Low-e windows can make a home more energy-efficient and have unintended consequences such as high humidity or HVAC equipment short-cycling.

How Can Installing Low-E Windows Hurt a Home?

To answer that question, one needs first to understand that today’s HVAC systems are designed specifically for the homes that they’re in. Several factors, including solar orientation, number of windows, number of appliances, number of people, and quality of insulation and construction, go in to properly sizing both the air conditioning and the system’s heating aspects.

If low-e windows are installed during the initial build, as is done on almost every home today, most any reputable HVAC contractor or architect will size the system accordingly using the U-value (heat resistance) and SHGC numbers (Infrared resistance) of the windows, as well as what type of materials used for the frame. However, suppose a homeowner is remodeling an existing house. In that case, there is a possibility that with a good enough increase in the window efficiency, the existing HVAC system will now be too large for the home.

This can result in excessive humidity in the Summer months due to short-cycling, or the system only running for very brief amounts of time each time it cycles on. As the air conditioner is also the dehumidifier for the home, this short-cycling can cause a build-up of moisture in the air, leading to mold or mildew problems developing. It can also lead to high power or gas bills in the winter due to the same short-cycling problem, as the heat will not have to run as long each cycle to maintain the interior temperature. This is because a not-insignificant portion of either the electricity or gas bill, depending on the type of system, is used to bring the heating element up to temperature each cycle before the fan even comes on.

Homeowners Should Request a Heat Load for their Home Using the New Low-E Window Data

As many homeowners are unaware of this potential problem, I decided to write this companion article so that anyone reading this can make an informed decision. I would recommend homeowners consider upgrading their windows, take the new window data to a licensed HVAC contractor and request a Heat Load be done on their home to determine whether the new windows are compatible with the existing sized system.

As an example of just how much difference the windows can make, I’ve created three Heat Loads (images below) using the same home with every variable the same except for the windows. As you can see, the recommended tonnage goes from 3.95 tons using regular windows to 3.22 tons using a bottom line low-e window to only 2.75 tons using a typical middle of the road low-e window. This clearly shows that the more energy-efficient the window, the less HVAC system tonnage required to handle the load. While some variation is okay, it would be best to have a professional opinion as to whether the new windows will adversely affect the home.

What if the Low-E Windows Won’t Work with the Existing HVAC System?

Suppose the existing system isn’t compatible with their current choice. In that case, the homeowner can either go with a less energy-efficient type of low-e window, or possibly, and this would be the best option, replace the existing HVAC system at the same time. If this option is out of the budget, they could consider installing an in-system dehumidifier to handle the excess moisture if necessary. This won’t help with the short-cycling during the winter, but it may help prevent excessive mold and mildew growth. Remember, having an energy-efficient home is a good thing, but you should guard against any unintended consequences.