A replacement window is a window that’s installed in an existing frame from the inside of a home. Hopefully, you have an existing window already. Sometimes, the replacement window – you’re not really replacing it. You may need to add or open an area, but it’s still going to be something like modifying it. You have to have a certain amount of sizing or ventilation. There’s a few rules that apply to being able to install windows and making sure that they meet proper codes.