New windows are almost always a great investment, but it’s easy to get overwhelmed by the options available on today’s market. Balancing curb appeal, energy efficiency, and your budget is no small feat!

In this article, you’ll learn how a window works as well as the primary differences between full-frame and insert replacement windows. Ultimately, a quality installation is every bit as important as the product you select, so be sure to hire a reputable, experienced window replacement contractor to help guide you through the process of updating your home with brand-new windows.

There May Be A Few Solutions to Your Replacement Window Needs

Your replacement window project will depend on several factors. As you get further along in your window buying journey, you’ll find that you’ve got lots of decisions to make. Throughout the process, you’ll decide on window brands, styles, sizes, materials, functions, hardware, finishes, and more.

There are many elements in the planning process, but working with the right team can make it feel easy—even fun!

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A good place to start is to figure out if you need a full-frame replacement or if you can get by with an insert replacement window. (Hint: Our company only does insert replacements for wood-framed windows.)

Let’s start by looking at the construction of windows, and then we can jump into the difference between full-frame and insert replacement windows.

What Are the Main Parts of An Operable Window?

An operable window is used for ventilation. It consists of moving parts (sashes) that slide vertically or horizontally so that the window can be opened and air can easily flow in and out.

To keep things simple, an operable window is made up of a few primary elements.

Main Parts Of A Window: Mainframe, Sash, Jamb Liner. Photo of a wood clad double hung window with mainframe, sash, and jamb liner called out.
Photo credit: Brennan Enterprises


The outermost part of a window unit is the mainframe. It holds the main parts of the window together within the opening in the wall of a structure. The mainframe is sometimes called the window reveal or jamb. Each word essentially describes the same area.

Jamb liner

Moving inward, the next part of the window is the jamb liner. Modern operable windows don’t use pulleys or weights to support the sash. Today, we place jamb liners on two opposite sides of the mainframe.

On vertically and horizontally sliding windows, the jamb liners act as runners or tracks for the sashes to slide on. These hide the jamb (also known as the window reveal). The term ‘jamb’ describes the inner perimeter of the mainframe.


The sashes are the parts of the window that hold the glass. Depending on the style of the window, you may have one or two (or more) sashes. In a standard double-hung window, you’ll have two operable sashes, one on the top half of the opening and another on the bottom half.

What Are Full-Frame Replacement Windows?

The installation of a full-frame replacement unit requires complete removal of the existing window down to the studs of the wall.

That means taking out every element of the window, including the mainframe and existing trim (and sometimes siding). A full-frame replacement window looks similar to a new construction window and provides lots of benefits as well as opportunities to change the look and feel of your home.

Illustration of Full Replacement Window composition: rough opening, existing studs, new drip pan, new window unit, new exterior window facing.
Photo credit: Brennan Enterprises

When are full-frame replacement windows the best option?

  • Rot is present in the window frame, trim, casing, or sill
  • You want to change the size or shape of your window
  • Your existing window is made from vinyl, fiberglass, or aluminum

What are the benefits of full-frame replacement?

  • If rot is present on any part of the window frame or casing, it is removed to avoid spreading
  • Professionals can inspect the rough opening for signs of water damage
  • Perfect sash-to-frame fit, which increases energy efficiency
  • Provides an opportunity to add insulation if needed around the perimeter of the window opening
  • Unless you change the style of your window, you shouldn’t lose any glass surface
  • If you choose to replace your windows with a cladded wood window, the exterior becomes almost completely maintenance-free

What are the disadvantages of full-frame replacement windows?

  • More expensive than insert windows
  • The removal and installation process is more complex

What is the removal process for the existing window?

  • Remove the lower sash
  • Remove the upper sash
  • Remove trim and casing
  • Remove sill and mainframe

This is a big project and is definitely one that should be left to experienced window installers. With a good installation, the investment in your windows should yield a major return. Proper installation results in a good-looking product and improves your energy spending.

A full-frame replacement window should provide excellent energy savings because each window is built as a single unit. Each part of the window is constructed at the same time to work together, which means the sashes will fit perfectly within the mainframe (thus minimizing air leakage that can drive up heating and cooling costs).

