A typical retrofit window installation starts with removing the operating part and the stationary part of the existing window and leaving the main frame. Next we install caulking on the outside of the old frame and install the new window with a retrofit flange inside of the old main frame opening, shimming and squaring up the new window and securing it in the opening. Then we install foam in the gap between the old frame and new window
We install new vinyl interior flat trim around window to cover any gap and caulk. Then we caulk the outside around the new flange of the window. The type of caulking we use is Polyurethane, the best brand we have tested that doesn’t get brittle and crack over time and it is paintable. When the installation is done properly like this, there shouldn’t be any leaks.