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4. Insert Vs. Full-Frame Window Replacement: What Your Home Actually Needs

When you start shopping for replacement windows, you’ll run into two terms almost immediately: insert replacement and full-frame replacement. The difference between them is one of the most important decisions in the entire project—and it’s the one that the least number of companies will explain to you honestly.

Here’s why: insert replacements are faster to sell, faster to install, and generate quicker revenue for window companies. Some companies only offer inserts. Others will recommend whichever approach pays them better. Very few will sit down with you, assess your frames, and give you a straight answer about which approach your home actually needs.

This page breaks down both methods so you can walk into any conversation—with Forshaw or anyone else—knowing what you’re being sold and why.

What Is a Window Insert?

A window insert (also called a pocket replacement or retrofit) involves removing the old sashes—the moving parts of the window—and sliding a new, self-contained window unit into the existing frame. The original frame, exterior trim, and interior casing all stay in place.

How It Works

The installer removes the old sashes and any hardware, cleans the frame opening, and sets the new insert unit inside. The new unit is shimmed, leveled, sealed, and insulated around the edges. From the inside, you get a new operable window. From the outside, the original frame and trim remain unchanged.

When Inserts Make Sense

Insert replacement is the right call when the existing frame is structurally sound—no rot, no water damage, no significant warping, and the frame is still square. If the frame is in good condition, an insert gives you a new, energy-efficient window without the disruption and cost of tearing into the wall.

Inserts also tend to work well in homes where the original frames haven’t had decades of weather exposure, and in situations where the homeowner wants to upgrade glass and hardware without a major construction project.

The Tradeoffs

There are real tradeoffs to understand. Because the new unit sits inside the old frame, the visible glass area gets slightly smaller—you lose about a half-inch to an inch on each side. More importantly, an insert doesn’t address anything behind the frame: failed flashing, deteriorated insulation, hidden rot, or gaps where air and moisture have been getting in for years. If those problems exist, an insert hides them rather than solving them.

What Is Full-Frame Window Replacement?

Full-frame window replacement removes everything—the sashes, the frame, the exterior trim, the interior casing—all the way down to the rough opening in the wall. Then a completely new window system goes in, including new flashing, insulation, and trim.

How It Works

The installer removes the entire old window assembly, exposing the rough framing. This is where hidden problems become visible: rot in the rough sill, water damage in the framing, failed or missing flashing, and gaps in insulation. Any of these issues get corrected before the new window goes in. The new unit is then set, shimmed, flashed, insulated, and sealed from scratch. New interior and exterior trim completes the professional window installation.

When Full-Frame Makes Sense

Full-frame is the right approach when the existing frame shows any signs of failure—rot, water staining, warping, difficulty operating, or visible gaps. It’s also the better choice for homes where original frames have endured decades of weather cycles and may have hidden damage that’s not visible from the surface.

Full frames are easiest to install on siding homes, since the existing siding can be removed, the window installed and properly flashed with a nail fin, and then the siding re-installed up to the new window. Full frames can also be installed in stucco, masonry, or brick openings, but this does require removal of the masonry, which can dramatically increase cost and time to install.

The Tradeoffs

Full-frame replacement costs more than an insert because it involves more labor, more materials, and more time. It’s also more disruptive—interior trim needs to be removed and refinished or replaced, and the work takes longer per window. But the result is a completely new window system with no hidden issues behind it, full-sized glass area, and proper weatherproofing that starts from scratch.

Insert vs. Full-Frame: Side-by-Side Comparison

	Insert Replacement	Full-Frame Replacement
What’s removed	Old sashes and hardware only	Entire window assembly: sashes, frame, trim, and casing
Frame condition	Existing frame stays in place—must be structurally sound	Existing frame is removed; rough opening is inspected and repaired as needed
Flashing & insulation	Not addressed—original flashing and insulation stay behind the old frame	Replaced from scratch with new flashing, insulation, and weatherproofing
Glass area	Slightly reduced—new unit sits inside old frame, losing ½” to 1” per side	Full original opening is used—maximum glass area
Hidden damage	Not visible or addressed—rot, water damage, and gaps remain behind the existing frame	Exposed and corrected before the new window goes in
Installation time	Faster—typically a few hours per window	Longer—additional time for frame removal, inspection, repair, and new trim
Disruption	Minimal—interior trim usually stays in place	More involved—interior and exterior trim are removed and replaced
Cost	Lower upfront cost per window	Higher upfront cost, but addresses underlying issues that inserts can leave untouched
Best for	Newer homes with sound frames; quick upgrades when frame condition is confirmed good; masonry exterior homes	Older homes, homes with frame damage, siding exterior homes

This table is a general guide. The right answer for your home depends on the specific condition of your frames—which is why an honest on-site assessment matters more than any rule of thumb.

Why So Many Window Companies Default to Inserts

If full-frame replacement is the better option for many homes, why do so many window companies lead with inserts? The answer is straightforward: inserts are faster to sell and faster to install.

