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How to Replace a Window: A Step-by-Step Guide

How to Replace a Window: A Step-by-Step Guide

Wondering how to replace a window? You’re not alone.

Thousands of homeowners do this job themselves each month. While we think you should leave this to a professional, we’ll show you the basic steps to take.

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How to Replace a Window in 12 Steps

To replace a window, there are 12 basic steps that you’ll need to follow. In a nutshell, these include:

  1. Gathering your tools and materials
  2. Understanding replacement windows
  3. Measuring for your new window
  4. Removing the old window
  5. Preparing the window opening
  6. Installing the new window
  7. Adding interior insulation
  8. Installing exterior trim
  9. Waterproofing the window
  10. Reinstalling exterior cladding
  11. Reinstalling all interior trims
  12. Applying caulk

Step 1: Gather Your Tools and Materials

Do you have the necessary tools and materials to complete the replacement job on your own? To properly replace your window, you’ll need:


  • Level
  • Caulk gun
  • Drill
  • Waterproof shims
  • Small pry bar
  • Putty knife
  • Tape measure
  • Utility knife
  • Drop cloth
  • Work gloves
  • Safety glasses
  • Dust mask


  • Wood filler
  • Silicone caulk
  • Paintable latex caulk
  • Flashing tape
  • Window spray foam insulation
  • Shims
  • Backer rod weather stripping
  • Finishing nails

Step 2: Understanding Replacement Windows

Replacement windows must be custom-made to ensure they fit in existing openings.

Many standard sizes and materials are available, including wood, vinyl, fiberglass, vinyl-clad wood, and aluminum-clad wood. They range from 12 to 68 inches wide.

Sash kits, insert replacements, and full-frame units are the three basic types of replacement windows.

Sash-replacement kits replace the moveable portions of an old window frame, such as the jamb liners and sash. 

To replace them, simply:

  1. Insert replacement windows, also known as pocket windows 
  2. Slide into the existing opening 
  3. Fasten to the existing side jambs

Window inserts have a head jamb, side jambs, and sill. Full-frame replacement windows have all three.

Old window frames, sills, and jambs are replaced when they rot. To do so, you’ll need to strip the window opening to its rough framing, inside and out.

Step 3: Measure for Your New Window

Measure the interior width of the old window frame in three places: top, center, and bottom, jamb to jamb. Take the smallest of the three measures and write it down.

Next, take three measurements of the frame’s height from the top of the sill to the underside of the head jamb: left, center, and right.

Record the smallest measurement once more. Finally, check for uniformity by measuring the diagonals.

The objective of this is to ensure that you buy a window that fits within the existing opening. You’ll have to make the window opening smaller or, worse, widen it to match the new window if you don’t measure it precisely. 

The extra work includes replacing all external trim, siding, and interior wall surfaces. When you go shopping, remember to bring all of these numbers with you. The salesperson will decode them and match you with the right window.

Step 4: Remove the Old Window

Remove the old window by following these three steps:

  1. Remove all the existing trims carefully while exercising caution to avoid damage, then run your utility knife around the edges of your window to loosen the seal. Proceed to remove the window panels.
  2. Using a pry bar, pry the window out of the frame. Remove the sash system by cutting the cord. If possible, remove the sash weights too.
  3. Using a tool knife and prying away, remove the interior or exterior stops like you removed the window. Whether you’re installing the new window from the inside or outside will determine which stop you remove. 

Once the window is installed, replace the stops. Then, move on to step 5.

Step 5: Prepare the Window Opening

Removing the old window will cause minor damage to the existing frame due to the removal of weights, nails, and screws. Scratch out the old caulk, paint the wood, and fill in all the holes with high-quality wood filler.

Allow the filler to dry for a while according to the manufacturer’s directions. Then sand the area using 120-grit sandpaper to smooth it out before painting over the mess. Your opening is now as good as new.

Examine the window sill; it should slope uniformly to the outside throughout the window. If required, adjust or replace the sill.

