The main difference between replacement and new construction windows is that new construction windows have a nail flange all around all four sides of the frame. When installing new construction windows installers drive nails through the nail flange into the studs. Next they apply proper flashing in order to prevent leaks. Once windows have been installed and flashed, exterior siding material of your choice can be installed. Vinyl siding is the most popular choice for exterior cladding in America, today. Therefore, most new construction windows come with built-in J-channel that acts as a receiving channel for siding. In some situations, however, you can order your new construction windows without receiving channel. For example, some homeowners who want to add distinction to their homes choose to trim their windows with special trim such as: Azek trim boards, aluminum casing, or vinyl lineal. In this case, you don’t need a J-channel. Another situation where you would order your new construction windows without a J-channel is when stucco or brick is to be used as an exterior cladding.