How to Install an Insert Replacement Window in 12 Steps

Large windows in a country home

The Spruce / Sophia Reay

Project Overview
  • Working Time: 2 - 4 hrs
  • Total Time: 2 - 4 hrs
  • Yield: 1 window replacement
  • Skill Level: Intermediate
  • Estimated Cost: $250 to $600

Few home improvement projects make a bigger difference than replacing old, drafty windows. The single-pane, double-hung windows that were once standard in older homes are no longer practical in times of high energy costs, and new windows with double or triple panes may pay for themselves in just a few years through lower heating costs.

Window replacement is often done as a major home improvement project hired out to a professional contractor. Such projects can cost many thousands of dollars and are often financed through a home improvement loan. But by using insert replacement windows, a DIYer can do this work themselves at a fraction of the cost. Replacing three or four windows a year using inserts can allow you to update your entire house within a few years without emptying your bank account.

Insert replacements windows are designed with DIY installation in mind, and with a little preparation and practice, you will find that it can be done in just a couple of hours.

New Construction Windows vs. Insert Replacement Windows

Residential window units come in two main types: new construction windows, and insert replacement windows.

New construction windows have nailing fins attached to the window perimeter that allows the window to be nailed flat against the outside of the house. New-construction windows are best suited for new homes or for extensive remodels that involve radical exterior changes, but some contractors will choose them even for window replacement projects. When used in a replacement project, new construction windows will require you to remove the outer case moldings and portions of the siding in order to attach the nailing fins to the sheathing. This is normally a job for professionals. New construction windows are also the logical choice where a window opening is being enlarged. Home centers and builder's supply houses generally stock a good inventory of new construction windows.

Insert replacement windows have no nailing fins and are meant to fit into existing window frames once the old sashes are removed. Using insert replacement windows minimizes the construction work since it doesn't require the removal of the outer case moldings or siding. Insert replacements are a good choice for DIYers since they require only basic carpentry skills and are relatively inexpensive. Replacement windows are meant for retrofits only—you can't install them in a newly framed opening.

But be aware that insert windows do slightly reduce the overall glass exposure of the window since they add an additional frame component. The overall glass exposure of a window shrinks by at least 2 to 3 inches in both directions when you install an insert window. The ease of installation and affordable cost, however, make insert windows a good choice for DIYers. It's not uncommon to pay as much as $1,500 per window for a professional replacement, while a DIYer can often install an insert window themselves for as little as $250.

Insert replacement windows may be stocked in limited supply at home improvement centers and building supply centers, but it's likely you will need to take measurements and have the windows custom-built for your window openings. You can expect this to take several weeks.

Professionals sometimes scoff at insert replacement windows, arguing that they are less durable and weatherproof than new construction windows. Many homeowners, though, find that carefully installed insert windows perform quite well for many, many years.

Before You Begin

Insert replacement windows are best for replacing standard double- or single-hung windows that open and close vertically. They are not a good choice if you are replacing sliders or casement windows that pivot open horizontally. For those types, it may be best to hire professionals to install new construction windows.

You will have several choices for frame materials and glazing. Frames can be made of solid wood, aluminum, wood-clad with aluminum or vinyl, or from a pure composite material with no wood at all. Glass glazing options now include a full range of options, ranging from single-pane glass suitable for mild climates to triple-pane insulated glass with low-E coatings and inert gas filling the gaps between panes.

Do your research before buying insert windows. There are many manufacturers, and the quality of the warranty is often a good indication of the durability of the window. Also, study the installation instructions; easy installation is an important quality for DIYers.

Proper sizing is critical when you are buying replacement windows. Insert replacement windows are designed to fit snugly with only the slightest of gaps to allow tight insertion within the existing window frame. Measure your window frame opening exactly as the manufacturer instructs, down to 1/16-inch increments. Ideally, the insert window unit should be only about 1/8 inch shorter and narrower than the window opening. In an older home, however, this can be tricky since window frames may be slightly out of square. Consult with the window specialist at your home center to ensure you're buying the right size.

When buying vinyl or vinyl-clad insert windows, you should be able to order the windows to match the color of your window trim. If the right color isn't available, make sure you are ordering windows that can be painted. Vinyl windows may not be paintable.

Safety Considerations

When you are replacing an old double-hung window, the process usually involves removing heavy pocket weights that counterbalance the window sashes. These weights are often made of lead, which needs to be disposed of in an approved manner. Never throw lead weights into household trash. Most communities now have procedures for disposing of hazardous materials such as lead. It may involve delivering the lead weights to a drop-off center equipped to handle them.

When installing insert replacement windows, take care to properly caulk the windows with a good-quality silicone caulk. Moisture seepage is the main cause of failure in replacement windows.

