If you are considering getting retrofit windows for your replacement, there aren’t too many options available for an effective exterior finish for your windows. In retrofit installations, your new windows get placed directly into your existing wooden frames. These kinds of replacements are only advisable if your frames are healthy and aren’t showing any signs of water damage or rotting.

Because the frame itself remains untouched during the installation, the only way to protect it from the outside elements is by covering  it up. Aluminum capping or cladding is put on around the window and is then sealed with caulking. But while it spares your frames from exterior moisture, capping actually contributes to further deterioration of wooden frames. Once the capping is put on around the window it seals the air out, but also seals the moisture in. As the temperature changes and the aluminum warms up and cools down, the moisture underneath has nowhere to escape and stays on the wood. This greatly contributes to rotting.

With a full frame replacement the exterior of your frame gets replaced with a vinyl brickmould. Unlike wood, vinyl doesn’t absorb or retain any moisture and is a much more efficient and longer lasting alternative.

https://www.youtube.com/watch?v=PxkUdfLboY0

You can find out more about the ways capping affects retrofit windows here..