Installing Replacement Windows Correctly

Older homes often have windows that for one reason or another are poorly insulated. Sometimes there is only one pane of glass. Other times the seal connecting a dual-pane insulated window has broken over time. This can lead to draft windows and inflated energy bills during the heat of summer or the cold of winter. Another failure that is common happens when the caulking used to seal around the window was of poor quality and has dried up and cracked due to thermal contraction.

When your older windows are ready for replacement, here are the steps you can take to replace them yourself or at least understand when hiring a professional to do it for you.

Installing Replacement Windows Correctly

Step 1 – Measure the Window Openings

Most replacement windows are different from new construction windows since they only include the window unit, not the surrounding frame, explains Brian, who runs a replacement window company in Seattle. The new replacement window is placed within the existing window frame. This is why many replacement window manufacturers offer custom sizes as window frames come in all shapes and sizes, especially in older non-standard home builds. These windows come pre-clad in any number of materials including vinyl (most common), wood, fiberglass, aluminum and composite. Before you order new replacement windows, you must measure the existing opening of each window you intend to replace. Start with the inside width of the frame measured at the top, middle and bottom of the frame. Continue with the height and finish off by measuring diagonally to ensure the window frame is square.

Step 2 – Remove Sash and Jamb Liners

So, you’ve measured the window openings, and the window units have been ordered and are waiting to be installed. The next step is to remove the old sash from inside the window frame. You’ll probably have to unscrew and pry off the lower sash as it is often stuck fairly securely. If the windows have never been replaced previously, you’ll also need to remove the parting beads to free the upper section of the sash. The jamb liners will also need to be removed. These are best taken out with a flat pry bar. Be careful when removing the jamb liners not to damage the exterior casings.

Step 3 – Prepare the Window Frame Opening

Now that the sash and jamb liners have been removed, you can prepare the window opening by scraping off all loose paint. You should also patch any cracks and/or holes in the frame using a quality wood putty. Once the putty has dried, sand everything down and prime and paint the frame’s interior surfaces.

Step 4 – Spray Foam and Caulk the Opening

To complete the preparation process, spray some polyurethane foam around the window frame for insulation. This will provide a better seal and reduce the air leakage around the window unit. Once the foam has hardened and dried, break or cut off the excess foam to ensure a flush area around where the replacement window will be inserted. Finally, apply a continuous bead of high quality caulk to the inside area of the exterior window casing as well as along the bottom window sill where the window unit will rest.

Step 5 – Install the Window

Always install the window unit from the inside if possible. Set the bottom edge of the unit on the sill, gently tipping it up and into the window frame. Then push the window tightly into the frame to ensure a complete fit.

Step 6 – Fasten, Shim and Seal the New Replacement Window

Once the window is pushed into place, use several screws to hold the upper side jambs against the window frame. Don’t tighten the screws at this point. Now, shim the window up or down as necessary to ensure the window unit is perfectly centered and square in the frame. Once this is done, use the same caulking from earlier to seal all gaps around the window on both the exterior and interior. Then prime, paint or stain the interior of the window sash and frame, depending on your finishing preference.

A properly installed replacement window should give you a solid 30+ years of good use. If you happen to be looking for replacement windows in the Seattle area be sure to check out Einar Johanson & Glass at

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