Back to basics: insulating glass seals are not hermetic
Architectural specifications often refer to insulating glass unit (IGU) seals as hermetic. But this is misleading. It gives a false impression of IGU performance and what is required to manage service life.
Hermetic Definition
A hermetic seal is one sealed so tightly that air and water cannot permeate. This is not the case for IGUs. There is always a finite, albeit (hopefully) small, permeation of water vapor and air into—and argon out of—the cavity through the seals. This is why desiccant is placed inside the IGU spacer—to absorb in-coming moisture vapor.
An IGU fails when the desiccant capacity is consumed and can no longer absorb additional moisture coming into the cavity. At this point, moisture will condense on the glass panes and/or damage the silver-based low-E coatings that are typically used to manage heat flow. A cloudy residue on the glass is a sign of coating degradation from moisture exposure. Coating degradation and cavity condensation will obscure the view to the outside and signify a failed IGU. Coating degradation may occur in many cases before visible water condensation is observed.
The goal of IGU seal design is to increase the unit’s lifetime by selecting spacer, sealant and desiccant types and quantities that mini-mize moisture and gas permeation rates while maximizing desiccant absorption capacity. It is also critical that the seal design is sufficient-ly durable to withstand in-use environmental stresses that can degrade the edge seal and increase permeation rates over time. This is why silicone, which retains its strength under high-temperature extremes, ultraviolet light and water exposure, is the go-to for commercial IGU secondary seals.
Box spacer with a solid metal vapor barrier, such as aluminum, stainless steel and plastic hybrid stainless steel warm-edge spacer, are optimum for use with silicone. First, they typically have high desiccant carrying capacity. Also, the metal vapor barrier is critical because it compensates for the relatively high water vapor and gas permeation rates through silicone. It also supports adhesion to silicone.
Fabrication quality, in-service climate stresses and installation quality, can impact moisture penetration and gas loss. To maximize service life:
- Use a durable spacer with high desiccant capacity and application-appropriate sealants with a track record of in-field performance.
- Certification programs, such as the IGCC®/IGMA® and IGMAC® programs, can help manage risk.
- Design edge seals to manage application-specific climate stress.
- Contractor certification programs, such as the North American Contractor Certification program, can help manage this risk.