September 4, 2018
by Michael J. Louis, PE
The appearance of condensation between the panes of an insulated glass (IG) unit is an indication the hermetic seal of the unit has been breached or compromised, thereby allowing outside moist air to infiltrate the air space where it condenses on the interpane surfaces of the glass. One could argue semantics, but this is the generally accepted meaning of the word “failure” as it relates to IG units. In this case, the failure is characterized by an ever-present condensation between the glass panes.
However, what happens when the condensation comes and goes with changes in outdoor temperatures and appears for only a handful of days when outside temperatures are very cold? Is this also failure?
Before addressing these questions, a basic explanation of IG unit construction is needed. IG units are composed of sheets of glass separated by a hollow metal spacer located around the glass perimeter (Figure 1). The air space between the glass sheets is hermetically sealed by liquid-applied sealants placed at the glass-to-metal spacer interface. The interpane air is dehydrated by desiccants (i.e. moisture-absorbing gels or beads) placed within the hollow spacer. Many types of spacers are available on the market today, including warm-edge designs using flexible materials and composites; all offer the same function: separate glass plates, hold a desiccant, and provide structure for the sealed air space. However, for illustrative purposes, the author is only referring to the most common style of hollow metal spacer.
If construction is done properly, the IG unit should provide a reasonably long service life, or at least, that is the expectation. The reality, however, is the life expectancy of the finished assembly is not guaranteed. If the perimeter seal of the unit is broken or compromised, a significant amount of outside air—carrying moisture vapor—filters into the air space. The dehydration capacity of the desiccant is quickly consumed, and excess moisture condenses on the inside glass surfaces (i.e. the unit fogs).
Even though the average building owner expects that IG units have an equivalent service life to the fenestration system it is glazed into, the durability of an IG unit’s edge seal and its service life is highly dependent on the quality of fabrication and glazing conditions in the fenestration assembly. For purposes of this article, the assumption is that IG fabrication is not an issue and thus, the only conditions affecting the IG seal are environmental conditions in the fenestration assembly.
Water in contact with the edges of an IG unit promotes degradation and loss of adhesion of the perimeter sealants, resulting in premature seal failure and glass fogging. Two essential design features helping preserve the integrity of the IG’s hermetic seal include:
Of course, a good glazing seal design preventing water from entering the glazing pocket in the first place should not be overlooked as an essential feature. The Glazing Manual by The Glass Association of North America (GANA) provides basic recommendations for a reliable water managing design that has as its base objective the preservation of the IG unit’s hermetic seals.
Most window manufacturers design their windows to have good glazing seals, but do not follow or observe all of GANA’s guidelines. One of the reasons for not following GANA guidelines is water in the glazing pocket of wood-framed windows for any length of time is not desirable for preservation of the wood. Therefore, a better external glazing seal is the preferred design method for wood assemblies. The biggest reason for not incorporating features such as weep holes to drain incidental moisture is weep holes tend to lower a fenestration’s air infiltration performance, and this is not a good marketing strategy in a culture demanding better and better thermal performance. Whatever the driving factors, the author finds wept glazing designs are becoming scarce in fenestration products.
Without wept glazing pockets, small amounts of moisture making their way into the glazing pocket are not given the opportunity to completely dry out and can cause a rise in the relative humidity (RH) in the glazing pocket at the perimeter of an IG unit. Additionally, changes in vapor pressure caused by heating and cooling the window frames through seasonal cycles can cause vapor in the glazing pocket to diffuse through the perimeter edge seal to the sealed air space—the seal does not need to be breached for this phenomenon to occur.
When an IG unit is first fabricated, it will have a very low internal dew/frost point. The dew/frost point is the temperature below which moisture can be forced to be released from the desiccant in the perimeter spacer and condense on the air space side of the IG unit. With newly fabricated IG units the dew/frost point is in the range of –51 to –40 C (–60 to –40 F), or lower. By design, this dew/frost point level intends to produce an IG unit with the ability to provide long-term thermal performance. Elevation of the dew/frost point above this initial level indicates external moisture is infiltrating the sealed air space from either a breach in the hermetic seal or by diffusing over a long period of time.
The difference between moisture infiltration by breach and moisture infiltration by diffusion is with a breach, moisture enters the sealed air space and fogging occurs rapidly (e.g. in a matter of days to a week or less, following the onset of the breach), while by diffusion, it could take many years to reach a state where condensation occurs. Another difference is with a breach, the fogging is permanent and irreversible. When moisture vapor is slowly diffusing into an IG unit, intermittent periods of fogging are likely to occur (Figure 2). As mentioned earlier, fogging occurs when the external ambient temperature falls below the dew/frost point of the IG unit. When external temperatures warm to above the dew/frost point, moisture is re-absorbed by the desiccant in the perimeter spacer causing the fogging to temporarily disappear. This is consistent with an IG unit in service for a long period of time in a glazing pocket that is not wept and has glazing seals not 100 percent effective at keeping moisture out. Long-term diffusion of moisture vapor into an IG unit tends to slowly raise the internal dew/frost point until permanent fogging occurs. At this stage the IG unit is deemed as having “failed.”
The official definition of IG failure was published by the Insulating Glass Manufacturers Alliance (IGMA) in 1989 in its Technical Bulletin 1205-89 which states:
Failed IG Units: an installed unit failure exhibits permanent material obstruction of vision through the unit due to accumulation of dust, moisture or film on the internal surface of the glass. Surface numbers 2 or 3 in dual-pane; surface numbers 2, 3, 4 or 5 on triple-panes.
The key words in this definition are “permanent material obstruction of vision.” Emphasis is on the word “permanent” because the IG unit cannot be classified as a failure if it fogs only when the temperature falls to –23 C (–10 F) and remains
clear the rest of the time.
