How Are Replacement Windows Installed? Step by Step

Replacement windows are installed in five broad steps: the old window comes out, the opening is cleaned, checked and repaired, the new window is set on shims and fastened until it is plumb, level and square, the gap around it is insulated and sealed, and the outside is flashed and trimmed so water drains away. How much of that happens depends on the method. An insert goes inside the old frame; a full-frame replacement strips the opening back to the framing.

The U.S. Department of Energy's window replacement checklist says all work should follow applicable codes, standards and the manufacturer's instructions, and it recommends full-frame replacement installed to the ASTM E2112 standard practice ¹. The steps below follow that checklist, DOE's flashing guide and one manufacturer's installation guide for both methods. For help choosing between the two methods, see full-frame vs. insert window replacement.

Before the crew arrives

Installation starts with measuring and ordering, often weeks earlier; how installers measure for replacement windows covers that visit. Andersen's installation guide says to check measurements before starting to confirm the unit is the correct size ³.

On the day, the crew needs room. Andersen suggests about 5 feet of clear space around each window, covering furniture against dust, and taking down blinds and window alarms beforehand, which it names as common causes of delay ⁴. Ask the installer a few days ahead whether the windows go in from inside, outside or both ⁴.

In a home built before 1978, the old paint may contain lead. DOE's checklist says to assume lead paint in pre-1978 homes unless testing shows otherwise ¹. EPA's Renovation, Repair and Painting rule covers anyone paid to perform work that disturbs painted surfaces in homes built before 1978, and requires lead-safe work practices ⁶. Full-frame vs. insert window replacement covers what the rule asks of the firm; on the day, ask how the crew will contain dust.

Step 1: Remove the old window

Insert installation

For an insert, the old frame stays. Andersen's guide for an interior insert lists the sequence for an old double-hung window: break the paint seal along the interior stops, remove the stops, cut the balancer cords, take out the bottom sash, the parting stops and the top sash, and remove pulleys and weights if present ³. The empty weight pockets are then insulated and the pulley holes filled with wood so screws have something to anchor into ³. DOE's checklist describes the same removal and calls for the opening to be cleaned and fully sealed ¹.

Full-frame installation

For a full-frame replacement, the whole window comes out. DOE's checklist calls for removing trim and finishes as needed to fully expose the rough opening on all four sides ¹. Andersen's guide says to remove the existing window, preserve the wall's existing water management system if there is one, clean the opening of debris and old sealant, and repair any surface that will be sealed or anchored to ³.

Step 2: Check and prepare the opening

This is where hidden problems show up. Fixr notes that once the old window is out, an installer may find water damage or shifting framing that must be repaired before the new window goes in ⁵. Andersen advises that the installer should tell you if rotted wood or water damage turns up, and that work may pause while it is fixed ⁴.

With the opening sound, the installer checks that it is plumb, level and square and that its size allows for flashing and fasteners ³. For a full-frame job, DOE's flashing guide then has the house wrap cut and folded into the opening, a back dam fixed along the inside edge of the sill, and self-adhesive or other approved pan flashing applied over the sill, wrapping its corners ². Citing ASTM E2112, that guide says sloped pans are required where the sill is deeper than 6 inches ².

Step 3: Set, shim and fasten the window

The new window is set on shims, not straight onto the sill. Andersen's guide says to elevate the window on shims under the side jambs and warns that a window that is not properly shimmed may not operate or perform as designed ³. The installer adjusts the shims until the window is plumb, level and square; Andersen's guide calls for the diagonal measurements, and the horizontal measurements, to be within 1/8 inch before the screws are tightened, and says not to shim at the head ³. The guide says fasteners must attach to a structural framing member ³.

On a full-frame job with a flanged window, DOE's guide has the outside edges of the head and side jambs caulked before the window is set, with the sill left uncaulked so water can drain ¹².

