Windows play an important role in the appearance of your home, while also providing light, air circulation and visibility. However, when windows are not installed properly, they can allow water to enter which will cause rot to form around them. This is a common problem in houses, even those that are relatively new. The perimeter of the window’s opening must be sealed with a waterproof membrane that is self-adhering, as well as caulk and metal flashing. Generally speaking, it is better to retain and fix old windows than it is to replace them completely. The costs for installing a new window can be twice as much as renovating an existing one.