As I write this blog, a good portion of the nation is experiencing very cold temperatures with high winds. I’m currently sitting at -9°F with a windchill or feels like temp of -38°F. Chatting with a good friend in Kansas City recently, he was impressed by the below zero temperature readings. (He was actually begging me to stop sharing and keep the nasty weather in Minnesota.) Because of the weather, I’ve been receiving emails and DMs on Instagram about how people’s houses are reacting to this weather extreme. Window condensation has been one of the more frequently asked questions, so I figured I’d cover that topic in a blog post.
Window condensation in the form of both liquid and frozen moisture is a product of temperature and water vapor. Think of it this way, what happens to an ice-cold drink on a hot and humid day? Moisture collects on the exterior of the glass, the temperature of the glass itself is cold enough to condense moisture out of the air, the dew point temperature has been reached and the water vapor in the air is being changed to liquid moisture. There’s a little more to it than that, but let’s keep it simple. This same principle happens to windows during cold weather, the surface of the interior glass becomes cold, moisture in the air condenses on the glass surface, usually near the bottom, but the moisture can also be distributed across the entire surface such as the photo above shows. The colder the glass temperature, the less interior humidity is needed to start the condensation process.
How do we eliminate, or at least reduce the amount of moisture on the windows? The answer is pretty simple, raise the temperature of the glass or reduce the humidity level inside the home. Raising the temperature of the glass is easiest to accomplish when building new or replacing old windows, purchase windows that have better insulated glass. Triple pane windows are great for condensation control. The issue is they are rarely used, even in my very cold climate simply because of the increased cost, which adds up quickly when you are purchasing dozens of windows.
Another method to raise the surface temperature of the glass is a technique us in cold climates have used for years, install heating systems directly under each window. Whether the heating system is hot water or electric baseboard, or some sort of forced air heating system, having heat flow across the surface of the glass can reduce condensation.
Closed window blinds will often increase window condensation. Blinds can act as insulation, further reducing glass temperatures and prevent heated interior air from moving across the glass surface. Some of the most frosted windows I’ve seen are due to the homeowner closing blinds overnight.
Interior humidity loads are the second contributor to window condensation. My old home with lots of air leaks has absolutely no window condensation, even at -40°F actual air temperature. That’s because most of the winter my home’s indoor humidity hangs in the teens, I’ve seen it as low as 8% during very cold weather. Unfortunately, this very low humidity level causes some comfort issues and may contribute to an increase health risk due to certain bacteria and viruses thriving in low humidity levels.
New homes will have higher humidity levels during the first year or two, until the building material moisture content have stabilized. Wintertime dehumidification may be needed those first couple years.
Other causes of high indoor humidity levels. Using either whole house humidifiers or room units will, of course, raise indoor humidity which in turn can cause window condensation. Window condensation is usually the least of your worries when using a humidifier during cold weather. There is a reason a home has low humidity, just like mine, uncontrolled air leaks. When you humidify, the air is still leaving the building and taking that humidity with it. Chances are, it will find some cold surface inside a wall cavity or in the attic where it can condense. In my climate, this condensation will be in the form of frost. When that frost melts, the building materials will become damp, damage may be the result.
Other sources of indoor humidity are the people living in the home, we cook, clean and shower which all adds moisture to the air. We also breath, which releases more moisture. The things we keep in the home can also add to humidity levels. Plants, open aquariums and pet watering are all sources. Storing green firewood can be a huge problem. As a kid, I remember hauling freshly cut wood into our basement, so it had a place to dry over the winter and was closer to the basement woodstove.
How to control indoor humidity. There are a couple methods that can help control humidity levels in the home, dehumidification is the first, but this method will add to the electric bill. Most dehumidifiers I test cost around $40 per month if they run often. Balanced mechanical ventilation is a controlled way for maintaining humidity levels. We’ve been using heat recovery ventilation, HRV’s for years in Minnesota. Not only can the units reduce indoor humidity, they also improve indoor air quality by a controlled exchange with outside (hopefully fresh) air. The cost to operate either an HRV or ERV will be substantially less than operating a dehumidifier.
The window can be the problem. Something I hear all the time, I have cheap windows, that’s my problem. Well, sometimes it’s a problem with the window itself.
These two older windows have lost the gas fill between the windowpanes, this can cause the windowpanes to move slightly closer together which lowers the center of glass temperature. The results are shown in the photos. Replacement of the glass or entire window may be needed to fix this condition.
The window in this new home has a faulty seal (there were a couple more windows in this home with the problem as well). I found this issue during a code compliant blower door test. These photos do show that sometimes it’s a problem with the window, but window condensation is much more common because of outdoor temperatures and indoor humidity levels that with the fault of the window.
How can you test? An inexpensive temperature/humidity monitor (a humidity monitor is called a hygrometer) placed near a window will tell us if the humidity is the issue. If you have one window that is more problematic than others, I would check humidity levels at several different windows.
Thermal imaging is another useful tool, though most people do not own a thermal imaging camera. You need to know that glass is reflective and thermal imaging may not give you an accurate temperature reading. A way around this is to place a piece of tape on the window and measure the surface temperature of the tape. A laser temperature tool is a less expensive way to measure the temperatures of surfaces. From there you can enter both the temperature and humidity level into a dew point calculator or application on a smart phone, tablet or computer to figure out what the dew point temperature is. If the actual temp is below the dew point temp, you will have condensation.
Hopefully this has been a helpful explanation of the what, why and how’s of window condensation. Now, get out there and enjoy this typical Minnesota winter happening throughout the country.
Thank you for the informative blog and post re: condensation. Can you recommend a quality HRV for northern MN and/or an HVAC contractor?
Hi Darren,
There are several really good HRV/ERV manufacturers, the best being Zehnder, but they are also the most expensive. Zehnder is going to start at around $10,000, I’ve heard of installations more than $20,000 in upper end homes. I like the Broan AI series, a much less expensive unit with a feature that keeps the unit balanced, even if filters or the exterior intake hood becomes partially plugged with debris. As far as HVAC contractors, I only know the ones in the Grand Rapids, MN area. Where are you located?
Randy