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The Condensation Conundrum

Here at, we regularly get questions about what some you might think are defective windows or perhaps failed insulating glass units (IGUs). These windows look like they’ve failed because moisture is forming on the interior side. We also regularly get questions concerning frost or ice on the inside surface of window glass, particularly in cold climate areas during the winter season. As the reader is probably not aware,, through its Glass Detective program, regularly offers glass-related technical consulting services and also sometimes gets asked to serve as an expert witness in lawsuits involving glass or glass-related problems. This is referenced here because a recent case that the Glass Detective Team was involved with was related to condensation issues. We hope that by discussing this case, a better understanding can be achieved of what we are herein calling the “Condensation Conundrum.”


The Window Condensation Case

The case in question involved a newly constructed four-story luxury condominium project in a Midwestern city. The units offered a wide-open floor plan and what was described as a very energy-efficient construction methodology. One of the exterior walls in each of these units was made primarily of glass, with the windows going from just a few inches above the floor line to the unit’s ten-foot ceiling. Because the building was surrounded by other mid- to high-rise buildings, window treatments were installed on these glazed wall areas to provide privacy, particularly at night. Without window treatments, neighbors could easily look into these units. The building’s construction was completed during the mid-summer, and by early winter, the building was completely occupied.

As the temperature dropped, the tenants started complaining that their glass window walls were leaking terribly. When they would open the drapes after a chilly evening, there would be water dripping from the glass and framing system; so much so that the hardwood floors adjacent to the window system were staining and, in some cases, starting to warp. A quick water test of the windows (performed from the exterior) concluded the problem was not that of improperly installed windows or glass. It was a condensation problem. Not unexpectedly, the finger-pointing from, and at, everyone involved soon started.



What is Condensation?

Before we go any farther with this little story, let’s talk about what condensation is, or is not. Condensation is not a leak in a window. If the condensation is appearing between two pieces of glass in an insulating unit (typically referred to as a “fogged unit” or a “leaker” by people in the glass industry), that is a defective insulating glass unit and needs to be replaced. A discussion of the reasons this fogging or seal failure in an insulating glass unit occurs will be reserved for another time. For now, we’ll deal with condensation.

Condensation is water vapor—humidity if you will. This water vapor in the air will collect on any surface at or below what is known as the dew point (that temperature at which air can no longer hold all of the water vapor that is in it). The warmer the air temperature, the more water vapor it can hold. When air with a lot of moisture in it comes in contact with a cold surface, it needs to give up some of that humidity; it just can’t hold it. Water vapor (moisture) starts depositing itself on cold surfaces. This is condensation.

We could get into a whole long discussion here about relative humidity and more. For now, we want to deal with our window condensation problems. Typically, the coldest surface in a home or building of any type is the interior side of a window. When the water vapor in the air of a house or structure comes in contact with that cold glass surface, it will deposit itself there, and that becomes condensation. If cold enough, it may even form frost or ice.


What Can Cause Condensation Inside a Home

Stop and think for a moment. When people go to bed on a cold winter night, they often turn down the thermostat on their furnace. If a parent had just cooked a big spaghetti dinner and filled the house with lots of humidity, and the humidifier on the furnace is still cranking out humidity, and a couple of the kids just took a shower or bath before going to bed, a condensation problem can develop rapidly. While all of this is going on in the house, the sun has set, and the temperature outside has dropped several degrees, lowering the temperature on the inside glass surface of all the windows as well. Now we can say hello to the collection of condensation that is slowly but surely forming on all of our windows. Condensation, which again, is water that came from the humidity in the air, can cause staining, warping of framing or flooring, musty odors, and at its worst, mold.


Continuing the Window Condensation Case

OK, let’s finish off that story we started. This particular condo building was in a very upscale area, and the buyers had paid a lot of money to be in their units. Some of them formed a group of angry homeowners and hired a very high-priced attorney to represent them in a lawsuit filed against the architect, the general contractor, and every subcontractor who had worked on the building.

As difficult as it is to see these types of things happen, it is difficult to argue against such actions being taken. The condo owners are not builders or contractors. They had expectations of a sound, efficient, and comfortable home. They have a legitimate concern and, if no one steps forward and says, “it’s my fault, and I’ll fix it,” the owners don’t have many other options or courses of action to follow. The lawsuits and countersuits dragged on for almost three years with no resolution until a wise judge suggested that the parties agree to find and hire an independent engineering firm to define the problem and develop a plan to fix them. Blaming each other and taking defensive positions to keep from being cited as the ultimate “culprit” was obviously not getting anywhere. At this point, thousands of dollars had already been spent.

As an aside, a growing number of subcontractors around the country today will not do condominium work because, when a problem occurs at the end of the project (and there almost always are a few), the new owners tend to take legal action against everybody who worked on the project as well as the architect, general contractor, and developer.

Reducing Window Condensation Inside a Home

So, the reader may ask, what was decided? The outcome was predictable—at least to the people who had some experience with these matters. The first step was to reduce the amount of water vapor in the air to begin with by reducing the output levels of the built-in humidifiers and using exhaust fans when cooking or showering. Humidistats, devices that measure the relative humidity in the air, were provided for each unit, and a target humidity level was established. Also, by moving warm air across window surfaces, the glass surface will get closer to room temperature and will be less inclined to become a landing place for water vapor. Some of the supply air ducts were strategically relocated closer to the windows. It was suggested that furnaces (the blowers/air handlers) be allowed to run continuously at night while opening/reconfiguring the heavy drapes so air would constantly be flowing across the exterior wall surfaces. After a few weeks of experimenting and working with the unit owners, the problems were under control.

Condensation is a real problem in buildings, and it needs to be dealt with in a reasonable yet aggressive manner. Like virtually all problems, it can be solved. The first step to solving a problem is always identifying the cause or source of the problem. Once that is done, the solution is only a matter of time and effort.

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Lyle Hill

Lyle Hill has been in the glass and metal industry for more than 40 years. In this time he has managed glass retail, contract glazing, mirror, architectural window, window film, and automotive glass businesses throughout America. He obtained an MBA from IIT with a focus on Technology and Engineering Management. Hill is also a columnist for glass industry trade magazines and often called the “face” of the glass industry. He has also authored books including “The Broken Tomato and Other Business Parables,” which is available through Amazon. Find out more about Lyle on Linkedin.

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4 Responses

    1. Hi Indra,
      The optimal temperature to prevent condensation will depend on many factors such as what kind of windows you have (single pane, double pane, or triple pane), outside temperature, and inside humidity levels. The optimal temperature will primarily be a personal preference that keeps you comfortable through the cold months.

  1. Great information. I have a 63 year old house that has large built in picture windows that consist of 9 separate glass units. They are set in a rather deep frame with about 5 inches of interior clearance from the glass panel. I am getting a fair amount of condensation that is causing damage to the wood frames and some mold/mildew issues. I have plenty of room to add a second insulated window panel to the interior of each unit. My thought is that would keep the inner most glass surface warmer and mitigate the condensation problem. Not only that, but would provide better insulating qualities to the large area of glass making the house more efficient. I could provide pictures if needed. Am I on the right track or is my thought just plain wrong. Thanks!

    1. Brian,
      Thanks for your remarks and question! Retrofitting an additional pane of glass into your current window frames is possible. It sounds like your frames would be a candidate given the 5 inch depth. However, retrofitting old frames is often labor intensive, and therefore costly. You’ll want to have a professional glazier come out to inspect the windows in person and discuss the options.

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