How to Install Insert Replacement Windows

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  • 4-24 hours
  • Advanced
  • 125-18,000
What You'll Need
Tape measure
Pry bar
Utility knife
Shop vac
Flashing tape
Spirit level
Spray insulation
Putty knife
Finishing nails

Replacing a window in your home can seem like a daunting task. With all of the different window replacement options on the market, it can be easy to become overwhelmed when shopping for the right replacement, let alone finishing the project. Fortunately, replacing a window is a relatively simple task that requires few tools to complete.

Determine the Best Replacement Option

There are generally two types of window replacements: full-frame and insert replacements. The full-frame option removes the entire window down to the home’s frame and is recommended if the old frame is falling apart. Insert, or pocket replacements, leave the existing frame in place and involve installing a new set of windows, called sashes. This guide will teach you how to install insert replacement windows, which are much less involved than full-frame projects.

Measure for the New Window

Someone measuring a window.

Before you purchase the replacement unit, you need to get exact measurements of the existing frame. Because the old frame is left intact, the new sashes will need to fit snuggly inside the frame. Start by measuring the width and height of the opening, making sure to take the measurements from inside the frame. Take three measurements for the height and width—from the right, middle, and left of the inner frame—and go with the smallest value you find. Then, take half an inch off the final measurement. This will ensure that the replacement window actually fits inside the opening.

Remove the Old Unit

Most older windows operate with either weights or springs, both of which need to be removed to take out the sashes. Cutting the cord will allow the weight to fall while springs should be removed while the sash is at the top of the frame. The springs are sometimes covered with plastic, so you might need to take these out before proceeding. Next, you need to remove any window stops holding the sashes in place. Only remove stops from one side of the frame, not both. With the stops out, simply lower the sash and take it out of the frame.

Prepare the New Window

With the old sashes out, prepare the frame before installing the new treatments. Start by vacuuming the frame with a shop vac to eliminate any dirt and debris. Next, install flashing tape on the inner part of the window sill. The tape should cover the entire length of the sill. Then, check the entire inner frame and make sure it's level. If the surface is not level, correct it with shims. Make sure the shims are secured with flashing tape and screws so that they do not move around. Finally, dry-fit the sash and make sure everything is working properly. You should notice a tiny gap around the entire window.

Secure the Windows

From the outside, use silicone caulk on the top and sides of the sill to secure the upper window in place. When placing the upper window, use shims to ensure a tight and level fit. The shims should be installed on top of the preexisting screw holes in the frame. From inside the house, install the screws through the shims and frame holes. At this point, you need to ensure that the window is resting level and plumb to the frame. Use additional shims to make sure everything is properly placed.

Install the Sill Adapter

Working on the outside, spray insulation around the window to fill any gaps. Then, measure the height and width for the sill adapter and cut it accordingly. The adapter will be secured in place at the bottom of the window via weep holes, which will need to be drilled into the adapter.

Finishing Touches

Caulking around a window.

Use finishing nails to install the exterior trim. Latex caulking should be used to seal gaps on trim edges, but avoid filling in the weep holes. On the interior, install a layer of latex caulk to completely seal the new window.