FAQ – Why are my windows foggy some mornings?

By: Brian Schutt  |  February 17, 2015


A common question we get during the winters, especially on cold days is “why are my windows fogging up?” While I think most of us have some intuition that is has to do with humidity in the air, it’s not always clear of when it might happen and what we can do to avoid it.

What I’ve learned since becoming a part of the heating and cooling industry is that ideal humidity level is relative. So as the temperature drops outside, the ideal humidity level drops with it. Which explains why on mornings we wake up and it’s zero degrees outside, we also wake up with windows coated with moisture.

On a recent morning I found every window coated in moisture. I checked my humidistat which controls my whole house humidifier and it was set to 45%. A simple reset to 35%, meant that the next morning my windows were clear.

If you don’t have a whole house humidifier, there still may be some mornings where your windows are foggy. Chances are this has to do with a disparity in temperature between outdoor air and your indoor temperature. I would be surprised if the moisture was on the inside in your situation, but if it is, you might consider turning your fan to the On mode, to increase your circulation.