Volume 35, Number 1, January 2000


Sealed with a Hiss

Improving the Durability of IG Units

by Jim Plavecsky


No matter how well-designed a window may be, it will never perform better than the insulating glass (IG) edge-seal, because if the edge-seal fails, so does the window. The consumer cannot see through a fogged IG unit, making the window useless. So, to ensure long-term customer satisfaction, the window manufacturer needs to place major emphasis on the long-term durability of the IG unit.

The major factors to consider in building IG with long-term durability in mind are: wpe18.jpg (7158 bytes)Figure 1: Chemical fog.

1. IG unit design, including spacer selection and type of construction—single-seal versus dual-seal design;

2. Sealant selection and amount used;

3. Desiccant selection and amount used;

4. Type of framing system–continuous vs. cut and keyed;

5. Argon retention;

6. Workmanship; and

7. Proper glazing practices.

 wpe19.jpg (8365 bytes)Figure 2: The hot box test.

Spacer Selection

As a result of increasing concern over maximizing thermal performance, many warm-edge spacer options have entered the market. Many of these spacer systems are comprised of organic materials, which possibly could decompose when subjected to elevated temperatures. It is very important, therefore, to be aware of chemical fog when choosing a new warm-edge spacer type. Chemical fog is caused when chemical vapors are released by organic materials as they are subjected to heat (see figure 1). Depending on what type of desiccant system is used, these vapors may linger within the IG unit where they will condense onto the glass resulting in a permanent discoloration. Low-E glass types are particularly sensitive to chemical fog, and this is the type of glass most often used in combination with warm-edge spacers. IG units can be tested utilizing a “hot-box” test, where the unit is heated with a high-intensity bulb, and any resulting chemical vapors are allowed to condense by attaching a cold-plate to the outside surface of the glass (see figure 2). Chemical fog may then be seen by placing the IG unit in a chemical fog viewer.


Single-Seal Versus Dual-Seal

Single-seal units are constructed of only one type of sealant, which is used to perform double-duty. Not only must the sealant retard the infiltration of moisture vapor, but it must also hold the unit together under a wide variety of both high and low temperatures while withstanding the effects of high-humidity and ultraviolet exposure. A dual-seal unit is constructed using a combination of a sealant which functions mainly as a high-strength adhesive and a second sealant, which is used primarily as a moisture vapor seal. Employing two components which combine the best of both worlds, dual-seal units generally are reported to have lower failure rates than single-seal units, but workmanship can be an overriding factor here. Even the best-designed unit can meet an early demise if quality workmanship is lacking.


Sealant Selection and Amount

When choosing a sealant, the most important factor to consider is the Moisture Vapor Transmission Rate (MVTR). The MVTR is the rate at which moisture passes through the sealant at the bond-line interface to enter the IG unit. Assuming that the sealant maintains its structural integrity, and has the proper amount of desiccant, the unit with the lower MVTR will last longer. The amount of sealant used is also important. This is measured as the MVTP or Moisture Vapor Transmission Path, which is the distance that moisture must travel across the sealant barrier in order to enter the IG unit. Units with longer MVTPs will generally be more durable. If you are fabricating a single-seal unit, try to look for a sealant that combines low MVTR with some degree of structural properties since you are relying upon one sealant to do it all. The new Dual-Seal Equivalent (DSE) sealants definitely are worth considering for added durability.


wpe1A.jpg (7654 bytes)Proper Desiccant Selection and Amount

The type and amount of desiccant used is also important. Some desiccants, such as the 3A types, absorb moisture only. Others, such as 4A or 13X, will absorb some degree of nitrogen and argon (see figure 3). This can cause negative pressure within the IG unit adding stress to the bondline. A unit with negative pressure, which is then shipped to a lower elevation, will experience even more stress and possibly severe

The amount of desiccant used is another critical factor to consider. A window manufacturer providing a full-replacement warranty for 20 years should consider filling all four sides of spacer with desiccant or using a continuous spacer with desiccant contained within. As moisture works its way into the unit over the course of time, the unit with twice the desiccant can last twice as long, assuming nothing else goes wrong.


Framing System

In general, a continuous framing system offers improved durability versus cutting spacer bar and using plastic corner keys to hold the framing system together. As the unit expands and contracts with temperature and barometric pressure changes, corner keys can eventually pull apart, stressing the sealant and leaving a gap in the corner. As a result, there is a much higher moisture vapor infiltration, the desiccant also becomes saturated quickly, and the unit fails. Therefore, continuous framing systems such as those offered by Swiggle®, Super Spacer®, and metal spacer-bending generally result in improved durability ratings.

Argon Retention

Believe it or not, argon retention is a significant factor to consider for ensuring long-term durability. I sometimes hear the comment, “If the argon leaks out, who will ever know—you can’t see it!” Well, the problem here is that because of the Law of Partial Pressures, argon leaks out of a unit faster than air re-enters to replace it. A unit with poor argon retention eventually will develop severe negative pressure within the unit. This results in glass deflection with reduced thermal performance (smaller air-gap), visible glass-distortion, and puts a high degree of stress on the glass and sealant bond-line.

As a result, units with poor argon retention stand the risk of early failure or replacement due to consumer complaints of increased condensation on the inside window surface or a distorted view through the window itself.