Specifiers should note when the dew/frost point is elevating, the IG unit may reach a point (before permanent condensation appears) where it is not able to meet the desired requirements for interior conditioning or thermal performance for a specific building. This condition may be considered “failure in performance” as opposed to failure of the edge seal because permanent condensation has not yet appeared.
Even if an IG unit begins to have an elevated dew/frost point, it still provides some insulating value. Two panes of glass separated by an air space—even if they are part of two different window units (as in the case of single pane glass window with an outside storm window)—provide insulation value, though not as much as a sealed IG unit with a fully intact hermetic seal. It is accepted when an IG unit begins to exhibit intermittent fogging and clearing at the design level cold temperature for a specific area of the country, the unit is no longer providing insulating value for the user. However, the unit can still continue to provide insulating value at cold temperatures above the design level condition. A quick test to evaluate the insulating value of an IG glass unit is to run an infrared scan of a building at cold temperatures. Units with elevated dew/frost points look the same as the ones with their dew/frost point down around –40 C (–40 F). A distinct difference in thermal value will be visible only when the unit begins to show fogging as condensed moisture conducts more heat than a glass unit with fully intact edge seal.
Elevation of dew/frost points is an indication of imminent, but not present, failure of the IG unit’s hermetic seal. Predicting the time-to-failure based on the elevation of dew/frost points is neither reliable nor practical, even though studies have been conducted on the topic. One notable paper by G. R. Torok was presented at the 11th Canadian Conference on Building Science and Technology, 2007.1 Torok presents in his paper an involved scientific approach for calculating time-to-failure, but the methodology is not (on first inspection) a practical or cost-effective means easily applicable to random IG units. The fact is, if dew/frost points are continuing to elevate permanent failure is imminent.
So why is it important to differentiate between a performance-based failure and one characterized by permanent fogging? The answer in most cases is it really does not matter—when the IG unit fails to perform its intended function it has failed regardless of whether or not there is permanent or intermittent fogging. The differentiation is key when there is an appraisal of a warranty claim on an IG unit or an evaluation regarding the validity of a failure purported to have been caused by a catastrophic one-time event such as hurricanes. The author has reviewed insurance claims where people look to have an insurance company replace all of their IG units regardless of age in the aftermath of a hurricane even if they are not physically broken and there are no visible breaches in the hermetic seals of the units. The argument being fogging has been observed only after the hurricane, even though fogging happens only when temperatures are unseasonably cold.
Most of the time telltale signs are obvious. For example, hard water stains appearing between the panes of an IG unit are an indication the unit has been fogged for many months and often many years, as hard water stains cannot develop between the glass panes overnight or in a week.
Another example where evaluation of IG unit seal failure can be important is in the case of seemingly unrelated events such as leakage caused by ice dams forming on a roof. This latter example is the subject of the following case study.
The winter of 2014-15 was a particularly harsh one in New England. Many regions in the state experienced low temperatures and high snowfalls. As a result, roof collapses and ice dam freeze-ups wreaked havoc on local residences as well as on commercial properties. Leakage and water damage from ice dams was a common problem from the end of January through the middle of March that winter.
One particular residence had extensive water damage on one side of the home as a result of ice dams forming on its roof. The owners also began to observe condensation forming between the glass panes on all the windows. The owners reported condensation would occur on different windows at different times of the day, but disappear when temperatures rose above –18 C (0 F). At the time of the ice dam problems the owners noted the existing windows were more than 25 years old and they had always been pleased with the windows’ performance. The owners remarked they had seen nothing like this and all the windows were performing as expected before the ice dam leakage. During the spring and summer of 2015, condensation was observed on only a handful of windows in the residence.
The owners acknowledged these windows had failed even before winter. Nevertheless, since the mass appearance of condensation coincided with the ice dam leakage, the owners reasoned the ice dams must have caused a complete failure of all the IG unit seals and the cost for new windows needed to be added to their insurance claim for the leakage.
By applying the principles described earlier in this article to the IG units in this home, one can understand the windows were indeed in the process of failing because their dew/frost points were elevated and hovering around –18 C (0 F). It is also clear the windows were no longer meeting the homeowner’s expectations and needs for thermal performance. From a pure performance perspective, the windows had failed. However, there was no evidence water entered the glazing pockets of any of the IG units to cause failure or breaches in the hermetic seals. The owners’ noted leakage never occurred through or around the windows either before or after the 2014-15 winter, even though the region experienced numerous storms later in the year. The author’s observations of the windows in question noted glazing seals appeared to be intact and the window frames themselves were meticulously painted and appeared to be in good service. The most convincing piece of evidence was there was no difference in IG fogging at the site of ice dam leakage and elsewhere in the house. This is a classic example of performance-based failure caused by long-term moisture diffusion. The author has no doubt permanent failure would eventually result, but it could still take years to move to that point. In this case study, ice dams did not cause the IG units to fail. The age of the glass and long-term diffusion of moisture were the culprits. Extreme cold temperatures brought attention to the elevating dew/frost points of the IG units, and not the presence of ice dam leakage as assumed by the owners.
IG units do not catastrophically fail from a single weather event. There is always a rational explanation for failed IG units. The point of failure, though, is often a subjective discussion and under most circumstances is not important. Relevance only matters if someone else is being asked to pay for the failure.
1 Read the paper here.
Michael J. Louis, PE, is senior principal at Simpson Gumpertz & Heger Inc. He specializes in glass-and-metal fenestration systems, and has extensive experience with below-grade and terrace waterproofing, exterior walls, and roofing. In his over 30-year career, Louis has investigated existing systems as well as participated in the design, construction, and testing of new and custom work. Louis can be reached at email@example.com.
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