February 14th, 2014
Fourth Surface Low-E Coatings – A Prescription for ‘Pane’ But Not Without Side Effects
During my recent travels to Michigan with the Polar Vortex temperatures in full effect, it seemed as though every window company I was visiting was getting phone calls from angry and confused customers about condensation. It seemed like condensation complaints were on everybody’s mind. “Yes we like when the phone rings, especially this time of the year when sales are slow, but not to hear about condensation complaints! We want to be out selling more windows but it seems like we are driving around visiting angry and frustrated customers to investigate condensation complaints,” said one frustrated window dealer.
With outside temperatures hitting record lows and inside temperatures remaining toasty, the driving force for heat to escape across a window boundary is very high, and “heat goes from where it is hot to where it is not.”
As heat migrates from the inside of the window to the outside, the inside pane of glass becomes cooler than room temperature. Now take this situation and add in a fourth-surface low-E coating. Many window manufacturers are using this technology to achieve very low U-value ratings which they can post on their NFRC labels and consequently compete in a thermal performance league usually reserved for windows made with triple-pane insulating glass construction. As you know, the NFRC label is required to list the window U-value but Condensation Resistance (CR) ratings are optional.
Now, when a fourth-surface low-E coating is employed, radiant heat from inside the room is immediately reflected right back into the room before it even has a chance to warm that inside pane of glass. Therefore, what you get is a dual-pane window with a very low U-Value, usually in the neighborhood of 0.20 (when combining a soft coat low-E on surface number 2) but with an unusually low surface temperature on the inside pane of glass. Add in the heat transfer to the outside being caused by very cold wintertime temperatures, and you have a recipe for an unusually high degree of condensation to occur on the inside pane of glass. In some cases, the temperature of the inside pane of glass can fall below 32 degrees F, and this means ice formation. Yes – ice on what is supposed to be a high-performance window system. I was actually in a dealer’s showroom when a call came in with a homeowner complaining about ice, and the salesman did not even know what to say other than, “Are you sure ma’am that it is really ice?” Next, I could hear the shrill voice of what sounded like Charlie Brown’s teacher coming through his phone!
I happened to have a few hygrometers with me on this trip along with a chart showing homeowners what inside humidity level they could start expecting to see condensation occur for a given outside temperature (assuming an indoor temperature of 70°F), and the window salespeople bought me lunch in return for these hygrometers. It seems that educating the consumer is a better long-term remedy. One thing that must be explained to the homeowner, however, is that, no matter how good your window might be in terms of resisting condensation (the CR rating), there will be a certain humidity level that will trigger it, and the colder the outside temperature, the lower this trigger point (indoor humidity level) will be.
The point to keep in mind is that with the use of fourth-surface low-E coatings, this trigger point can be a relatively lower indoor humidity level even though the window is advertised as being a premium product posting a very low window U-value. Now, most homeowners might expect a premium window system it to perform better with respect to condensation during cold wintery conditions—not worse.
So what’s the solution? If you are marketing windows where outdoor temperatures are likely to get very cold, which also happen to be the same market areas where the lowest U-values are most desirable, perhaps triple-pane construction is the way to go when it comes to fabricating your high-end windows. Sure, fourth-surface coatings make it easy—no worries about pocket size, extra weight, heavy duty balance systems, etc., but you know the old saying, “there is no such thing as a free lunch.”
Another way to look at it is to reconsider the relative importance of U-value in relationship to other factors such as CR rating. Perhaps you want to market a dual-pane window with a higher U-value than some of your competitors while boasting a higher CR rating than your competition. It is not mandatory, but CR ratings can also be listed on the NFRC label and featured in your marketing literature. Perhaps you can educate your customers about the advantages of higher CR ratings, such as clearer view, less moisture (and ice) buildup on inside window surfaces, less chance for mold formation and in the case of wood windows, a lower propensity for rotting to occur.
Sure, U-value is important, but what degree of value are we offering (or not) to a consumer when selling him or her a window boasting a 0.20 U-value only to have the same customer become extremely unhappy with their “premium windows” when these cold winter days descend upon us?
So, just consider an overall balanced package of benefits to the consumer when deciding whether or not to market the fourth-surface low-E glass products in your geographic region—they might look like a great “prescription for pane” but not without some side effects.