Before we go through our replacement steps, let's be clear that we're talking about an insert or pocket window; it's a fully assembled window that can fit in existing jambs (the vertical part of the window frame). There are a couple other options. Sash kits give an old frame new sashes (the frame that holds the pane in place) and jamb liners along with the window. And full-frame windows have a complete frame and new sashes, jambs, liners and sill.
Full-frame windows are their own beast. They're essentially the same as installing new construction windows, in the sense that you're removing everything down to the rough opening and replacing every part of the window -- frame, jambs and sills. It's the only option if the window frame is rotted or compromised.
The sash kits aren't much more difficult than the replacement, except instead of keeping old sashes and jambs intact, you'll be switching them out for new ones. That means that along with removing sashes and stops, you're taking out the jambs, installing new ones and then basically following the insert steps below to put in your new window:
- When you're installing a window, it's the measurements that will either make the project a snap or leave you in an angry stupor of home-improvement rage. From inside your house, measure from jamb to jamb at the bottom, middle and top of the window. Use the smallest measurement; that will be your width. Measure vertically from the sill to the head jamb (the horizontal part across the top) left, center and right. Use the shortest measurement for your length.
- Because you're replacing a window and not getting an entirely new frame, we're just removing the sashes. To do that, remove the inside stops; these are what we laypeople think of as the inside trim. Keep 'em in good shape; you'll be using them after you replace the window.
- Remove the lower sash; that means you're basically taking the lower part of the window out. You'll probably need to slide it up a little and then tilt it out. Now lower the upper sash a bit and tilt that one out as well. (Don't remove any of the outside stops.)
- Now you can tilt the bottom of the window into the jamb; secure it so it fits snugly. Some recommend putting a small beading of caulk around the inside stops on the jamb to create a nice seal. Either way, you can now screw the window into place through the existing holes. Replace the inside stops by hammering them in as they were before. Now go outside and apply a line of caulk between the stop and the window for a good seal.
And you've done it! Your replacement window is set.