Apply an adhesive waterproof membrane to the sill and studs. A self-adhesive membrane should cover the vinyl cladding nailing strip.

Step 6: Install the New Window

Check the fit of your new window in the frame. It’s common to have a space between the window and the frame. At the confluence point of the nailing strip, run a bead of silicone caulk on the seal and around the window’s outer face.

Install the replacement window by first pressing the underside of the window into place, then pushing the top of the window into the external stops. Make sure it’s in the center of the frame. To keep the window in place, put a 5-cm screw into both sides of the jamb.

Make sure the window is level on the top, bottom, and sides with your level. You can now adjust it with the shims until the entire window is even. Once you’re sure everything is working, cut the shims that stick out of the window.

Check for smooth sliding on both sashes. If the straps are too tight, adjust the operation using the adjustment screws. If this doesn’t do much, you’ll have to remove the shims and replace them with thinner ones. The protruding ends of the window now fit.

Step 7: Interior Insulation

Use fiberglass insulation in the inner parts of the window. Install a window backer rod made of neoprene.

Step 8: Install Exterior Trims

Use finish nails to install an outside trim around the window frame, and drive the nails using a nail set to avoid denting the trim. 

Weep holes should remain unobstructed. Apply latex caulk to the edges of the trim. Fill visible nail holes with plastic wood (wood filler), and paint the trim as needed to make it look nice and neat.

Step 9: Waterproof the Window

First, apply a self-adhesive membrane to each side of the window. Cover the frame’s edges and make sure the membrane is well bonded.

Under the air barrier, install run-off molding and a self-adhesive membrane on top of the winder. Cover the self-adhesive membrane with sheathing tape and glue all air barriers.

Step 10: Reinstall the Exterior Cladding

Next, you’ll install the exterior cladding. To do so, first take measurements around the window. Add two inches to the width of the molding for the bottom of the window.

On the section resting on the window sill, cut a 1″ notch on each side of the nailing flange, leaving the front intact.

Then nail the molding to the underside of each overhang, leaving a 1″ overhang on each side. Cut the J-trims and nail them in place.

Make a 45°-angle miter cut in part before the J-trim and nail it to the roofline. Create a rainwater run-off by inserting the upper molding strips into the notches of the two lateral moldings.

Step 11: Reinstall all Interior Trims

Any trim or stops that you removed when removing the old window should be replaced. Place the wooden stops in their final positions, then secure them with three clean nails at the top, center, and bottom.

Carry out the same procedure for all of the frame’s stops. If your new window design calls for it, you should also install stops on the outside.

Step 12: Apply Caulk

Apply some exterior-grade caulk beads to the gaps between the window and the frame from the outside. Apply them evenly using one-centimeter beads, and make sure there is no caulk between the sashes and the jambs.

Fill any gaps surrounding the window on the inside with a little expanding foam. The interior of the window frame and sash should then be primed, painted, or stained.

Things to Consider

While we’ve shown you how to replace a window, there are some important things we think you should consider during the process:

  • Replacing windows is a delicate process. Equip yourself with the proper tools to avoid damage.
  • Wear protective clothing when removing the old window
  • You’ll need to contact a waste disposal company for the proper disposal of your old window
  • Windows are constantly exposed to the elements; don’t choose materials that will rot soon
  • Ensure that everything you removed is replaced so that the window sits securely in the frame.
  • Do not proceed to replace the window if the wood is rotten. Call an expert.
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We partnered with Networx to save you big money on your next home renovation project. And quotes are free!

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We may earn a commission when you click this link, at no extra cost to you.

So, How Do You Replace a Window?

So there you have it—a complete guide on how to replace a window. It’s a lot of steps. We know.

For this reason, we highly suggest contacting a contractor. Bids are free, and we make it easy to find one.

Just click on the green button above, enter some basic information, and you’ll be paired with a few contractors for a free, no-obligation bid.