A septum is installed in the spacer system allowing insertion of a gas-tight syringe to sample the unit. Units are sampled immediately after production, and secondly, upon completion of both high-humidity exposure and weather cycling tests. Units with the lowest averages exhibited the best overall retention rates. The “best” number is simply the best or lowest gas-loss that the units are capable of, assuming the best workmanship, and the standard deviation is a measure of how consistent the results were. Units with high standard deviation were likely assembled with inconsistent workmanship – some good and some bad, whereas units with low standard deviation were more consistent in terms of workmanship.

This leads me to a discussion of the last, but certainly not the least significant factor, which is workmanship itself. Over the years, I have seen manufacturers of single-seal units manufacture IG units with relatively low failure rates and I have seen dual-seal units manufactured that exhibit extraordinarily high failure rates. In both cases, the main factor involved was workmanship. Therefore, there is something to be said for manufacturing methods that involve fewer manufacturing steps and/or automation.

Proper Glazing Practices

Although IG units are designed to withstand the effects of water vapor, they are not always the best systems for withstanding the effects of water itself. Even a perfectly made IG unit can fail early if the bond line is subjected to standing water. Insulating glass units must be glazed properly into the window in a manner that prevents water from coming into long-term contact with the edge of the unit.

 Test Methods

ASTM E773. IG unit durability is evaluated with several test methods. The ASTM E773/774 test is a 15-week test where units are preconditioned in a high humidity chamber at 140 F and 100 percent relative humidity (RH). Units are then weather-cycled from temperature extremes of –20 F to 135 F while exposed to UV radiation (see figure 6). Upon completing the first stage of testing and exhibiting a –30 F or lower frost point, the units receive a “C” level rating. After completing the second stage with a –20 F or lower frost point, units are certified at the “B” level rating. Completing the final stage with a –20 F or lower frost point results in a “A” level rating. The Sealed Insulating Glass Manufacturer’s Association (SIGMA) has an ongoing field correlation study that does show a correlation between this test and actual field results.

P-1 Test. The P-1 test is also used internally by many window manufacturers as a measure of long-term durability. Units are subjected to 140 F and 100 percent RH with a bank of high-intensity UV bulbs beaming radiation directly against the bond-line interface. Chemical reaction rates will double with each 10 F increase in temperature. Therefore, one week at 140 F is generally believed to be equivalent to two weeks at 130 F, four weeks at 120 F, eight weeks at 110 F, 16 weeks at 100 F, 32 weeks at 90 F, and 64 weeks or 1.2 years at 80 F.

Single-seal systems will generally last only six to 12 weeks in a P-1 Test, but dual-seal configurations have withstood over one hundred weeks of
exposure in this test (see figure 7). Once again, workmanship is critical, and units with inherent flaws will most surely fail early in this test.

IG unit manufacturers have also shown that success in the P-1 test also correlates to real world performance, assuming that workmanship remains consistent on a day-to-day basis.

In addition to test data, it is very useful to ask a number of manufacturers about their failure rates and compare this real-world data for different IG edge-seal systems.

 Investment in Durability

Money spent to make a higher performance IG unit represents a very high return on investment. The Net Present Value (NPV) and Internal Rate of Return (IRR) can be calculated for the investment in superior quality, especially when full replacement warranty programs are in place. An Excel spreadsheet program can be used to evaluate the financial benefits of manufacturing IG with improved durability:

NPV = PV – I, where PV is the present value of the cash savings in warranty expense for each successive year, and I is the cost of the initial investment. The investment could be one or all of the following:

• The cost of superior materials (spacer, desiccant or sealants) to construct a higher performance IG unit.

• The cost of capital equipment to simplify and improve the manufacturing process.

• The cost of quality assurance programs to improve workmanship practices.


Internal Rate of Return

The Internal Rate of Return (IRR) is the true interest yield provided by the investment in more durable IG. The IRR is the interest rate at which the present value of future warranty expense savings is equal to the initial investment.

In the spreadsheet example, a window manufacturer is considering a switch to a more durable edge-seal system, which will reduce his failure rate by one half of a percent. This system costs him $.05 per foot more in materials and requires an equipment investment of $100,000, but it will reduce his warranty expense by $50,000 per year over the life of the warranty. The NPV, the present value of the cash savings less the initial cash investment, is worth $35,944 if he offers a five-year warranty and $142,934 if he offers a ten-year warranty. The corresponding internal Rates of Return on Investment (ROI) are 19.9 percent for the five-year program and 31.1 percent on the ten-year program.

The following year, when his equipment is paid off, it gets even better. Now the investment to make more durable IG is only $55,000, which includes the $.05 increase in materials per perimeter foot of IG plus $5,000 in maintenance costs on the equipment. Another 100,000 IG units are made and installed in windows. Now, the NPV with a five-year warranty is $122,308 and it is $229,298 for a ten-year program. Corresponding IRRs are now 86.9 percent and 90.8 percent respectively.


With the emergence of the information age, homeowners are becoming more knowledgeable than ever before. The result is an increased level of knowledge at the consumer level, and a demand for improved warranty programs.

Because of the potential liability involved with servicing these warranty programs, the IG unit is definitely not an area to skimp on when it comes to choosing the proper materials and process equipment to make units that will withstand the test of time. The financial benefits of investing in materials and methods for improved durability can be tremendous.

Jim Plavecsky serves as vice president of marketing and sales for Edgetech IG, based in Cambridge, OH.



© Copyright 2000 Key Communications, Inc. All rights reserved. No reproduction of any type without expressed written permission.