Volume 39, Issue 4, April  2004

Hazy Shades
Fogging Potential in Insulating Glass Units
by Robert Spindler

According to certification information from the Insula-ting Glass Manufacturers Alliance (IGMA) the inability to pass the required fogging test is the number one reason insulating glass (IG) units do not obtain certification. Chemical fogging occurs when any material in the IG unit airspace has volatized and condensed on the number-two or number-three glass surfaces. Materials in the airspace and/or those used in manufacturing the IGU can degrade and volatize under ultraviolet (UV) -light and at high temperatures (100 degrees Fahrenheit to 140 degrees Fahrenheit), depositing a microscopic film on the glass airspace surfaces. Some of the new low-E and reflective coatings are more absorptive to solar energy than older types were, making the glass and airspace warmer. When these high temperatures occur, materials in the airspace can volatize. This can cause a chemical fog on the number-two and number-three glass surfaces in the IG unit airspace.

Chemical fogging looks similar to condensation and is more apparent with low-E coatings and usually will not dissipate. 

Characteristics to Consider
When manufacturing IG units there are measures that can be employed to minimize fogging potential. One way is material selection. Spacers, sealants, grilles and desiccated matrix materials can all cause fogging and should be tested for their fogging potential.

When choosing fabrication materials the following should be considered:
• Gas and moisture barrier;
• Chemical fog resistance;
• Long-term durability; and 
• Thermal performance.

High temperatures on the glass and in the airspace can occur on the coatings and glasses, especially heat-absorbing ones. Manufacturers should be careful of the materials used in the airspace, as certain ones can volatize and cause fogging.

The unit’s geographic location can also add to the high temperatures of the glass and airspace. Sunbelt locations—those with high ambient temperatures and high solar intensity--can produce glass and airspace temperatures of 130 to 150 degrees Fahrenheit.

Most IG manufacturers warrant units against an obstruction of vision in the airspace for a period of time ranging from five years to a lifetime. Vision obstruction could be caused by a seal failure (moisture in the airspace) or from outgassing of materials used in the manufacturing of an IG unit (chemical fogging). Therefore, chemical fogging, in addition to moisture in the airspace, could be considered a warranty claim.

Determining Fogging Potential
There are ways for manufacturers to test IG units for fogging potential. Using the Canadian fog box method, there are three testing protocols that can be used to determine potential fogging. The test protocols are:
• ASTM E-2189;• CAN/CGSB-12.8-97; and
• Cardinal IG’s glass donut test 

The tests are similar, but two require a higher testing temperature.

Through the CGSB fog box method and our company’s protocol, a unit is tested for one week at 140 degrees Fahrenheit, at constant UV light and with a cold plate at 70 degrees Fahrenheit.

The ASTM method is also a one-week test. It requires a temperature of 120 degrees Fahrenheit, constant UV light and a cold plate at 70 degrees Fahrenheit. Test results should show no fog at arms length. 

This temperature differential in an IG unit could be caused by an overhang or other condition that causes a shadow pattern on the glass. 

While the tests are similar, some companies prefer the tests that have higher temperatures because they simulate better the temperature IGUs will see in the field. 

The glass donut test method (which was developed by our company) is another means for determining fogging potential. A number of different components (i.e. the spacer, desiccant, desiccated matrix, grilles and the primary and secondary sealants) make up an IG unit. It may not be apparent which component caused the fogging. Rather than testing a complete IG unit, the glass donut isolates individual components to determine specifically which part of the unit is causing the fog. 

The glass donut consists of a ˝-inch lite of glass with a hole cut in it, (this acts as the spacer), a lite of glass on each side (one with a low-E coating and one clear) and a bag of desiccant in the donut area to deepen the dew point to below zero degrees Fahrenheit for the duration of one-week.

If fogging does occur during the fog test, it will be more apparent on coated (low-E and reflective) glass substrates than on clear glass. With clear glass the fogging sometimes has no color, but with low-E or reflective glass products fogging can generate color problems on the glass. This is due to the added thickness of the fog on the glass. 

Some materials in the airspace of low-E or reflective coated glass units can cause the coatings to corrode. It is important to be certain that the outgassing materials will not affect these coatings.

Fogging Rating System 
When determining an acceptable level of fogging, the following is a rating system example developed by our company:
1. No fog;
2. Slight fog at acute angle;
3. Medium fog;
4. Heavy fog; and
5. Heavy fog with volatile droplets on glass.

Our company prefers a level no greater than two, but there are instances (such as with clear glass) that a level three may be permitted.

In summary, IG manufacturers and component suppliers should evaluate and test all components used in the fabrication of IG units. A fog test is now a standard practice to obtain IG certification either through IGMA or the Insulating Glass Certification Council. Having each component of the IG unit tested and passing the Canadian fog box or ASTM standard for fogging, manufacturers can be more assured that the materials used in the fabrication of the unit will not cause a potential chemical fog and a possible warranty claim. 


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