3. Evaluate cavity pressure and edge seal forces exerted by environmental loads:

• Since the lites do deflect to relief the pressure—at least to some extent—the un-equalized cavity pressure must be converted into a partially equalized cavity pressure. This should be done for the stiffest IGUs identified. There are several methods for calculating the partially equalized cavity pressure, from easy-to-use, commercially available, IGU structural analysis software, to a customized finite element analysis.
• The partially equalized cavity pressure can now be translated into a force on the edge seal of the IGUs. Below is a simplified equation to allow easier and faster estimates (see reference 5 for a more accurate equation):

q_e = ½ a F_d

• q_e = Edge seal force
• a = IGU width
• F_d = Partially equalized pressure

4. Compare expected in-service edge seal forces to qualification test edge forces. The force on the IGU seal can now be compared to those experienced by the edge seal when qualified according to the standard durability specifications, such as ASTM E2190. If the calculated force is lower than the one used to qualify the IGU durability, then no action is needed. If the force is higher than the one qualified, then a decision needs to be made about addressing the design of these stiffest units. The decision depends on the quantity of units affected, the degree of excess load, and the project team’s experience with the edge seal system.
5. Modify the IGU design. If modification is needed to reduce the load on the seals, there are several options to consider:

• Reducing the stiffness of the units. This can be achieved by either increasing the size of the units or adjusting the make-up by reducing the thickness of the lites or using smaller or fewer cavities.
• Increasing the IGU seal joint stiffness. This can be achieved by specifying a higher modulus silicone. These silicones stretch less for a given force to reduce the polyisobutylene (PIB) extension. However, use of such a silicone would have been qualified to the durability standards. The contact width of the silicone can also be increased, but it is not a linear impact, so it may not be possible to reasonably achieve the performance needed.
• Reducing climate stressors. If altitude is a contributor to the climate load, then these long-term forces can be reduced by implementing permanent cavity pressure equalization. If temperature is the driving climate load, changing the IGU design to reduce solar absorption is a strategy to reduce these loads.

In addition to designing for climate loads, spacer, and sealant material choices, there are also several other best practices for reducing service life risk and specifiers should be cognizant.

Managing fabrication quality risk

It is not sufficient enough to be able to meet the requirements of the IGU seal weathering standards once; fabricators need to be able to deliver the same quality consistently from their factories. IGU certification programs have been developed in North America to recognize fabricators who have quality management programs in place and can demonstrate that IGUs fabricated on their production lines regularly meet the requirements of ASTM E2190 or CGSB 12.8. The Insulating Glass Certification Council’s (IGCC’s) program and the Insulating Glass Manufacturers Alliance Certification (IGMAC) program are recommended because they require test IGUs to be made on the production line in the presence of an auditor. Specifying IGU certification through these programs is helpful in managing manufacturing quality risk.

Managing installation quality risk

Installation can have a significant impact on IGU lifetime. Excellent water management is essential in framing systems. If the edge seals are subjected to contact with water for long periods, the edge seal will degrade quickly (Figure 8). Therefore, it is important to follow the manufacturer’s instructions for frame fabrication and installation to ensure the effective functioning of weep systems.

Supporting the edges of all glass lites in the IGU is also critical (Figure 8). Without support, the weight of a lite can cause sheer of, and subsequent damage to, the edge seal, resulting in premature failure.