Façade Consultants
Jul, 25,2024 by facadeconslt

Insulated Glass Seal Failure

Condensation between glass panes can be a real pain for building owners

Insulated glass, sometimes called by the old LOF trade name Thermopane, has been commercially offered in the United States since the 1950s. For the ensuing 60 years, the fundamental concept hasn’t really changed.

Seal Failure

Seal Failure

Insulated glass still consists of two or more pieces of glass which are bonded to, yet separated from one another through the use of a spacer at all four edges. The space between them becomes an insulating “dead air space”. This sandwich is glued together with sealants designed to keep the glass together and moisture out. The spacer contains a dessicant that removes moisture which might be residual from manufacturing. Insulated glass seal failure occurs when moisture penetrates the edge seals and wets the airspace.

Thermal Improvement with Insulated Glass

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

Insulated glass seal failure

Insulated glass seal failure

Insulated Glass Seal Failure

Despite a respectable track record for insulated glass in general, naturally there are failures. A moderate insulated glass seal 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. This failure is also called “fogging glass”, “foggy window” or “window condensation between panes.”

A serious failure will involve the permanent presence of moisture in the airspace as you can see to the right. Animated glowing is just for entertainment and only occurs in real life when a cloud goes by very quickly, or during frat parties.

The worst insulated glass seal failures I have seen also involve degradation of the spacer. Sometimes the spacer corrodes or even rusts. Sometimes the dessicant becomes so saturated that it turns to mush and becomes visible on the spacer. I have even seen a failure in which, upon deglazing, one lite of glass completely separated from the spacer. See this disaster below.

Total Insulated Glass Seal Failure

Total Insulated Glass Seal Failure. One lite is completely delaminated. Spacer is rusting. Dessicant is overwhelmed. Water is present in the unit. Probable cause of failure: long-term immersion in water. There were no weep holes in the frame.

Fixing Insulated Glass Seal Failures

When seal failures occur, the glass must be replaced. There are vendors 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 (or even leaving it open), but future failures are likely. In some instances the glass cannot be cleaned of the stain from the condensation. It’s hard to imagine the total failure shown above repaired without replacement. If any readers have experience with this process, or if you make your living performing it, please write in with your experiences.

Types of Insulated Glass Spacers

Insulated glass spacer types

Insulated glass spacer types

Metal spacers
These ubiquitous spacers can be made from stainless steel or aluminum. Most are a rectangular tube in cross section, and the dessicant is inside the tube. Dessicant consists of tiny beads. Perforations in the airspace side of the tube allow moisture in the airspace to eventually migrate to the dessicant. Corners of these spacers are either bent or fitted into “corner keys”.

glass spacer 2

Hybrid spacers, Intercept
This PPG invention reduces the flow of heat through the spacer by changing it from a tube to a channel. Dessicant is exposed to view in the airspace, and is a plastic.

insulating glass spacer3

Thermal break spacers
This new family of spacers uses a polyurethane thermal break, just like aluminum window frames. Two separate tubes carry the dessicant beads. This is one variation of “warm edge” technology, which means that on cold days, the interior temperature at the edge of the glass will be similar to the best temperature, considered to be at the center of the glass. I have not yet seen wide adoption of this design here so far. One manufacturer is Azon. They are a force in the thermal break market and naturally evolved their product to work inside the glass as well as in the frame.

insulating glass 04
Thermoplastic spacers, TPS

These spacers are a solid rectangular cross section of a plastic material that has dessicant entrained within it (Edgetech). Sometimes these spacers also contain a thin corrugated metal fin that assists in maintaining the dimension between the two lites (Tremco Swiggle). Sometimes these spacers are literally extruded right onto the glass edge in a semi-plastic state (Bystronic). Once the thermoplastic spacers are applied to one lite, the second lite is then placed, and the entire assembly goes through pinch rollers into an oven for fusing and curing. Thermoplastic spacers are considered to utilize “warm-edge technology”.


Dessicants are necessary in insulated glass units because of the near impossibility of sealing an insulated unit with perfectly dry air, and to allow for small amounts of moisture permeance during the unit’s service life. Generally, dessicants adsorb, rather than absorb the free-floating moisture in the airspace. In adsorption, the water molecules adhere to the surface of the dessicant rather than permeating into the dessicant as would occur in absorption. I really like the technical name for this process: “molecular sieve”. Cool, huh?

Primary Seal

Polyisobutylene molecule, stalward defender against water vapor

Polyisobutylene molecule, stalward defender against water vapor

The primary seal is responsible for keeping water out. The hands-down favorite for this seal is PIB, or polyisobutylene. It is a synthetic rubber that is gas impermeable, and so it is able to resist airborne moisture (water vapor) for many years if applied properly. If the primary seal fails, the result will almost always be condensation between the panes of glass symptomatic of insulated glass seal failure.

Secondary Seal

The job of the secondary seal is to hold it all together. Strength, flexibility and a strong adhesive bond are essential. Polysulfides, a family of carbon and sulfur-based synthetic rubbers have been used for many years. In the last few decades, silicone sealants have experienced increasing popularity. Silicone is similar to quartz, except that some of the oxygen molecules are substituted with carbon-hydrogen groups (methyl – CH3). to create long strands of flexible, durable material. Both have two-part rapid-set versions for factory production use. If this seal fails, the primary seal can easily become disrupted, bringing about the fogging associated with insulated glass seal failure.

The versatile, strong yet supple silicone molecule

The versatile, strong yet supple silicone molecule is a favorite in the glazing industry

Edge Deletion

One key element of seal longevity is edge deletion. This is the grinding off of coatings such as low-e coatings for a stripe along each edge to allow for direct adhesion to the glass. Edge deletion also keeps the coatings, which are metallic, as far away from the outside air as possible to avoid the possible creep of corrosion beneath the adhesive bond line.

Types of edge seal failures

If you have been diligently reading these articles, especially this one, you will not be surprised to find that failures of insulated glass seals mirror failures that we see in sealants in general. They are:

  • Adhesive failure – the sealant stops sticking to the glass
  • Cohesive failure – the sealant tears or ruptures within its own boundaries
  • Localized puncture or absence of sealant
  • Sealant Reversion – the sealant decides to revert back to its prior form or degrade into a totally new and useless form, that is, if the sealant could “decide” things.

Why does seal failure occur?

If the failure occurs within the first few years, there was an error in manufacture or installation. The manufacturer may have used old material or failed to properly mix or apply the sealant. The glass unit may have been moved too soon, disrupting the seal before sufficient cure. There may be sealant incompatibilities between the various sealants, even the setting blocks, used in the glazing system. The manufacturer, shipper or installer may have punctured the seal at any point along the way.

Insulated glass destroyed by long-term immersion in water.

Insulated glass destroyed by long-term immersion in water.

If the seal failures occur after a couple of years following installation, it’s time to look at the drainage system beneath the glass. Weep holes are notoriously absent from most wood and wood-clad windows and even some aluminum windows.

As the glazing seals (between glass and adjacent frame) loosen up, water enters that area and accumulates beneath the bottom of the glass. Without a “weepage path”, this is DOOM, do you hear me? CERTAIN DOOM!!! Doom that could have been avoided if the manufacturers designed a way for the glass to drain. Insulated glass seals are not designed for immersion. These are not aquarium sealants.

In my humble, unbiased and completely objective view, absence of a weepage path beneath the glass has been the single most glaring, persistent weakness in window products since the advent of insulated glass. If you have been a victim of this design/manufacturing shortcoming, I hope you will contact me.

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