Additionally, if you’re working with a company that does custom sizing, the mainframe of the window should fit so that there is about a quarter inch of room around the perimeter. That little bit of space allows installers to properly insulate and shim the window within the existing rough opening. Windows that precisely fit the rough opening minimize opportunities for air leaks around the window unit.

The process for removing and installing full-frame windows is more extensive than that of insert windows, so you can minimize your costs by keeping your replacement simple and installing windows of the same size as the original openings.

Of course, if you want to transform your space, opting for full-frame replacement gives you the flexibility to choose different window styles and sizes, or even the option to increase the opening to fit a door.

What Are Insert Windows?

The installation process for insert replacement windows requires pocketing a new window unit within an existing frame. Insert windows are sometimes called pocket windows.

Before installing the window, the existing sashes and jamb liners are removed to make room for the new window unit. Essentially, a new window unit (sashes held within a frame) will be pocketed into the existing frame of the opening. This means that the overall glass surface of your new window unit will be smaller than the original.

Illustration of Insert Replacement Window composition: existing window frame, exterior trim, new window unit.
Photo credit: Brennan Enterprises

When are insert replacement windows the best option?

  • No rot is present in the window frame
  • The window frame is still square and level
  • You’re interested in a replacement window of the same size, shape, and operating style

What are the benefits of insert replacement windows?

  • Less expensive than full-frame replacement windows
  • Installation is typically quick and easy
  • You can preserve the original frame as well as exterior and interior trim (also referred to as casing)

What are the disadvantages of insert replacement windows?

  • Only an option on wood window frames
  • Reduction in glass surface
  • Leaving the existing wood casings means they’ll eventually have to be replaced anyway

What is the removal process for the existing window?

  • Remove the lower sash
  • Remove the upper sash
  • Remove the jamb liners

Whether or not you have insert replacement windows as an option depends on the material and condition of the existing window frame. For homeowners who want to keep their wood windows, insert replacement windows are a good option because they’re typically less expensive than full-frame wood replacement windows. Typically, we see insert windows replaced with solid wood windows or cladded wood windows.

Inserts are also a good option for homeowners with smaller projects and tighter budgets. You’ll still want to make sure you’re working with an experienced company so that your new window unit fits properly within the existing opening.

The better the installation, the better your window unit will perform in regard to energy efficiency. As we mentioned for full-replacement windows, you want to make sure you minimize the air leaks around your window unit.

Now That You Know the Difference Between Insert Replacements and Full-Frame Replacements, What’s Next?

As you decide which type of replacement window you’ll want for your home, also consider the scope of your project.

Here are a few things you’ll want to determine:

  • Are you happy with the current operating style of the windows you’re replacing?
  • Do you need to replace a couple of windows or all of the windows on your house?
  • What is the material of the existing window frame?
  • If the window frame is made of wood, is it in structurally sound condition?

If you’re considering full-frame replacement, ask yourself:

  • Are you comfortable paying more for a full-frame window and installation?
  • Do you want to replace your windows with the same material, or are there better options available for your home’s climate zone?
  • If your budget is tight and your wood window frames are in good condition, would you want to save some money by choosing insert windows with less glass surface?

If you’re thinking about insert replacement windows, ask yourself:

  • Are you comfortable losing some glass surface in order to spend less money?
  • Do you want to replace all the windows along a wall or just one? Remember, a newly framed window unit will be placed within the existing frame; this could look odd.
  • Is there any possibility for water damage behind the existing window frame? Existing mold or rot can spread to the new unit and lead to bigger problems and another replacement in the future.

Being informed about your window options will help you make a better purchase for your home. With all of this information, you’ll be able to narrow down your options and feel confident as you interact with your window retailer.

Ariana Martinez is a Product Advocate on the marketing team at Brennan Enterprises. Brennan Enterprises is a family-owned-and-operated home exterior remodeling company. Since 1979, Brennan Enterprises has helped homeowners across North Texas improve the curb appeal and energy efficiency of their homes. Brennan Enterprises specializes in replacement windows, front doors, patio doors, outdoor living, replacement roofing, and replacement siding. You can learn more at