Some signs that it is time to replace a skylight include distortion of the light that comes through the window and leaking water. If the window is older than 15 years or if condensation is gathering in the window, it is likely time for a new skylight, even if nothing else is wrong.
If a skylight is acrylic, it is likely not very energy efficient. The window sits on an aluminum frame, which brings unwanted heat into the home. A homeowner may see condensation collecting on the inside of the skylight, which runs down into the chase of the window. Velux glass is a good replacement material; it has two panes, and is about 45 percent more efficient than an acrylic window.
It is also a good idea for homeowners to consider a skylight replacement when getting a new roof. Since the homeowner is already investing into the home, the addition of a new, improved skylight is unlikely to make a large impact. If the skylight is more than a decade old, it is likely to have enough sustained enough damage that the roofer may suggest a replacement. Windows become weaker over time, so a major storm can easily break an old skylight.