Window Moisture

Very few people ever take note of the window glass in their home, which is not surprising considering the nature of the material - it is there specifically NOT to be seen. It is, however, an important component of a building's exterior cosmetically and integral to its energy efficiency.

Insulated glass is a unit formed by bonding two glass panes together with a sealant (more on this later.) Generally, this material only gains the attention of homeowners once it starts to break down and make itself visible in the form of condensation build-up. We've all seen it, water droplets forming a halo in the middle of a window - it might even strike you as beautiful at first, but as time passes the moisture becomes permanent and eventually gives way to an ugly chalky build-up that muddles your view and reduces the value of your home.

In this two-part blog posting, we'll take a look at the make-up of insulated glass units (IGUs), what causes moisture infiltration, and what cost saving measures are available for restoring your windows to their original clarity.

What Am I (Not) Looking At?
Like I mentioned above, the basic components of an IGU are two pieces of glass separated by a spacer and held together by sealant - not exactly exciting stuff, but important to energy conservation in your home. Let's look a little closer at the individual parts and how they can effect your energy bill.

Glass
IGUs found in homes typically use 3/32" to 1/4" glass. The thicker the glass, the less thermally conductive the unit. When the temperature outside drops thinner, more flexible glass bows inwards, decreasing the distance between the two panes thus facilitating heat transfer. In some extremely thin units the panes actually flex so severely that they touch in the center.

Another important feature to note is low-E coating, which is an add-on component, not found on every IGU. This reflective coating, applied to the glass during manufacturing, greatly reduces the heat absorbed through the unit by deflecting a portion of the sunlight, keeping your house cooler during the summer months. Low-E is applied to the inside of the exterior pane so it can provide the most protection possible without allowing it to degraded by the elements. Low-E is particularly useful in windows located near a valuable piece of furniture or a treasured family heirloom, because it blocks out harmful UV rays.

The last thing to bare in mind with IGU glass is whether or not it is tempered. Tempered glass is heat strengthened and therefore less susceptible to breakage due to heat stress. You'll know your glass is tempered if you find a small block of white print in a corner referred to in the glass industry as a "bug". Like low-E, tempered glass will substantially increase the cost of the unit.

Spacer
As the name would imply, the spacer is used to separate the glass panes, but it also plays a role in reducing thermal conductivity. Spacers are typically made from either aluminum or structural foam and there are advantages unique to both. Aluminum is a durable material but also highly conductive when there is no barrier between it and the glass. Our company uses aluminum, but the units are dual-sealed with butyl tape which forms an insulated barrier between the materials and resists energy transfer. Structural foam is able to achieve the same low thermal conductivity, but lacks the strength of aluminum.

In the diagram above, you'll notice that the spacer is hollow and packed with minuscule balls. This highly-absorptive material is called, desiccant, and prevents moisture build-up in the unit.

Around the perimeter of the spacer and between the edges of the glass, a polysulfide sealant is used to hold the unit together and further insulate it against energy exchange.

Air/Monatomic Gas
The space between the two panes is filled with either dry air or a gas that has low thermal conductivity. The most common gases used for insulation purposes are argon and krypton; argon has 67%  the thermal conductivity of air, while krypton has half the conductivity of argon. These gases can increase the energy efficiency of your home, but you definitely pay for that benefit up front.

Next posting: What's causing moisture build-up and what can be done about it.