How to Install a Replacement Window DIY

Man installing window

Homeowners should have their broken windows repaired or, in the worst-case scenario, replaced, but professional window replacement can be quite expensive. If the price tag is turning you away, then why not do it yourself? Learning how to install a replacement window might take you a few hours, but it gets easier once you’ve replaced one.

DIY vinyl window replacement requires some experience working with tools. It can take you the better part of an afternoon to replace a few windows, and you might also need a friend to help you out. 

Here’s everything you’ll need to know to replace your own window:

Tools and Materials

Man installing window with drill
Photo Credit: Pexels

Before you get started, make sure you have all the necessary tools and materials for replacing a window, including: 

  • Tape measure
  • Work gloves
  • Safety glasses
  • Hearing protection
  • Oscillating tool
  • Utility knife
  • Putty knife
  • Pry bar
  • Hammer
  • Cordless drill
  • Screwdriver
  • Wood filler
  • Flashing tape
  • Level
  • Caulk
  • Caulk gun
  • Shims
  • Window and door expanding foam and/or fiberglass insulation
  • Screws
  • Finishing nails
  • Trim paint
  • Paintbrush

These materials and the following steps will be for a retrofit sash window replacement, which preserves the existing window frame. If your frame is rotting, then consider hiring a professional to perform a full-frame window replacement, which is more difficult and more like a new construction installation. Without further ado, here’s how to install a vinyl replacement window.

Window replacement illustration
Illustration Credit: Juan Rodriguez

Step 1: Measure the Height and Width

Before anything else, you should measure your existing window to find out how big your replacement window should be. Use a tape measure to get the dimensions of the pocket opening. Make sure to measure the inside, not the outside, of the frame. 

To do this, measure your window’s height from the head (the top) to the sill (the bottom) at three points. You want to measure the height at the left side, middle, then the right side.

Then, measure your window’s width by going from jamb to jamb. The jambs are the vertical sides of your window frame. Measure at three points again, this time at the top, middle, and then bottom.

If the three measurements for the height and width are all the same, then that’s great! If they’re not, then use the smallest measurement when you order your replacement windows. You want them to fit the frame.

Step 2: Check if Your Frame is Out of Square

Measure your frame’s diagonals. You want to check if they have the same measurements. If there’s more than a ¼-inch difference, then you may need to replace your whole frame.

Double measure your window to be sure. Window manufacturers usually have instructions on what the size difference should be between your frame and your replacement window. Use these and your measurements to order your new window.

Step 3: Remove Any Storm Windows That You Have

Working from the outside, use a utility knife or any other sharp tool to score along the edge of the storm window’s frame. Then, remove it with a putty knife or a pry bar. If your storm window’s frame has been screwed in and painted over, then use a Grabit to remove the screws.

Step 4: Remove the Trim, Molding, and Sash Stops From the Inside

Working from the inside, use a utility knife to score the paint and caulk in between the trim, sash stops, and other moldings to separate them. Then, use a putty knife to widen the gap enough for a pry bar to fit through. Carefully remove the moldings and trim, especially if you want to reuse them. 

If you’re having trouble removing them, use an oscillating tool to cut them out of the frame carefully. Be mindful of the nails. If they were screwed in place, unscrew them first – either with a drill or a screwdriver – before removing them.

Note: If your home was built before 1978, then it’s highly recommended to have your house checked for lead paint during home renovations like replacing a window. If professionals find that your house has lead paint, then it’s best to have it removed. Lead paint is a health hazard, especially for young children.

Step 5: Remove the Sashes, Sash Weights, Sash Springs, Jamb Liners, and the Parting Stop

If you have sash springs, then unscrew them first if possible. Be careful while removing them, as they might snap due to tension. After that, you can remove your window sashes. For some windows, it’s easier to remove the lower sash first. For others, it’s better to remove the upper sash first or even both at the same time.

After you’ve removed your sashes, use pliers to remove the parting stop attached to the head of the frame.

If you have sash weights, cut the cords while removing the sashes. Once they’re out, open the sash pockets and remove the sash weights, sash cord, and pulleys.

If you have jamb liners, pry them off after removing the sashes and parting stop.

You should be left with just the window frame.