Step 4: Insulate and seal the gap

There is always a gap between the new frame and the opening. Andersen's guide calls for a continuous seal around the full perimeter, filling the interior third of the gap with low-expanding foam or backer rod and sealant, and warns that overfilling with foam or overpacking backer rod can bow the jambs ³. DOE's flashing guide describes the same seal, using backer rod or non-expanding foam and caulk ². Bowed jambs may make a sash bind, so this is a step where restraint matters.

Step 5: Flash, trim and finish

On a full-frame installation, flashing is layered so each piece sheds water over the one below it. DOE's guide sets the order: jamb flashing over the window flanges, extending 4 inches above the head flange; head flashing over it, extending 1 inch past the jamb flashing; then the house wrap folded down over the head flashing and taped ². Trim goes back on last, with cap flashing above exterior trim ². DOE's checklist asks for replacement trim to be primed on all six sides ¹.

On an insert, the joint between the new window and the old frame is sealed, and the old exterior frame may be covered with trim or bent aluminum coil stock; Andersen lists coil stock among the supplies in its installation guide ³.

Two final details are easy to miss. Andersen's guide says not to cover the window's weep holes, which let water out of the frame ³. It also says not to remove the NFRC performance label until after final inspection, since removing it could delay a code official's sign-off ³. Whether a permit and inspection apply is a local question; see do you need a permit to replace windows.

How long it takes

Andersen says a full-frame replacement window on the ground floor typically takes 4 to 6 hours, with upper-story windows taking longer ⁴. Inserts are generally quicker because the trim and siding stay in place. Rot repair, size changes and second-floor work all add time. Ask your installer for a timeline and whether the job runs over more than one day.

What you can check on the day

You do not need to know how to install a window to spot problems. Things to ask or look at:

• Before work starts: Is the crew installing an insert or a full-frame window in each opening, as the contract says?
• When the old window is out: Was any rot or water damage found, and what does the repair cost?
• Operation: Does each sash open, close and lock smoothly, without rubbing?
• Gaps: Are the reveals around the sash even? Uneven gaps can point to a frame out of square or bowed by foam.
• Outside: Is the bottom edge of the sill left uncaulked, and are the weep holes open ²³?
• Label: Is the NFRC label still on the glass if an inspection is due ³?
• Walkthrough: Andersen recommends a walkthrough with the contractor to look over the finished work and learn how to operate and clean the windows ⁴.

What installation adds to the price

Installation is a real share of the bill. Fixr reports labor of $150 to $300 for smaller windows and up to $800 for large bay or bow windows, and puts labor at about 30 percent of the total ⁵. Full-frame work, rot repair and exterior trim add to that. The window replacement cost guide covers totals, and how to compare window quotes shows how to line up what each quote includes.

Installation quality matters as much as the window. The how to choose a window installer guide lists what to ask, starting with: Is the installer licensed in your state? When you are ready, compare quotes from window installers and ask each one to describe its installation steps for your openings.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Official record: DOE Building America Solution Center window replacement checklist, including its recommendation of full-frame replacement to ASTM E2112, removal steps, lead paint assumption for pre-1978 homes, head and jamb caulking with the sill left uncaulked, and six-side priming, checked October 26, 2026.
• ² Official record: DOE Building America Solution Center flashing guide, including the 6-inch sloped sill pan rule, 4-inch jamb flashing and 1-inch head flashing overlaps, and interior air sealing, checked October 26, 2026.
• ³ Reported: Andersen 100 Series flangeless installation instructions for full-frame and insert installations, including the 1/8-inch tolerance, shimming, sealing, weep hole and label instructions, dated September 25, 2025, checked October 26, 2026.
• ⁴ Reported: Andersen installation day checklist, including the 5 feet of clearance, the 4 to 6 hours per ground-floor full-frame window and the walkthrough, checked October 26, 2026.
• ⁵ Reported: Fixr labor ranges, the 30 percent labor share and its note on hidden damage, updated January 21, 2026, checked October 26, 2026.
• ⁶ Official record: EPA Renovation, Repair and Painting program scope for paid work on painted surfaces in pre-1978 homes and its lead-safe work practices, checked October 26, 2026.