Replacement Window Installation Guide

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How to install a replacement window in an existing window jamb.

If you are considering replacement windows for your home you can refer to this guide before you begin to help get a handle on the steps involved and the tools and materials required to not only improve the look of your home, but also improve the energy efficiency.

As long as your existing window frames are sound and square, you can install replacement windows inside the jambs of the windows. If you have any rot or water damage around the window, you will need to remove the window and repair any damaged framing and flashing around the window.

Replacement windows are constructed to fit into existing window openings and many manufacturers have dozens of standard sizes. You will be able to purchase wood, vinyl, fiberglass, vinyl-clad wood, and aluminum-clad wood replacement windows that will conform to the original look of your home as well as your budget. Replacement windows can cost from $100 to $600 per unit depending on the size and materials used.

Replacement Window Types

The three basic types of replacement windows are sash replacement kits, insert replacements, and full-frame units. Sash-replacement kits reuse the old window frame and replace the movable parts, including jamb liners and sash. The liners are fastened to the side jambs of the window opening, then the sash are slipped in between. For these to work, the existing window frame must be level and square. An insert replacement window consists of a fully assembled window in a ready-to-install secondary frame. Sometimes called a pocket window, an insert replacement slips into the existing opening and is then fastened to the old side jambs. Because you’re adding new jambs and liners, the glass area will be slightly smaller than it was before.

Full-frame replacement windows are similar to inserts, except that they have a complete frame that includes head jamb, side jambs, and sill. These are the only option when the existing window frame, sill, or jambs are rotted. To install these, you must strip the window opening down to its rough framing and remove the interior and exterior molding.

Tools and Materials

Tape measure

Angle square or framing square – to determine sill angle


Flat bar

Putty knife

Caulking gun

Cordless drill/driver

3/8-inch spade bit

Paint scraper

Utility knife

Replacement windows

Exterior-grade wood putty

Polyurethane foam

Wood shims

100-grit sandpaper

2-inch Philips head wood screws

Foam-rubber backer rod – ¼-inch

Interior window stops

Primer, paint, and paintbrush  


Replacement Window Diagram



1. Measuring Window

A critical step when replacing windows is measuring the dimensions of the existing window frame to ensure proper window replacement size.

Start by measuring the inside width of the old window frame, jamb to jamb, in three places: across the top, middle, and bottom. Use the smallest of the three measurements.

Next, measure the window frame's height from the top of the sill to the underside of the head jamb in three places: at the left jamb, in the middle, and at the right. Use the smallest measurement.

Check that the window frame is square by measuring the diagonals from corner to corner. The two dimensions should be the same. If the frame is out of square by 1/4 inch the replacement unit can be shimmed to fit. Anything more may require adjustments to the frame. If the original frame is out of square by ½-inch or more the new unit wouldn’t look right after it has been installed and you should opt for a full-frame replacement.

Use an angle-measuring tool to determine the slope of the sill as some replacements come with a choice of sill angles. You can use an adjustable angle square to copy the sill angle and then use a protractor or framing square to determine the angle.

2. Remove the Sash

To remove the old sash from the window frame you will need to pry off or unscrew the interior wooden stops to remove the lower sash. If you need to reuse the stops, be careful as they snap easily. Next, take out the parting beads to free the upper sash. If the windows have been replaced previously there won’t be any parting beads.


3. Remove jamb liners

If the window frame was fitted with vinyl or aluminum jamb liners, use a flat bar to pry them free. In the case of an original window, remove any remaining wooden stops from the window frame. Leave the interior and exterior casings intact. Older windows will have spiral ballasts and aluminum tracks that need to be removed.

If the original sash weights are still in place, take this opportunity to remove them from their pockets and insulate behind the window frame and inside the pockets minimally expanding foam. (See Step 5) Unscrew the access panel on each side jamb and pull out the weights.

4. Clean up the frame

Scrape off any loose paint and patch any holes or cracks with exterior-grade wood putty. Sand the frame smooth then prime and paint the surfaces.

5. Insulate the frame

Use low-pressure, minimally expanding polyurethane foam intended for windows and doors; anything else will bow the frames and prevent the sash from working properly. Remove any existing fiberglass in the weight pockets. Drill 3/8-inch-diameter holes with a spade bit near each end and one in the center, down through the sill and up through the head jamb.

Spray the expanding foam into the holes until it begins to seep out. Also spray foam into the sash-weight pockets in the side jambs. Allow the excess to harden for at least 6 hours, then break or cut it off flush.



6. Caulk

In preparation for installing the window, apply a bead of caulk to the exposed inner face of the exterior casings or to the blind stops on the top and sides of the frame. Also apply two continuous beads of caulk along the windowsill.

7. Install the Replacement Unit

Follow the instructions for installing the new window from the manufacturer. Some vinyl replacement windows come with a vinyl channel that is placed on the top of the unit called a header or expander that is typically installed at the same time. Working from inside the room, set the bottom of the insert replacement onto the sill, then tip it up into the opening. Press the window tight against the exterior casings or blind stops.

8. Secure the Window

Hold the window in place with one 2-inch screw driven loosely through the side jamb a few inches from the top and into the framing. The screw should be in just far enough to allow the window to operate. Test the operation of the new window by closing and locking the sash.


9. Shim

Adjust the unit by inserting shims under the sill and behind the side jambs until it is centered in the space and opens, closes, and locks smoothly. Measure the window diagonally from corner to corner; the measurements should be the same. When the window is square, screw it in place through the predrilled holes. To avoid bowing the frame, slip a shim behind the jamb at each screw, then screw through the shim. Trim the shims flush with a utility knife.  

10. Finishing

From the outside, measure the gaps between the window frame and the casing. Fill gaps less than 1/4 inch wide with a high quality caulk. If you have wider gaps around the window frame you will need to insert a foam-rubber backer rod. On the inside, fill any gaps around the window with minimally expanding foam. Finish the inside by reinstalling the old stops or installing new stops; you may want to paint them first before installing them. Prime and paint the frame and sash if the unit is wood.


carol roach
Posted on Jan 11, 2012
Abdel-moniem El-Shorbagy
Posted on Jan 11, 2012
Roberta Baxter
Posted on Jan 11, 2012