What You'll Need

Equipment / Tools

  • Flat pry bars
  • Screwdriver
  • Utility knife
  • Screwgun
  • Torpedo level
  • Framing square
  • Hammer
  • Caulk gun
  • Miter saw (if needed)
  • Brad nailer
  • Paintbrush (if needed)


  • Insert replacement window
  • Latex silicone caulk
  • Shims
  • Stop moldings (if needed)
  • Trim paint (if needed)


You should expect a learning curve when installing insert replacement windows. You may spend several hours on the first window or two, but after that, each window may take just an hour or so. You'll need dry, warm weather, and it always helps to start early in the day.

Each manufacturer will have specific installation methods, but the process outlined here is typical. Make sure to research the manufacturer's instructions for measuring the window opening and sizing the new window.

  1. Measure and Order the Window

    Measure from the inside of one window jamb to the inside of the opposite jamb at the bottom, middle, and top, both horizontally and vertically. Choose the smallest measurement when ordering the window. The manufacturer's instructions will explain how the new window should be sized in relation to your window opening. The window unit is usually just slightly undersized to allow for insertion into the opening.

    If you are uncertain about sizing, consult the window specialist at your home improvement center.

  2. Remove the Window Stops

    Use a pry bar or putty knife and hammer to carefully pry away the window stop molding from the sides and top of the window frame. (Some moldings may be secured with screws). If the molding has been painted in place, slice through the paint with a utility knife before prying it away from the window frame.

    Careful removal may allow you to reuse the stop moldings after the new insert window is installed. But if the stop moldings are old and brittle, you may find it easiest to buy and install new moldings.

  3. Remove the Inner Sash

    Carefully remove the inner window sash from the opening, and remove the knotted weight cords from the holes in the sides of the sash frame. Sometimes, the sash cords are nailed into the sash frames.

  4. Remove the Parting Stops

    Use a small pry bar or screwdriver to remove the narrow parting stop moldings that separate the inner window from the outer window sash. These will not be reused, so it's fine to break them in order to free them from the window frame.

  5. Remove the Outer Sash

    With the parting stops removed, the outer window should now slide out of the window frame. Free it from the sash cords by pulling the knots out of their openings on the sides of the sash frame.

  6. Remove the Counter Weights and Pulleys

    Open the weight pockets on the sides of the window frame and extract the heavy weights. Cut the sash cords with a utility knife, then remove the cords.

    Unscrew the weight pulleys at the tops of the window frame. Discard the weights, cords, and pulleys. (The metal weights and pulleys can usually be taken to a recycling center; do not discard in household trash).

  7. Insulate the Weight Pockets

    Fill the weight pockets with loosely packed fiberglass insulation. These empty cavities can be a substantial source of heat loss, so insulating the pockets will greatly improve the energy efficiency of the windows.

    Replace the covers on the weight pockets and screw them in place.

  8. Test-Fit the New Window

    Fit the new insert window with whatever expansion strip or header that is required, then test fit the window in the frame opening and loosely shim it in place. Use a carpenter's square and torpedo level, as necessary, to adjust the window so it is level and square.

  9. Secure the Window

    Once you are confident the window will fit properly, remove it from the opening, then apply caulk to the window frame as directed by the instructions. Reinsert the window and adjust it once again for level and plumb, using a framing square and torpedo level. Use shims to lightly wedge the window in place.

    Test both sashes for smooth sliding. If the sashes are tight, use the adjustment screws located in the side channels to adjust the operation. If this does not fix the issue, you may need to remove the shims and try thinner shims.

    When the window fits properly and the sashes move freely, drive mounting screws through the side channels and into the window frame to secure it in place. Most insert windows have drilled openings where the screws should be driven. Make sure not to over-tighten these screws, as it can pull the sash frame out of square.

    Trim off the shims so they are flush with the sash frame.

  10. Caulk the Joints

    Slide the header piece at the top of the window up to eliminate gaps, then apply the recommended caulk to the joints on both the outside of the window and the inside. Carefully follow the manufacturer's instruction on this; some windows require leaving a gap at the bottom sill for drainage.


    If you need to paint over the wood trim, make sure to choose a paintable caulk. Most latex caulks are paintable, while pure silicone caulks are not.

  11. Install the Window Stops

    Reattach the window stops around the sides and top of the new insert window, using a brad nailer or screws. (If using new stop moldings, cut them to size using a miter saw, and paint or stain them before installing).

  12. Caulk and Paint

    Complete the installation by caulking the joints around the stop moldings, then doing whatever final painting is needed.

  • Do you install replacement windows from the inside or outside?

    It depends. Installing from the outside can be easier (and cause less disruption indoors), especially if the window opening on the outer facade is larger than the opening inside. However, you might be able to get a better seal when installing from the inside. 

  • Are replacement windows hard to install?

    With intermediate DIY skills, you should be able to tackle installing insert replacement windows. Installation of the first window might take several hours for you to work out the process, but after that the project should go more quickly and smoothly.

  • Can you replace just the window and not the frame?

    Yes, insert replacement windows allow you to keep your old outer case moldings and siding.

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  1. Management of Lead-Based Paint Waste. New Hampshire Department of Environmental Services.