Many companies have built their entire business model around insert replacements. The in-home sales process is designed to close on the first visit, and the installation is designed to be completed as quickly as possible. Inserts make that possible—there’s no need to open up the wall, inspect framing, or deal with the unknowns that come with full-frame work.

That’s not to say inserts are always the wrong choice. When the frame is genuinely in good shape, an insert is a perfectly valid approach. The problem is when a company only offers inserts—or only recommends inserts—because that’s what their business model is built around, regardless of what the homeowner’s frames actually need.

A company that does both approaches and prices both options for every project has no incentive to push you toward one or the other. They can assess your frames honestly and let the condition of your home drive the recommendation.

What This Means for St. Louis Homes

St. Louis’s climate is particularly relevant to this decision. The metro area experiences temperature swings from well below zero in winter to triple digits in summer, combined with significant humidity and rainfall. That cycle takes a real toll on window frames over time.

Homes in neighborhoods like Kirkwood, Webster Groves, Clayton, and Ladue—many built in the early-to-mid 20th century—have original wood frames that have been through 50 to 100 years of this weather. Even when the surface looks intact, the wood behind the trim may be soft, the flashing may have failed, and insulation may have settled or deteriorated.

Newer communities present different challenges. In areas like Ballwin, where rapid development in the 1990s and 2000s led to widespread use of budget-grade vinyl windows, the frames may technically be younger but were never built for long-term durability. These homes often need full-frame work sooner than their age would suggest.

In either case, a proper on-site assessment of your existing frames is the only reliable way to know whether an insert will hold up or whether full-frame is the smarter investment.

Questions to Ask Any Window Company

Whether you work with Forshaw or someone else, here are the questions that will help you figure out whether you’re getting an honest recommendation:

Do you offer both insert and full-frame replacement?

If the answer is no—if they only do inserts—their recommendation will always be inserts, regardless of what your home needs. A company that handles both approaches can assess your frames and recommend based on condition rather than business model.

Will you price both options for me?

A company willing to quote both approaches is a company that’s confident enough in its recommendation to let you compare. If they only quote one option and pressure you to decide quickly, that’s a signal.

What happens if there’s hidden damage?

Ask how they handle surprises. With inserts, the answer is usually “we don’t open the wall, so we won’t know.” With full-frame, the answer should be: “We’ll find it, show it to you, and fix it before the new window goes in.”

How Forshaw Handles It

Forshaw does both insert and full-frame replacement from Marvin Windows. We do equal numbers of both, so we don’t push a particular approach or product. We do what the house needs.

We price both options for every project where both approaches are viable. That means you see the cost difference, understand what each approach includes, and make the decision yourself based on facts rather than a sales pitch.

During our free on-site consultation, we assess the condition of your existing frames and give you a straightforward recommendation. If an insert makes sense, we’ll tell you. If your frames need full-frame work, we’ll explain why and show you what we’re seeing.

Not sure which approach your home needs? Call (314) 993-5570 to schedule a free consultation.

Frequently Asked Questions: Insert vs. Full-Frame Replacement

Q: What is the difference between insert and full-frame window replacement? An insert replacement removes the old sashes and places a new window unit inside the existing frame. The original frame, trim, and casing stay in place. Full-frame replacement removes the entire window assembly down to the rough opening, allowing for inspection and repair of any hidden damage, new flashing and insulation, and a completely new window system. Inserts are faster and less expensive; full-frame addresses underlying issues and delivers better long-term performance.

Q: Is full-frame replacement worth the extra cost? In most cases, yes—particularly for St. Louis homes built before the mid-2000s. Full-frame work exposes and corrects problems that inserts leave hidden: rot, failed flashing, deteriorated insulation, and water damage. Paying more upfront for full-frame typically prevents more expensive repairs down the road and results in better energy performance and a longer-lasting installation.

Q: How do I know if my frames are in good enough condition for an insert? The only reliable way is an on-site assessment. However, warning signs that suggest your frames may need full-frame work include: soft spots in wooden frames, peeling paint that keeps returning, visible water staining, difficulty opening or closing windows, and drafts even when windows are fully closed and locked. A company that offers both approaches can assess your frames honestly and recommend based on what they find.

Q: Do inserts reduce the size of my window opening?Yes, slightly. Because the new unit sits inside the existing frame, you lose approximately a half-inch to one inch of visible glass on each side. With full-frame replacement, the entire original opening is used, so you maintain the maximum glass area.

Q: Can I do inserts on some windows and full-frame on others? Yes. In fact, this is a practical approach for many homes. Windows where the frame is in good condition—often on protected sides of the house or in newer additions—may be fine for inserts, while windows with more weather exposure or visible frame damage may need full-frame work. A company that offers both can tailor the approach room by room.

Q: Does Forshaw offer both insert and full-frame replacement? Yes. Forshaw handles both approaches and prices both options for every project where both are viable. Full-frame work is the majority of what we do because it’s what most St. Louis homes need, but we won’t recommend full-frame if your frames are sound and an insert would serve you just as well. We assess your frames during a free on-site consultation and give you a straightforward recommendation.

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