Window Condensation: Why & How

//Window Condensation: Why & How
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Window Condensation: Why & How

Do you have window condensation?

Window condensation is so. not. fun. First of all, there goes your view! And it’s not good for the windows or walls: all that moisture can lead to rot and problems with mold and mildew, as well as lift up paint and damage plaster.  To address the problem of window condensation, it’s important to understand why it’s happening.


Left side with Indow window insert, right side without.


How does it form?

Window condensation forms when the interior surface temperature of your windows is at or below the dew point temperature of the surrounding air in your house. What’s the dew point? That’s when the air is no longer able to “contain” all the water vapor mixed with it. Some of that water vapor has to say “see ya”, so it condenses into liquid on the window pane.

Think of the air as a sponge holding water: when the temperature drops, that squeezes the sponge onto the interior surface of your window.


What environmental factors affect it?

The inside and outside temperature, as well as the relative humidity of a space, determine whether your windows get foggy. As the air cools outside, it may cause the interior surface of your windows to fall beneath the dew point.

If all other conditions are equal, condensation will occur more quickly in a space that’s more humid. And all kinds of things increase the humidity of a home including breathing, pets, showers and plants to name a few.


Solving window condensation with Indow Window inserts.

Indow inserts are made of acrylic and press into the interior of your existing window frames. The silicone edging creates a near airtight seal so the moist interior air of your house can’t get to the glass window pane. The air in the cavity between the acrylic insert and your glass window also buffers the insert from the cold outside temperature

High-grade acrylic insulates better than glass: acrylic holds more energy and stays above the dew point more easily.

People with single-pane windows often experience more window condensation than people with double-pane windows, but it can happen to both types of windows. Indow inserts can help solve condensation on both single and double-pane windows.*

Here is a great picture a customers sent of a clear window with an Indow insert right next to a foggy, wet window without an insert:

window condensation

Window condensation in Boston.


Video: Indow inserts solving condensation.

And here’s a video our creative director did. Michael loves the inserts so much he actually came to work at Indow after buying them for his mid-century modern home in Portland. This video shows how the windows with inserts don’t steam up after a shower. Those without inserts get foggy!


Other methods of reducing it.

  • Lower the relative humidity in a room.
    Lowing the relative humidity can help, although you don’t want to go below 30 percent for reasons of comfort. To check the humidity levels in a house, you can use a hygrometer. According to the U.S. Energy Star program, indoor humidity levels between 30-50% are ideal. Below or above that can lead to problems with bacteria, viruses, fungi and respiratory infections among other problems.
  • Be careful of blinds and drapes if you notice condensation.
    Air moving over a window surface slows the ability of moisture to form. Window coverings can reduce the surface temperature of your window below the dewpoint. And we know what happens then. Condensation!
  • Install and use ventilation fans.
    Control moisture in kitchen and bathrooms with ventilation fans. These fans will pull the warm, moist air out of your house so the “sponge” doesn’t get too full.
  • Buy a dehumidifier or make your own. If you buy one, follow best practices by placing the dehumidifier away from walls and in as central a location as possible in a room. Also, shut windows and doors so it doesn’t have to work to dehumidify the outdoors.

Graphic of Optimal Relative Humidity levels that shows them to be 30-60 percent.


I don’t have window inserts but I do have storms: why do I have condensation?

Exterior storms contain “weep holes” and don’t provide a near airtight seal. In wetter, southern climates, the outside air can be more humid than the indoor air. In this case, the air pocket between the exterior storm and the primary window pane will have high relative humidity and so condensation can occur on the primary window pane when it cools to the dewpoint, typically due to air conditioning inside the
 In the case of interior storms made of glass, that can lead to condensation in the cavity on the interior storm itself as moist, exterior air gets into the pocket between the storm and the window pane through the leaky original window.  If the glass of the interior storm window is cooled to the dewpoint by air conditioning,  condensation happens. Indow window inserts, on the other hand, are made from acrylic which is a better insulator than glass and will not drop as significantly in temperature, thereby reducing the chance of condensation.
In northern climates with lower relative humidity, if an interior storm, regardless of glazing type isn’t airtight, moist interior air can enter and condense inside the cavity between the storm and the primary window pane.  Condensation will occur more readily if the glazing material is glass.  Indow window acrylic inserts prevent this from happening. They have a near airtight seal and the acrylic holds more energy so it doesn’t cool to the dewpoint as easily as glass.

What if I have condensation between the glass on my double-pane windows?

We were recently inside a building with vinyl double-pane windows that had condensation between the panes. This was caused by a crack in the exterior pane that allowed moisture to get into the space between the two panes of glass:


Window condensation between the panes in a double-pane window.


If you have condensation between the glass panes on a double-pane window, then the seal is broken. There are people who specialize in trying to fix this particular problem. Often, though, the window must be replaced. Call your window manufacturer for advice.

*Indow window inserts will reduce or eliminate condensation in most cases, but not all. Sometimes structural issues can interfere with the ability of the inserts to solve the problem.

By |2018-09-24T19:46:01+00:00February 14th, 2018|Blog|0 Comments

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