How to Install Replacement Windows

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Things You'll Need

  • Reciprocating saw

  • Chisel or ice pick

  • Utility knife

  • Level

  • Drop cloths

  • Replacement windows

  • Caulking gun and caulk, shims

  • Pry bar

  • Hammer, nail set and finishing nails (1 1/2)

  • Tape measure

How to Install Replacement Windows. Installing new windows can help make a house more energy efficient and improve its appearance. However, many homeowners are reluctant to replace their windows because of the cost of the windows themselves plus installation costs.

"Replacement windows," though, are designed to fit right into existing window frames. They are different from a "new construction" window (which has a nailing flange around the outside of the window frame that requires removing the outside trim to mount the window). Replacement windows are installed from the inside and don't require removing any exterior trim or stucco.


Step 1

Determine the style and material you want for your new windows. Replacement windows are built from vinyl, wood or composite and are available in virtually any style (such as double hung, single hung and casement).

Step 2

Measure the height and width of your existing windows. Measure the width from jamb to jamb (side to side) at the top, middle and bottom of the window. Measure the height from the sill to the top of the window at the left side, the middle and the right side. To ensure the replacement window will fit into the window frame, use the smallest measurements for both height and width.

Step 3

Order your replacement windows with these measurements in hand.

Step 4

Determine that your windowsill is solid and free of any wood rot. It doesn't make any sense to install replacement windows into a rotten frame.

Step 5

Carefully remove the inside stops of your existing window. You can use them again when you install the replacement window.

Step 6

Remove the window sashes themselves (cut through any nails holding them in) and clean up the window frame. Remove any old caulk and drive in or pull out any nails.

Installing the Replacement Window

Step 1

Check to ensure that the window will actually fit into the opening by placing it into the frame and then remove it.

Step 2

Run a bead of caulk along the inside edge of the outside window stops and along the windowsill.

Step 3

Place the window back in the center of the frame and check that it is plumb and square. (Use a level for plumb and measure diagonally from corner to corner (both ways) to check that it is square). Shim the window to keep it in place.

Step 4

Reinstall the inside stops using new 1 1/2 inch finishing nails and set the nails.

Step 5

Caulk where the stops meet the window frame.

Step 6

Check the outside of the window. The window should come with a sloped frame to match the slope of the sill, or it should come with an insert that you use to cover the gap. If neither is there, you can cover the gap with a piece of wood. Caulk the seams where the insert goes against the frame.


Measure carefully. The replacement window needs to fit inside the existing opening. Some professionals suggest that you use the smallest measurement and then take another 1/4 inch off of it for your window size. This ensures that the window will fit, and shimming around the window can eliminate any gaps. Check your windows carefully when they arrive. Are they the size you ordered and the window style and quality that you paid for? Remember, mistakes happen. Use a small chisel or an ice pick to determine that the wood in your window frames is sound.


Since replacement windows fit into an existing window frame, they do slightly reduce the size of the window opening. You may notice that your window is slightly smaller.