Install Your Own Windows

Enlarge Image Image Gallery New windows can make your home more energy efficient — and add charm and style. With a little guidance and practice, even beginning DIYers can master this weekend home-improvement project. Len Churchill

New windows can breathe fresh life into an older home, and they are one of the deciding factors in the longevity of any new structure. Successful window installation is possible even for beginning DIYers, but the approach you take depends on whether you’re replacing old windows in an existing structure or installing windows in a new building. I’ll show you both methods — the processes are similar, but there are key differences in techniques and products.

The time it takes to install a window depends on its size. Start with smaller windows; they are easier to install. Excluding the time taken to remove an old window, which can vary widely, I estimate installation takes two to four hours for someone with no previous experience.

Out With the Old

Replacement windows are designed to slip within existing window jambs (the framed opening of a window) after the old sashes (the frame that holds the glass pane or panes of a window) have been removed. This is the simplest way to upgrade your windows, and you won’t have to add new trim or flashing because they were installed with the old window.

Start by measuring the width and height of your window opening, and then subtract an allowance for the gap required to let the new window fit easily and receive caulking. How much gap is enough? That depends; use a 24- or 48-inch level to determine if the top and bottom of your existing window opening are plumb and level.

If the opening is square and true, then a quarter-inch gap on all four sides will do nicely. But if your window opening has a bowed side or an area that’s not level or plumb, then give the gaps a larger width. Few things are more discouraging than suddenly discovering that your brand-new windows are too big to fit into the openings. If you think you’ll have trouble like this, order replacement windows that are 1 inch shorter and narrower than the opening. As added insurance, cut some rigid cardboard or thin plywood to the size you think your replacement windows should be. Try them in the opening and see how well they fit before you order new windows. Do you have a few windows that should be the same size on the same wall? Size them all to fit the smallest opening — that way you’ll maintain a consistent look and avoid the confusion of having many different window sizes to sort out on installation day. For your first window installation, choose an area that’s not a prominent location in the house.

Start by removing the old sash. There are infinite numbers of possible sash removal methods, but taking them out won’t be difficult; the idea is to create a clear path for your new window. You’ll probably need to pry off strips of wood or metal that previously guided the old sash. A thin-bladed “cat’s paw” pry bar is the ideal tool for demolition operations like this. A wire cutter and a putty knife are handy for pulling out old nails. Grab the nail with the cutter, place the putty knife underneath the jaws to protect the wood and then lever the cutter up, bringing the nail with it.