Despite warnings from industry experts, some installers and homeowners still continue to remove the existing frames when installing flush-fin replacement windows. We always leave the old frame intact on our installations and strongly suggest that all of our customers follow this practice as well.
Why remove them?
The only reason a homeowner wants to remove the existing frame is to gain a few inches of daylight opening for the new replacement window. While this gain is minimal, usually 1"-2" in each direction (see image), the reasons for not removing the frame are compelling.
Why NOT to remove them:
When your windows were first installed, the nail fin was flashed to your stucco paper to create a relatively seamless water barrier. The reason for this is that stucco is actually quite absorbant and moisture works its way through to the stucco paper where it is shed away from your house. When old frames are removed, in order not to crack or break out the stucco, the frames are collapsed into the opening. This is done by ripping out the nail fin along with the window, from between the stucco and the framing. This process tears the stucco paper that was acting to waterproof your house. Even after new windows are installed, water that seeps through your stucco is able to get through the holes in the stucco paper barrier, causing leaks and all of the problems associated with them.