How to Install Replacement Windows in 15 Steps

It’s important to note that there are two main types of replacement windows—full-frame replacement windows and insert or pocket replacement windows. 

A full-frame replacement removes the entire window, frame, molding, and jambs, whereas an insert or pocket replacement window replaces the window itself, leaving the framing and woodwork in place.

Using a tape measure, start by identifying the height of the pocket by measuring the inside of the frame. It’s helpful to take the window measurements two times to ensure you’re purchasing the correct size.

Measure the height in three places—the left side, middle, and right side of the frame. Take the shortest of the three measurements to determine the height.

Repeat this process to measure the width, noting the dimensions of the top, middle, and bottom sections. Use the smallest measurement of the three to determine the width.

Once you have the dimensions, you can purchase the replacement window. The new window should be 1/4- to 3/4-inch smaller than the opening; however, this can vary depending on the manufacturer. Most windows come in standard sizes, but you may have to order custom windows if you have a uniquely sized opening.

Safety note: If your home was built before 1978, pause before doing anything else because there’s a good chance your windows contain lead paint. Make sure to wear a P100 respirator and follow proper lead-safe work practices before moving forward.

Start by removing any storm windows. You’ll either need to lift them out or unscrew them, depending on how they’re installed. Then, locate the sashes, which are the parts of the window that move and hold the glass. You’ll need to remove these from the inside or outside of the house, depending on the window’s setup. If layers of old paint are sealing things in place, you can use a utility knife to score around the edges and loosen it up. 

Some windows have spring-loaded mechanisms that help the sashes slide. These can snap back when released, so wear protective gear and carefully unscrew or cut them out, along with their plastic covers.

Next, remove the sash stops. If you’re working from the outside, use a chisel, hammer, or oscillating saw to remove the exterior sash stops first. Then, use a putty knife or chisel to remove the parting stops, which are the narrow strips between the upper and lower sashes. If you’re removing the window from the outside and the interior stops are still in good shape, feel free to leave them in place.

Once the stops are out, both sashes should slide free. Gently pull them out of the frame and set them aside.

Remove the old trim, molding, and window stops by scoring the edges with a utility knife, and prying them out with a putty knife, pry bar, or chisel. You can reuse these pieces if they’re in good condition; otherwise, you can discard them once removed.

Sometimes, older aluminum or vinyl windows will have cords or weights that also need to be removed. You can cut them away from the frame using a pair of scissors.

The rough opening is the hole in the wall where the window frame will fit. It needs to be properly prepared for your new window to sit snugly and securely in place. But you don’t want to install a new window in a spot that is worse for wear since it can compromise the efficiency of your new windows. So, after the window has been removed, determine if you need to make any repairs to the rough opening. Inspect the wood, remove any areas with damage or rot, and cut and install new pieces of wood to replace them. You should also check the opening for any holes and fill them in with wood filler.

You may notice securing staples used to hold the old window’s aluminum tracks in place. You can remove these with a pair of pliers. If you notice chipped paint or caulk in the window opening, remove it with a paint scraper, then sand it down to create a smooth finish.

You also want to make sure the rough opening is clear of any debris that could prevent the frame from fitting into the wall tightly. Vacuum the opening and sill with a shop vac, and wipe it down to remove any dirt, grime, and debris.

Flashing tape helps protect your home from air and water leaks. You’ll install this material around the window frame. It creates a protective barrier between the window and the wall.

Apply your flashing tape across the length of the window sill, ensuring that it extends over the window stool (the horizontal surface at the bottom rail of the window sash). Press the flashing tape firmly into the seam and corners for a tight fit.

After all of the preparations have been made to the window opening, use a level to ensure the surface is even. If you notice bowing or unevenness, you may need to add wood shims to the low spots to create a level surface. You can secure the wood shims into place using flashing tape and screws.

Dry-fit the new window to ensure it fits in the opening (there should be a small gap of about ⅛-inch between the window and the frame). As replacement windows are heavy and bulky, you may need to enlist a helper during this step.

Now it’s time to caulk your window opening. Set the window aside and apply exterior caulk to the outside stops and head jambs. Apply a bead of caulk along the edges where the window stool meets the window sill. If there are any small gaps less than 1/4-inch wide around the window frame, casing, or interior window sill, you can fill the gaps with caulk to minimize drafts.

After applying the caulk, lift the window and position it into the opening, ensuring it’s centered. Gently press the window into the bead of the caulk, allowing it to rest on the interior blind stops.

After the window is set, identify the pre-drilled screw holes in the window, and set the shims behind the screw holes between the window and the jamb. You can also set shims at the bottom or top of the window to ensure the window is level and square. 

Measure the window from corner to corner, diagonally on each side. If the measurements are the same, the window is square. You may need to adjust the shims a few times to get equal measurements. Drill the screws into place (or use a screwdriver), driving the screws through the shims as well.

Check that the window is level and plumb (a perfectly straight vertical line), and make any final adjustments. Once the window is square, level, and plumb, trim excess material away with a utility knife.

Look for gaps in the window and apply a spray foam insulation to prevent hot or cold air from escaping your home. Opt for a spray foam insulation rated for windows and refer to the manufacturer's directions for dry time and the recommended installation technique.

Reinstall the old trim, molding, and window stops ‌you removed using finishing nails or caulk. Or, install new trim, molding, and window stops if you discarded them.

Apply caulk to the interior trim and the exterior joints to fully secure the window. It’s important to refer to the window manufacturer, as you may need to leave a gap at the bottom of the sill to promote drainage.

Depending on the window you purchased, you may need to paint or stain the interior window sash and frame. Wood, in particular, requires paint, stain, or sealant to protect against moisture, which can lead to rot or warping. After the caulk and spray foam are completely dry, you can begin painting or staining using an angled paintbrush, ensuring the room is well-ventilated.