Step 6: Prep Your Window Frame

Before installing your window replacement, you should prep your frame to ensure it goes in smoothly. Start by scraping off any blistered and loose paint. Next, check your frame for holes and cracks, and then fill them in with wood filler or wood putty.

Sand the window jambs if you wish before priming and painting the frame. After prepping the frame, wipe, sweep, or vacuum it to remove dirt, dust, and debris.

If you have sash pockets, you can stuff them with fiberglass insulation or expanding foam to fill in the gaps and maintain energy efficiency.

Now that your frame is ready, it’s time to install your replacement windows. At this point, you can measure your new window and the frame again to be completely sure that your replacement window will fit.

Step 7: Apply Flashing Tape to the Window Sill

Take your flashing tape and measure out a strip that’s long enough to cover not just the window sill, but also a small part of the bottom of the jambs. Flashing tape will protect your window sill from rotting when it rains.

Step 8: Check if the Sill is Level

Use a level to check the sill for bowing or curving. If it’s not level, use shims to make it level. Once your sill is level, secure the shims with flashing tape to prevent them from moving, and then screw them in.

Insert your window to check if it fits into the frame. Then, set your window aside.

Step 9: Apply Caulk to Your Window Frame

Use your caulk gun to run a ⅜-inch bead of caulk at the head and jambs, and then two at the sill. This caulk will serve as your sealant to keep your window in place.

Step 10: Install Your Replacement Window

Take your window and pop it into the window frame starting from the bottom, pressing it against the sill. Then, tip it up to the head of the frame. Press the window tightly so that it comes firmly into contact with the caulk.

Look for the pre-drilled holes and add shims in between the new window and the existing window frame to adjust and secure the window. Check if the sashes are level, plumb, and square. One way to check is by raising and lowering the sashes with one finger. If you have trouble doing this, then your window needs to be adjusted.

Once you’re satisfied with your window’s placement, screw your window at the pre-drilled holes and through the shims. Use your utility knife to score the shims poking out of the frame and snap them cleanly off.

If you have a sill adapter, measure the distance between the bottom of the window and the window sill and cut the sill adapter based on this measurement. Drill weep holes at the bottom of the adapter and lock it into place.

Step 11: Insulate the Gaps

Using expanding foam insulation made specifically for windows and doors, spray the foam into the gaps between the new window and the frame. Don’t spray flush to the edge, as the foam will expand. If there are still gaps after the foam sets, apply the expanding foam again. 

Step 12: Install Trim and Caulk

Install exterior trim, whether it’s new or recycled from the old window, with your hammer and finishing nails. If you had to remove interior trim, reinstall it the same way you did your exterior trim.

Caulk along the exterior and interior trim to fully seal the window. If you have a sill adapter, be careful not to caulk along the weep holes.

You’re done! Pat yourself on the back and admire your newly installed replacement window.

FAQ About Replacement Windows

Do replacement windows install from inside or outside?

Most replacement windows can easily be installed from the inside, but it’s good to have a friend to hold the window in place from the outside.

Are replacement windows smaller than the original window?

Retrofit replacement windows are typically slightly smaller than the original window, as they need to fit into the existing window frame. These replacement windows – also called insert windows – are also typically smaller than new construction windows.

How do you install a non-sash window?

Replacing non-sash windows, like casement windows, is quite similar to installing a sash window replacement. You strip the window down to its frame, remove the old window, install the new one, and reinstall the molding. 

However, with a casement window, you’ll need to remove the siding and shingles covering the roofing nails along the window flange to remove the window. You’ll also need to apply a water seal gasket all around the frame. Casement windows also have extension jams that you’ll need to remove and reinstall carefully, as there’s a gap between the window frame and the molding.

DIY Window Replacement vs Professional Window Replacement

DIY window replacement can be a fulfilling home improvement project that can save you money, especially if you have most of the tools and materials already. If you don’t fancy yourself a handyman or don’t have the time, then there’s no shame in calling in a window specialist to replace your window. Contact a window pro near you if you would prefer a professional window replacement, window installation, or window repair.

Main Photo by: Andersen Windows / Flickr / CC BY-ND 2.0

Janine Caayao

Janine Caayao has always been fascinated with growing plants, from fruits and veggies to bonsai trees and orchids. Now, she’s interested in urban gardening with her family. She loves finding new tips and tricks to keep their plants thriving.