Your Windows and Condensation
- What is condensation. How does it form?
- What causes condensation on windows?
- How can I stop condensation on my windows?
What is condensation. How does it form?
Condensation: the conversion of a substance (as water) from the vapor state to a denser liquid or solid state, usually initiated by a reduction in temperature of the vapor. Yeah, yeah . . . we know that. Get to the good stuff.
First of all, don’t be afraid of condensation!
Water is the most miraculous substance on the face of the earth. The mere fact that condensation occurs is a direct result of the same inter-molecular forces that make all life possible on this planet. Think about it: what other substance can exist as a gas, a liquid, and a solid, in your living room, at the same time? WOW.
Yes, I know what you’re thinking . . . “I don’t care about the miracles of life, I just want my windows to stop sweating.”
Let’s look at it from a “real world” perspective. We will have to get a little scientific though.
Sweating soda cans, your car in the morning, the mirror in the bathroom after a hot shower. These examples all show how water vapor in the air will condense on a surface that is cooler than the air itself.
Have you ever taken a shower with the fan on versus the fan off? Have you noticed the difference? What is going on here?
When you step in the shower, water is being sprayed through the air, usually at high temperatures. Some of this air will be hot enough, or spread thin enough, to become water vapor. The air in your bathroom will become saturated with water vapor, and the temperature inside your bathroom will continue to support this saturated air. Condensation will happen on any surface that is cold enough to force the water vapor in the adjacent air below the dew point. In other words, the surface is cold enough to force the vapor to cool down, and as a result, turn into water. That is usually everywhere in the room, but the mirror is the most visible place. What is happening?
When we boil water, it is very clear that when it reaches a certain temperature, the water becomes vapor (and that vapor is REALLY hot). The same situation is true in reverse. When that water vapor becomes cool enough, it will turn into liquid. In your bathroom, the water vapor in the air comes in contact with the solid objects in the room, which are much cooler than the heated air, and that causes the air to cool down and turn to liquid water. That process is what we call condensation.
Great, so how does the fan stop this?
Two ways: 1. It evacuates the room of some of this warm air that is rich in vapor. 2. It creates air movement, which helps to evaporate standing water. The same rules that apply to the vapor-filled bathroom and the exhaust fan can apply to your home and your windows.
What causes condensation on windows?
One or more of these factors causes condensation on your windows:
- Cool window surface
- High relative humidity levels
- Lack of air movement
Let’s look at each of these from a window perspective.
Cool window surface. If there is vapor in the air, whether it be on the inside of your home or the outside, it will condense if the surface of your window is cool enough. Most often, condensation becomes an issue during winter, when there is a greater difference between inside and outside temperatures.
During winter, we all heat our homes to remain comfortable inside. With thermally inefficient windows, there is no barrier for heat transfer. As a result, the glass becomes as cold as the outside temperature. These windows do not stop the heat produced by your furnace, warming the inside of your home and windows. Instead, they allow the heat to escape, creating cooler temperatures in the house and on the surfaces of your windows. With the glass surface getting colder and colder, eventually the air next to the windows will be cooled below the dew point, and condensation will form.
Well insulated windows will stop this heat transfer, increasing the temperature of the air and the glass surface. When the temperature is raised, the relative humidity is decreased. This means there is less chance of condensation. The higher the surface of glass temperature, the better the condensation resistance.
If your window is accumulating condensation because the glass inside your home is very cold, consider scheduling a free estimate for highly energy-efficient windows.
High relative humidity levels. Relative humidity is a measure of the amount of water vapor the air is holding, compared to the amount that it will hold at a maximum AT THAT TEMPERATURE. When air reaches a 100% relative humidity level, it is said to be saturated. It will hold no more vapor, and the vapor turns to liquid. Warm air can hold more vapor than cold air. This means that if the air temperature is increased, the relative humidity will go down. How does this affect the real world? The answer is simple: if your home air temperature is heated, the relative humidity goes down. This lowers the chance of condensation on interior surfaces, such as your windows. What it actually does is lower the dew point. Here’s a real world example of how temperature and relative humidity interact with a window and condensation:
A one-story home with 1,000 sq. ft. has an average inside air temperature of 66 degrees F. The outside air temperature is 0 degrees F while the relative humidity inside is 45%. This would set the dew point at 44 degrees. In this particular case, the windows in the house are made up with double-pane glass, with no low-e or insulating gas fill. As a result, the inside glass temperature is 42 F. Since the glass temperature is below the dew point of 44 degrees, condensation occurs. If we raise the temperature of the air in this house, the relative humidity will lower (remember: warm air will hold more moisture). Warming the air temperature to 70F decreases the relative humidity to 35%, thus lowering the dew point to 41F. Now, the glass temperature is above the dew point, and condensation will not occur on this surface. Let’s take a look at the data: