What causes sweating (condensation) on the inside of windows in the winter?

Monday, July 9, 2018

The science behind the problem is fairly simple. Warm air expands and can hold more humidity (moisture) than cool air, but when it comes in contact with window glass that is significantly colder than the air, the surface chills and shrinks the air volume. This squeezes out some of the humidity as condensation water on the glass surface. 

    So, the two things that are necessary for condensation on window glass are 1), a significant temperature difference between the interior surface of the glass and the air and 2), high humidity in the air. You can eliminate, or greatly reduce, sweating window glass by reducing both of the factors that cause it.

    We see the solution as something like a “Combination Plate Special” on a Chinese restaurant menu. Pick one from column A and one from column B, and your fortune cookie will read “Happy windows make happy home!” 

Also, see our blog post Why does condensation form on the outside of some windows and not others in the morning?

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  To learn more about doors and windows, see these other blog posts:

Is every exterior door of a house required to have a landing outside? 

 What are the small slots at the bottom of the outside of my window? 

Why is the garage door track a white tube? 

What is the raised metal plate on the floor under the garage door?

 Why do I have to hold down the button to close the garage door? 

How can I tell if a window or glass door is safety glass? 

What are the code requirements for safety tempered glass for doors?• 

Should a front door swing in or out? 


How many exit doors are required for a house?

 How many exit doors are required for a mobile/manufactured home? 

Are openable windows required to have window screens? Will windows with no screens pass a home inspection? 

Can a bedroom door open into the garage?

What are the building code requirements for a door from the garage to the house?

What is "low-E" window glass? 

What does ANSI 297.1 on glass mean?

Why is a double cylinder deadbolt lock on an exterior door a safety hazard? 

How can I check my garage door to make sure it is safe?  

What is an egress window?

Does a home inspector test all the windows and doors in a home? 

How difficult is it to change a window to french doors or a sliding glass door?

How do you determine if a door is left-handed or right-handed?

Why are window security bars dangerous? 

What are the common problems you find inspecting windows?

What is causing a foggy haze on my windows? 

What do those numbers on the manufacturer's stickers in new windows mean?

What does a home inspector check on an electric garage door? 

• What is the tempered label on glass at windows and sliding glass doors called?

Why is pressure washing double pane windows an expensive mistake? 

• Do I need to have two exterior exit doors in my house? 

• When is safety glass required for windows at stairs and stair landings?

   Visit our DOORS AND WINDOWS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

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