This is a vinyl replacement window on an older apartment building. The install on this window is particularly bad. Notice the large gap between the top of the window and the siding.

The window lacks a flashing to deflect the water away from the opening or a caulking to seal the top of the window from water penetration. The window not being correctly seated against the building further compounds the problem.

Water entry into the structure and interior as well as heat loss is an issue here.