How to install a replacement window: measuring, flashing and sealing, insert or full-frame

Insert or full-frame replacement: which do you need?

Use an insert when the old frame is sound and square. Go full-frame when it is rotten or out of square, or when you want the full glass size back.

An insert, also sold as a pocket or retrofit window, goes into the old window frame after the sashes, stops and hardware come out. The exterior trim, the interior casing and the wall stay put. It is fast and it does not touch the weather barrier of the wall. The cost is glass area: a new frame inside the old one shrinks the opening on every side.

Full-frame means removing everything down to the rough opening in the framing, then installing a window with a nailing flange and flashing it into the house wrap. It is the right call when the old frame is rotten or out of square, when you want the full glass size back, or when there is water damage around the window that you need to see. It roughly doubles the work: the calculator behind this site uses 3.2 hours for a double-hung insert and 6.2 for the same window full-frame.

This guide covers how to do the work. Whether new windows pay for themselves is a different question, answered in the window replacement payback guide. Short version: rarely on energy alone.

How do you measure for a replacement window?

Measure from inside: the width at three points and the height at three points, keeping the smallest of each, and check that the diagonals are within 1/8 inch. Andersen's method, step by step:

1. Check square first. Measure from the upper left corner, where the head meets the side jamb, to the lower right corner, then the other diagonal. Check: the two are within 1/8 inch of each other. If not, the frame is not a good candidate for an insert.
2. Width: jamb to jamb at the top, center and bottom. Use the smallest, rounded to the nearest 1/8 inch. Example: 36-5/8, 36-1/2 and 36-9/16 inches gives 36-1/2. Check: you wrote down three numbers and kept the lowest.
3. Height: head to sill at the left, center and right. Use the smallest. Example: 54-3/8, 54-1/4 and 54-5/16 gives 54-1/4. Check: same as above.
4. Jamb depth, the space between the inside and outside stops. Andersen's inserts need at least 3-1/4 inches. Check: your depth meets the minimum in the maker's sizing guide.

Why the smallest: a frame that is 1/8 inch narrower in the middle than at the ends will not take a unit sized to the ends. The window maker's sizing guide then says how much smaller than those numbers to order. Menards lists the stock unit priced below by a 36-1/4 by 54 inch opening, so check the maker's sizing guide to confirm it fits the 36-1/2 by 54-1/4 inch example before ordering. Stock sizes rarely line up that well, which is why most inserts are made to order.

Materials and tools for one replacement window

The declared case: one ground-floor double-hung window in a sound wood frame, house built after 1978, replaced with a vinyl double-hung insert without a nailing flange.

Two tubes because the sill, the side jambs and the exterior perimeter each take a bead; a 36 by 54 inch window has a perimeter of 180 inches, and the sealant goes on at least twice. For a full-frame job, add flashing tape (a 4-inch by 75-foot roll was $42.99 for 3M and $52.99 for DuPont at Ace), house wrap tape and a nail-fin window, which is a different product from the one priced here.

Tools you need: a tape, a utility knife, a stiff putty knife and a small pry bar for the stops, a drill and driver, a combination square, a level, a caulk gun and a foam gun or can, and safety glasses. Andersen's guide adds a trim nailer and a miter saw for putting the stops back, and a table saw for cutting stop covers to width; a friend's or a rental will do. Nice to have: suction cups to handle the glass.

How to install a replacement window insert, step by step

Summarized from Andersen's interior installation guide for double-hung inserts. Your window's own guide wins wherever the two differ.

1. Confirm the size. Hold the new unit's dimensions against your smallest measurements. Check: the unit is smaller in both directions, and the opening is structurally sound.
2. Remove the interior stops. Score the paint seal first, then pry them off without breaking them; they go back on. Check: stops off in one piece, numbered by position.
3. Remove the sashes and hardware. Cut the balance cords, lift out the bottom sash, pull the parting stops, lower and remove the top sash, then take out pulleys and weights. Check: an empty frame with nothing sticking out of the jambs.
4. Insulate the weight pockets and close them. Fill the pulley holes. Check: covers back on, no open cavity.
5. Clean the opening and lay the sealant. Andersen calls for 3/8-inch beads on the sill and a 3/8-inch bead on each side jamb. Check: continuous beads, no gaps at the corners.
6. Set the window. Raise the bottom sash, lift the unit into the center of the opening, tip the top in, and pull the sill tight to the interior stool. Hold it until it is fastened. Check: it sits against the exterior stops with the sealant squeezed out evenly.
7. Fasten, loosely. Drive the 2-inch screws through the predrilled holes at the top and center of each side, without fully tightening. Use a combination square from the interior trim to keep the reveal equal. Check: the same distance from the trim at the top and bottom on both sides.
8. Shim and square. Put shims all the way in behind every screw location, adjust until the unit is square and the jambs are straight, then tighten. Check: diagonals equal, both sashes slide and tilt freely, locks close. Do this before the foam, while you can still change it.
9. Seal the interior gap at the top and sides with low-expanding foam, or backer rod plus a bead of sealant. Do not overfill. Andersen skips backer rod at the sill for gaps under 3/8 inch. Check: sashes still operate with the same effort as before the foam.
10. Seal the exterior. Push backer rod between the frame and the old opening, then run sealant around the full perimeter, connecting to the sill bead. Check: one unbroken line of sealant.
11. Put the trim back. Install exterior stop covers if your unit has them, leaving 1/16 inch at the bottom for drainage, then reinstall the interior stops. Check: drainage gap open, stops tight to the frame without pushing it.

