January 18th, 2024
I am a member of a Facebook Group called Ask Replacement Window & Door Experts, to which an interesting post popped up recently. The member posted a picture of his icy window that looked like it was mounted on an igloo on the North Pole. The caption was, “It is -11 outside and it was 71 in this room when I woke up. Is this normal?” Well, it’s that time of year again!
This is the time of the year when many window fabricators and window dealers start getting phone calls from angry and confused customers about condensation.
With outside temperatures hitting extreme lows and inside temperatures 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 inside a 71-degree home through the windows to a -11 F outdoor environment, this creates very cold temperatures on inside surfaces of the window, causing condensation to form. The greater the humidity level in the home, the greater the amount of condensation that will form on the window surfaces.
As we all know, homeowners mostly keep their windows shut during the cold winter months, resulting in higher humidity levels, especially in modern well-insulated homes.
As I explained to the homeowner in the Facebook group: It’s not that his windows are defective, but rather they are just not that great at resisting the build-up of condensation. The build-up is due to their inherent level of performance and given the cold circumstances of the day, coupled with indoor humidity levels. Now, triple-pane vinyl and wood windows made with non-metallic insulating spacers and Argon (or Krypton) offer higher levels of condensation resistance, so I suggested that he look into these options for his next window purchase.
But then later in the week, after doing some research, he came back and told me that he found dual-pane windows that are labeled Energy Star-compliant in the Northern climate zone, so “they must be a viable solution, right?” However, the windows he found were made using 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. As you know, the NFRC label is provided to list the window U-value but condensation resistance (CR) ratings are optional and usually not listed on the label.
Now, when a fourth-surface low-E coating is employed, radiant heat from inside the home 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 window with a very low U-value, usually in the neighborhood of 0.20 (with a soft-coat low-E on surface number two) but with an unusually low surface temperature on the inside lite of glass. Add in the heat transfer to the outside, which is caused by very cold wintertime temperatures, and you have a recipe for a higher degree of condensation to occur on the inside lite of glass and frame. In some cases, this temperature can fall below 32 F, and this means ice formation!
One useful tool to help educate window consumers is to suggest the use of a hygrometer. Homeowners can use an inexpensive hygrometer to measure the humidity level inside their homes. Some dealers provide hygrometers as an educational tool along with a chart showing homeowners the inside humidity level that they could start expecting to see condensation occur for a given outside temperature, assuming an indoor temperature of 70 F. One thing that should 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 indeed trigger it. The colder the outside temperature, the lower the trigger point will be, in terms of indoor humidity level.
The point to keep in mind is that with the use of surface-four low-E coatings, this trigger point can be a lower indoor humidity level, even though the window is advertised as being a premium product posting a very low window U-value. So, most homeowners would expect it to perform better in 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, surface-four coatings make it easy – no worries about pocket size, extra weight of triple-pane glass, heavy duty balance systems, etc. But you know the old saying: “There is no such thing as a free lunch.”
This is precisely why the industry is so interested in the concept of skinny triples, a concept which provides triple-pane advantages without the extra size and weight. Skinny triples may very well represent a way of having one’s cake and eating it too, and very well could be a viable solution to the condensation conundrum!