New Construction vs Replacement Windows

A new construction window has a nailing fin, a flange around the outside of the frame that is nailed to the wall framing before the siding goes on. A replacement window is made to go into an existing opening without that step: either as an insert inside the old frame, or as a full-frame unit with no fin that fits the rough opening once the old window is out. Andersen describes all three and says the fin makes a new construction window easy to seal and flash ¹.

In an existing home, the choice comes down to whether the siding or trim around the window is coming off. If it is, as in an addition, a major remodel or a siding job, a new construction window is the natural fit. If the exterior is staying, a replacement window usually is. On cost, the window itself may cost a little more as a replacement unit, while the installation costs less; the figures are below.

What the nailing fin does

The fin, also called a flange, projects beyond the frame and has pre-punched holes for fasteners, Andersen says ¹. In a new wall, the window is set into the rough opening and nailed through the fin to the framing before the siding goes on ¹. Pella's installation booklet describes the same arrangement from the removal side: existing nailing fins are usually nailed to the studs in frame construction, with siding, brick veneer or other exterior material applied over the fin ³.

That is the catch in an existing home. Because the wall finish laps over the fin, Andersen says a flanged window must either be installed before the siding or the siding and trim must be removed or peeled back ¹. Andersen calls that the most invasive and costly option for a replacement project ¹.

The three ways to put a window into an existing home

These descriptions follow Andersen's comparison ¹. Andersen says a flangeless full-frame window offers the chance to fix air and water problems because it does not rely on the existing systems, while an insert relies on them and so will not improve those conditions ¹. It also says an insert narrows the opening and reduces daylight, since the new frame sits inside the old one ¹. The full-frame vs insert window replacement article goes deeper on those two.

Masonry and stucco walls have no siding to peel back. Pella's booklet covers full-frame replacement in masonry with stucco using block-frame windows, which are fastened with screws through the frame or with clips ³. Flush-fin retrofits are another option in stucco; see retrofit windows in stucco and brick.

When each is used in an existing home

New construction windows may suit you when:

• You are adding on. An addition has new walls and rough openings, so it is new construction in the plain sense.
• The siding is coming off anyway. During a re-siding job, the fin can be nailed and flashed into the new weather barrier before the new siding goes on ¹.
• You are changing the opening. Andersen notes flanged windows are used in remodels that change rough openings ¹.
• The old windows leaked into the wall. The Department of Energy's Building America Solution Center recommends connecting the window's membrane flashing to the building paper or other water-resistive barrier, notes that some siding typically has to be removed for a complete replacement, and says cracked or rotting sills need to be replaced before other work ⁴.

Replacement windows may suit you when the siding, brick or stucco and the interior trim are staying, the openings are the size you want, and the goal is new glass and sashes rather than wall repair. An insert needs a sound old frame; a flangeless full-frame unit needs a sound rough opening ¹.

The cost difference

There is no single survey figure for the difference, because it depends mostly on the wall work rather than the window. What the sources say:

• The window. This Old House says replacement windows cost about 10% to 15% more to purchase than new construction windows (updated May 22, 2026) ².
• The installation. This Old House says installation labor for replacement windows can be as much as 50% to 100% less, a figure it sources to Angi ². Treat that range as a rough indication that labor can be much lower, not as a figure to plan on.
• The order of the options. Andersen ranks inserts as the least expensive replacement option, a flangeless full-frame as costing more than an insert but usually less than a flanged window, and a flanged new construction window as the most costly in a replacement ¹. The Solution Center likewise calls complete window and frame replacement the most intrusive and expensive option ⁴.

What drives the gap on a real quote is everything around the window: removing and replacing siding or trim, flashing, interior drywall returns, paint, and any rot found once the wall is open. When you compare quotes, ask each company to price the same method, and ask for the siding and trim work as separate lines. The how to compare window quotes article has a template, and the window replacement cost guide gives per-window ranges by material and type.

Questions to ask before you choose

• Which method are you quoting for each window: new construction, flangeless full-frame or insert?
• If new construction, who removes and replaces the siding and trim, and is that in the price?
• How will the window be flashed into the weather barrier?
• If you find rot once the old window is out, how will that be priced?
• Does the work need a permit here? See do you need a permit to replace windows.
• Is the installer licensed where you live? The how to choose a window installer guide covers what else to check.

In older homes, removing old windows and trim may disturb lead-based paint; Pella's booklet carries that caution and points to the EPA ³. Ask how the crew will contain dust.

When you know which method fits your walls, you can compare quotes from window installers and ask each company to price it.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Reported: Andersen's descriptions of flanged full-frame, flangeless full-frame and insert windows, when each works, and their relative cost, checked October 26, 2026.
• ² Reported: This Old House figures of 10% to 15% more to purchase and up to 50% to 100% less for installation labor (the labor figure sourced to Angi), updated May 22, 2026, checked October 26, 2026.
• ³ Reported: Pella block-frame installation booklet on nailing fins under siding, block-frame fastening with frame screws or clips in masonry with stucco, and the lead paint caution, revised March 31, 2023, checked October 26, 2026.
• ⁴ Official record: Building America Solution Center on complete window and frame replacement, flashing to the water-resistive barrier, siding removal and replacing rotting sills, checked October 26, 2026.