Window Replacement: Measurement, Removal, and Installation

Insert vs. Full-Frame Replacement

Insert (pocket) replacement: the new window slides into the existing frame after removing the old sashes, hardware, and parting stops. The original frame, exterior trim, and interior casing stay in place. Fastest, least disruptive, no exterior work.

Full-frame replacement: the entire window unit — frame, sashes, exterior casing — comes out. The new unit installs into the rough opening in the wall framing. Requires exterior flashing, possibly siding modification, and interior trim work.

Choose insert when: the existing frame is square, solid (no rot), and the right size. Choose full-frame when: the frame is rotted, the window is severely out of square, or you are changing the window size.

Measuring for Insert Replacement

Measure inside the existing frame — not the sash size, not the rough opening. You need the width and height of the space where the new window will sit.

Width: measure at three points — top, middle, and bottom of the frame opening. Use the smallest measurement. Height: measure at three points — left, center, and right. Use the smallest.

Measure the depth of the frame from the interior stop to the exterior blind stop. The new window must fit within this depth. Most insert windows need at least 3-1/4 inches of frame depth.

Order windows 1/4 to 3/8 inch smaller than your measurements in both width and height. This gives shimming room for leveling. A window ordered to exact measurement will not fit.

Removing the Old Window

For double-hung windows: remove the interior stops (the strips holding the lower sash in place) with a thin pry bar. Lift out the lower sash. Cut the sash cords if present. Remove the parting bead (the strip between the two sashes). Lift out the upper sash.

Remove any remaining hardware, pulleys, and weights from the frame. Insulate the weight pockets with spray foam or fiberglass — these cavities are major air leak paths.

Scrape the frame clean of old paint buildup, glazing compound, and debris. The new window needs smooth, flat surfaces to sit and seal against.

Check the frame for rot by probing with an awl. Soft spots mean rot. Small areas of rot can be treated with wood hardener and filled with epoxy wood filler. Extensive rot means full-frame replacement is the better path.

Installing the Insert Window

Dry-fit the new window in the opening. Check that it fits with shimming room on all sides. If it is too tight, do not force it — sand or plane the frame. If it is way too loose, the measurement was wrong.

Apply a continuous bead of silicone caulk along the sill (bottom of the frame) and up each side where the new window flange will rest. This is the weather seal — gaps here let water in.

Set the window into the opening from the outside, pressing the exterior flange against the caulk. Center it in the opening with equal gaps on each side.

Shim behind the mounting screw locations — typically two shims per side and two across the top. Check level and plumb as you shim. Adjust until the window is perfectly level, plumb, and square.

Drive the mounting screws through the window frame into the existing wood frame. Tighten evenly — over-tightening bows the frame and the window will not operate smoothly. Check operation: both sashes should slide freely and the lock should engage without forcing.

Insulating and Sealing

Fill the gap between the new window and the old frame with low-expansion spray foam. Use foam labeled 'window and door' — regular expanding foam generates enough force to bow the frame. Apply in thin beads, not one massive fill.

Caulk the exterior where the new window meets the old frame. Use paintable exterior-grade silicone or polyurethane caulk. This is the primary weather barrier — do it thoroughly.

Replace the interior stops and casing. If the old stops are damaged, buy new stop molding at the lumber yard — it is stocked in standard profiles. Nail the stops snug against the new window frame without pinching it.

Tools for Window Replacement

Tape measure. Level (2-foot). Utility knife. Pry bar for removing stops and old hardware. Drill/driver for mounting screws. Shims (cedar or composite). Caulk gun with silicone caulk. Low-expansion spray foam. Hammer and finish nails for interior trim.

For old windows with painted-shut sashes: a paint zipper or utility knife to score the paint seal before removal. A reciprocating saw for cutting stubborn nails or frames that will not come apart.

Frequently Asked Questions