more resistant the unit is to internal fogging.
International Standards for Testing Unit Service Life
Insulating unit performance is tested per standards such as American ASTM E2190 and European EN 1279, which include accelerated weathering tests simulating years of thermal and humidity exposure within a compressed timeframe, along with repeated air pressure resistance testing. Manufacturers committed to these standards typically offer a warranty spanning ten to fifteen years against seal failure.
Signs of Unit Failure and How to Catch Them Early
Persistent fogging or condensation between the two glass panes, even on hot, dry days, is the clearest indicator of seal failure and desiccant exhaustion. This defect cannot be fixed by cleaning, no matter how often — it requires full replacement of the insulating unit. An annual visual inspection of large facades is recommended to catch any unit showing this sign early, before it spreads to a larger number of neighboring units.
