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Condensation from the differential temperature is of course on answer. The other and more concerning is the furnace or force air unit (FAU). I would recommend contacting a licenced heating contractor and ask them to check the replacement air going to the FAU. The old windows were letting in a lot of air. What air was acting as replacement air for the furnace. With the new windows you are no longer supplying replacemet air to the furnace.
See Pella’s Understanding Condensation fact sheet at:
HDR Remodeling Inc
You have condensation on your new windows. This is common with new windows that are air sealing your home better than the old ones. It also means you have humidity in the air in your home that is attracted to the coldest surfaces. A glass of cold water will do the same thing. Winter time will bring cold temperatures which will cool the glass of the windows. Check to make sure the humidity in the home is 30-50% and wide the water off the windows and window sills. Opening shades or blinds to allow air flow may also help.
Yes, we agree with other answers. Common household condensation, or "sweating" on windows is caused by excess humidity or water vapor in a home. When this water vapor in the air comes in contact with a cold surface such as a mirror or glass window, it turns to water droplets that is called condensation. All homes have occasional condensation, such as a little fogging on the windows, and is no cause for concern.
The window (frame) surface is cold, but the air in the apartment is humid. Humid air condenses on a cold surface (when it drops below the dewpoint temperature).
So you can either make the window frame surface warmer, or you can make the air less humid.
Another approach to making the air less humid is to run the air-conditioner, but that makes the house very cold. The dehumidifier usually makes the air warmer, so it's probably preferred, although way less capacity. Some fancy houses have dedicated whole house dehumidifiers (but maybe they also have better windows!).
Window films (the type that shrink with a hair-dryer) attached to the window trim/casing can keep the humid inside air away from the cold surfaces. Added bonus of keeping cold air leaking through windows out of the house.
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