Plavecsky's Ponderings By Jim Plavecsky
by Jim Plavecsky
January 16th, 2020

Insulating Glass Sealant—Critical to Keeping Windows Clear

I was recently asked by an insulating glass (IG) fabricator how much sealant he needed to use when making his IG units (IGU) to pass the insulating glass certification test. The IG certification test, E2190, is a very stringent test that simulates extreme conditions for the test units, trying to force failure in a matter of weeks. These test conditions force moisture vapor into the unit while also stressing the sealant to the brink of failure.

The IG sealant and spacer/desiccant are the belt and suspenders of an IGU. Basically, what you have here is a sealant that is trying to keep moisture vapor out of the unit. This is coupled with a spacer containing a finite amount of molecular sieve that is capturing the moisture vapor as it enters the IGU, inhibiting the creation of fog for as long as the drying agent holds out (hopefully the life of the window system). Once the moisture vapor overwhelms the molecular sieve, the fog moves in to destroy the clarity of the window unit and thus ruins the view for the homeowner.

So, given the fact that any sealant—no matter how good it may be—can not totally stop the flow of moisture vapor from getting into the unit, the molecular sieve or desiccant will someday be totally saturated and the IGU will then fog. The only question becomes, how long will this take?

Properties of the sealant are critical when it comes to the longevity of the IGU. One of the most critical of these properties is what is called the Moisture Vapor Transmission Rate (MVTR) and this is basically the rate at which moisture vapor can move through the sealant over a 24-hour period. The lower the number the better!

This is something that should be evaluated when it comes to choosing the sealant that you will use to make your IGUs.

Think of your sealant as a patch of woods between two open fields. You have a group of soldiers trying to get from one field to the other field to create havoc on the other side. They must go through the patch of woods (your sealant) to get to the other side. So, the thicker this patch of woods is, the longer it will take them. Over a given amount of time, fewer soldiers will make it across if you have a good sealant.

Then, once the soldiers get to the other side, think of the desiccant or molecular sieve as enemy allies capturing these enemy soldiers and putting them into cages so that they cannot do any harm on the other side. The more molecular sieve or desiccant that is used, then the more moisture vapor that can be contained or captured before it can start to do harm to the insulating class airspace by creating the fog.

When it comes to argon or any other exotic gas that you are trying to keep inside the unit, it must also pass through the sealant barrier to escape the unit. So, the other property of the sealant that is also crucial is the argon or krypton gas permeability rate—the rate at which the exotic gas in question can pass through the sealant over a given period of time. In this case, the lower the number, the better. The lower the gas permeability rating the better chance your unit has of keeping the energy-enhancing gas inside the unit so that the thermal benefits are long-lasting. Workmanship, of course, is also important.

An IGU is constantly expanding and contracting as temperature goes up-and-down and barometric pressure is changing. Ultraviolet (UV) radiation from the sun can destroy chemical bonds that keep the sealant flexible, so this intense UV radiation is also included in the ASTM E-2190 test protocol.

Not only does the sealant have to provide a moisture and gas barrier but it also must provide long term adhesion between the spacer and the glass and hold it all together. Remember, it must also do this while allowing flexibility as the unit is constantly being subjected to changes in size brought on by temperature and pressure changes. Flexibility must be ongoing and long-lasting or the sealant will snap and adhesion to glass and/or spacer will be lost!

So, to answer my customer’s question, I explained, “Yes, there is a minimum ‘cut-back’ which defines a minimum amount of sealant to be used to make a quality unit. But the choice of sealant is far more important when it comes to increasing your chances of passing the certification test.

When it comes to fabricating an IGU that will last the lifetime of the window system, your sealant selection becomes critical. It is that second half of the ‘belt and suspenders’ system that keeps your IGU clear for years to come!”

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  1. Well, I read this with interest, thinking I knew where you were going, but I was wrong. I expected the article to end with you pointing out that a manufacturer might talking about putting extra sealant on their test windows to pass the test and less on regular production! In other words, the question of how much sealant might only refer to making good units for the test. Of course, we’ve never seen a manufacturer actually do that, have we?

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