WINDOWS 101

Window Durability

The durability of a window has direct implications for its energy performance, ease of use, and appearance. There is a significant variation amongst products on the market today in terms of durability, even though in many cases the products appear very similar when new. There are two durability areas in particular that need to be addressed for a wise window purchasing decision. The first is the type of framing material, which will be discussed later in this section. The second is seal failure resulting in condensation between glazings.

EnergySmart Construction has, on a regular basis, performed window replacement projects for customers who have already replaced their windows in the past ten years. These customers unsuspectingly purchased products they believed would last for a prolonged period of time. The products, unfortunately, failed. The most common failure is condensation between the glazings.

To understand how this problem occurs, it is necessary to first understand how a sealed unit is assembled. A dual pane unit, also called an insulating glass unit (IGU), consists of two panes of glass, a spacer, and a sealant. The two panes of glass are separated by the spacer, and the unit is then sealed at the perimeter. The now permanently sealed unit prevents any exchange of air between the outside of the unit and the inside.

In the heat of the day, the temperature inside the IGU soars, causing the air to expand. Since there is no air being exchanged between the outside and inside of the IGU, that expansion causes pressure on the seal. When the IGU is new, the integrity of the seal contains the pressure easily. However, as the unit ages, the seal becomes weaker. Eventually, the seal is weak enough to be overcome by the pressure that builds up in the unit. Then, generally on an excessively hot day, the pressure punctures the seal and pushes air out of the unit. That evening, as temperatures cool, the air that's left inside the unit cools and contracts. Since now there is less air molecules in the unit, the contraction creates a vacuum, which draws air from the outside into the unit through the puncture. The air drawn in contains moisture, which then condenses on the colder surface of the exterior pane. When this occurs, the energy efficiency of the unit has been significantly compromised, and an unsightly film builds up on the glass.

Insulating glass failure will eventually occur on each and every IGU. Failure is simply a function of the degradation of the seal, which occurs over a period of time given weathering exposure. When IGU failure occurs on a permanently sealed unit, it is not possible to fix. Rather, the unit must be replaced.

Due to the cost of the custom sized IGU, and the labor involved in its replacement, the cost of replacement can often equal the cost of replacing the entire window. It is for this reason that EnergySmart Construction has often performed window replacement projects for customers who have replaced windows less than ten years ago.

Fortunately, there is a massive body of scientific data available on the subject of IGU seal failure, which can aid in the window selection decision. Specifically, a set of accelerated laboratory tests have been conducted over the past few years that can provide a very good indication of the longevity of the IGU before seal failure. The basic idea behind these studies is to expose the different IGU's available to extreme weathering conditions in the laboratory. That is, the units are placed in a machine that cycles through extreme heat, extreme cold, extreme UV, salt and moisture.

Figure 3 shows a table which summarizes the test results. You will notice that the test results are in weeks. The basic hypothesis is that each week the IGU lasts under extreme weathering conditions of the test, the IGU will last approximately one year under normal weathering conditions of the South/Central zone (1week/1year ratio). There have been longitudinal studies whose results buttress this hypothesis.


Figure 3: IGU Seal Failure Accelerated Testing Results.


Final Note on IGU longevity: Recent studies have shown that when grids are placed in between the two panes of glass, the IGU longevity is greatly reduced. There are a number of theories about why this is so, mostly focusing on temperature differentials of the grid material (aluminum) and movement of air particles around the grids. This fact should be taken into considerations when choosing a window product. In other words, it is preferable to choose a window whose longevity test results are at the higher end if grids are desired.

In order to make a wise window purchasing decision, it is pertinent to understand the type of IGU each product considered utilizes. A knowledgeable window professional can help provide such information.