The physics in one sentence
Condensation forms when warm, moisture-carrying indoor air contacts a glass surface cold enough to bring that air below its dew point, causing the water vapor in it to turn back into liquid droplets on the pane.[1]
Where the moisture actually comes from
Everyday indoor activity adds more water vapor to household air than most people expect: cooking, showering, running a humidifier, and simply breathing all raise indoor humidity, and that moisture has to go somewhere once it meets a cold enough surface.[1]
Why new windows can show more condensation, not less
A well-sealed replacement window reduces the incidental air leakage that used to let humid indoor air escape around an old, drafty frame. Tighten up the house without adjusting for that lost airflow, and the same moisture that used to leak out around old windows now has nowhere to go, showing up on the new, tighter glass instead.[1]
That means visible condensation right after a window replacement is often a sign the new windows are sealing well, not a sign they're defective.
Newly installed windows have their own temporary moisture source
Framing lumber, drywall, paint and other building materials hold a surprising amount of moisture after installation, and that moisture works its way into the air during the first heating season, landing on the coldest available surface, often a window.
What actually reduces it
Practical fixes focus on humidity, not the glass: track indoor humidity with a hygrometer, run bathroom and kitchen exhaust fans during and after cooking or showering, ease off on humidifier use in winter, and keep moisture-releasing plants away from windowsills.[1]
When condensation does point to a window problem
Fog or haze that forms between the panes of a sealed insulating unit, rather than on the interior or exterior surface, is a different issue: it usually means the seal holding the gas fill in place has failed. That's a warranty question for the manufacturer, distinct from the humidity-driven condensation this guide otherwise covers.
How to tell the two apart
Interior-surface condensation comes and goes with humidity and temperature, often worse in early morning or right after a shower. Between-the-glass fogging doesn't clear up on its own and tends to leave a visible film or streaking even after conditions change, which is the tell that it's a seal failure rather than routine humidity.
Where climate zone fits into the picture
A colder climate zone means a colder interior glass surface for a given indoor temperature, which lowers the humidity level needed to trigger condensation in the first place. That's part of why a Northern-zone home can see condensation at a humidity level that would never fog glass in a Southern-zone home, independent of anything the window itself is doing wrong.
Exterior condensation is its own, separate story
Occasionally condensation shows up on the outside face of the glass, usually on a cool, humid morning. That's outdoor humidity condensing on glass that a high-performance low-E coating has kept unusually cool relative to the outdoor air, which is, again, a sign of an efficient window rather than a flaw.