How to Replace a Window in 18 Steps

Identify the Replacement Window You Need

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.

Measure the Old Window

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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.

Buy a Replacement Window

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.

Find the Old Window Sashes

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Safety note: If your house was built before 1978, be sure to take the proper lead safety precautions if it contains lead paint before continuing to replace the windows. 

Remove any storm windows first by either directly lifting them out of place or removing the screws with a screwdriver.

Start by identifying the sashes—the moveable part of the window that holds the glass panes together. Remember, you can either remove the window from the interior or exterior of the home, depending on the location of the sashes. If there’s paint over the old window, you may need to score the seams with a utility knife to break the seal.

Remove the Sashes

You may have sash springs hung in the window (spring-loaded mechanisms that help the windows glide up and down). You can remove these and their plastic covering by unscrewing them and cutting them away. It’s important to note that the spring-loaded sashes may snap back when removing them, so be sure to wear protective gear during this step.

From the outside of the window, use a hammer and chisel or an oscillating saw to remove the exterior sash stops. Next, remove the parting stops (the strip between the upper and lower sashes) using a putty knife, chisel, or oscillating saw. If you’re removing the window from the outside, you may choose to leave the interior stop in place if it’s in good condition. 

Once the parting stops have been removed, the upper and lower sashes can be removed by pulling the window out of place.

Remove the Window Frame

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.

Make Repairs

You don’t want to install a new window in a spot that is worse for wear. Doing so could compromise the efficiency of your new windows.

After the window has been removed, determine if any repairs need to be made 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. Check the opening for any holes and fill them in with wood filler.

Prepare the Rough Opening

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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.

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.

Apply Flashing Tape

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.

Insert Shims as Needed

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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.

Test the Window Fit

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). You may need to enlist a helper during this step, as replacement windows are heavy and bulky. 

Add Caulk

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.

Set the Window

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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.

Secure the Window

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.

Make Sure Everything Is Just Right

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.

Add Insulation

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 Window Parts

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.

Paint or Stain the Frame

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.