Full-frame installation: the flashing sequence

On a full-frame install the water protection comes from the order of the layers. Each piece has to lap over the one below it, like shingles, so water running down the wall always lands on the outside of the next layer. The sequence from Andersen's flanged-window guide 105419:

1. Cut the house wrap at the opening, fold the sides in, and cut a flap at the head on 45-degree angles. Tape the flap up out of the way. Check: head flap free to fold down later.
2. Sill flashing goes first, turned up 6 inches at each side and lapped 2 to 3 inches onto the house wrap. Check: no bare wood at the sill or the bottom corners.
3. Set the window in a 3/8-inch bead of sealant on the back of the flange, shim, level and fasten as its guide says. Andersen warns the factory flange is not flashing. Check: level sill, equal diagonals.
4. Side flashing over the side flanges, overlapping the sill flashing and running 2 inches above the head flange and 2 inches below the sill flashing. Check: the side pieces lap over the sill piece.
5. Drip cap and head flashing: drip cap set in sealant, head flashing over it and over the tops of the side flashing. Check: the head piece is the top layer.
6. Fold the house wrap flap down over the head flashing and tape it. Check: from top to bottom, every layer laps over the one below.

Inside, fill the gap with low-expanding foam without overfilling, and leave the void at the sill open for drainage, as the same guide says.

How to seal windows without trapping water

Two products, two jobs. Foam insulates and stops air; sealant keeps out water. For foam, buy one labeled for windows and doors. DAP rates its low-pressure version to AAMA 812-04, the test for foams used around frames. Ordinary expanding foam keeps pushing after you walk away and can bow the jamb enough to bind the sash. For sealant, use one labeled for windows and siding, or the one your window maker names. Andersen's guide for flanged windows calls for silicone.

Both Andersen guides leave deliberate openings: the 1/16-inch gap at the bottom of the exterior stop covers and the unfilled void at the sill. Those are drainage paths. Caulk them shut and the water that gets behind the frame has nowhere to go but into the sill.

Code rules: lead paint, bedroom egress and safety glass

In a house built before 1978, window replacement disturbs old paint. EPA's RRP rule requires anyone paid to do that work to be a lead-safe certified firm. Window replacement is specifically excluded from the small-job exemption in 40 CFR 745.83, so the rule applies to a paid window job of any size. Homeowners working on their own home are outside the rule, unless the home is rented, used for child care, or bought to fix and sell. Outside the rule does not mean safe: Andersen's guide flags pre-1978 paint too.

For bedrooms, IRC R310 requires an emergency escape opening in every sleeping room: at least 5.7 sq ft of net clear opening (5 sq ft at grade floor), at least 24 inches high and 20 inches wide, with the sill no more than 44 inches above the floor. An insert shrinks the opening. As an illustration, a lower sash that opens 32 inches wide by 23 inches high gives 32 x 23 / 144 = 5.11 sq ft, short on area and 1 inch short on height. R310.5 exempts a replacement window if it is the maker's largest standard size that fits the existing frame or rough opening, of the same operating style or one with a larger opening, and not part of a change of occupancy. Check the net clear opening the maker publishes for the exact unit before ordering a bedroom window.

Under IRC R308.4.3, glass in a window is a hazardous location, and needs safety glazing, when all four conditions are met: a pane larger than 9 sq ft, bottom edge less than 18 inches above the floor, top edge more than 36 inches above the floor, and a walking surface within 36 inches. Low picture windows often qualify. Order tempered or laminated glass for those; Andersen, for one, lists it as a special order.

Window installation mistakes that cost money

Is it worth installing replacement windows yourself?

For one window, rarely. For ten ground-floor inserts in sound frames, the labor you skip is $1,000 to $4,000. Carpenters earned a median $29.12 an hour in May 2025 (BLS). Loaded with benefits at the 69.5% wage share of construction compensation, that is $41.90 an hour. At the calculator's 3.2 hours, the labor inside an insert costs a contractor $134.08; at 6.2 hours, a full-frame costs about $260. This Old House reports insert installation at $100 to $400 per window, which brackets that wage cost plus overhead.

The window itself costs the same either way, which is why the percentage saving on windows is smaller than on doors or springs. One window is rarely worth the tool purchases and the learning. Ten ground-floor inserts in a post-1978 house, where the sills are sound, is a real saving of $1,000 to $4,000. Upstairs, rotten, pre-1978 or full-frame, hire it and spend your time checking the quote. Run your own sizes, frames and labor rate through the window replacement cost calculator.