This is due to the lack of properly installed insulation between the window frame and rough wall framing or old window frame.
As you had mentioned, these are replacement windows. In most cases they are installed from the interior of the home and finished off on the exterior with aluminum trim.
- If no insulation was installed around the window frame during the installation, you now have a direct path of heat loss to the outside and air infiltration into the home, creating condensation and drafts.
- This continuous buildup of moisture will become a source for mold and mildew growth and in many cases an invitation for insects such as carpenter ants.
I would contact the company whom you hired to installed the windows to remove a section of trim in an area where you are experiencing the problem.
If there is no insulation present or improperly installed, it should be their responsibility to:
- inspect all windows they installed, and
- repair the defect and any associated material and labor costs free of charge to you.
For more information:
Read "Should we insist that our contractor use expanding foam insulation around our new windows?", a Q&A with Carl Seville.