Installing a Window Start to Finish

You’ve finally committed to turning that wall into a window. But how do you even get started with installing a window? What’s the first step? We’ve put together a guide to answer that question, along with any others that might arise while you install your window, start to finish.

A word of caution first—you will be working with glass, tools, and chemicals. Always wear proper protection, including a long-sleeve shirt and pants, eye goggles, a mask, gloves, and ear guards when using power tools.

Step 1 - Figure Out Size and Type of Window

If you’re installing a window from scratch, you’ll have more prep work to do than if you’re simply replacing an existing window.

Before you remove any wall material, calculate the size of your window and get the supplies on hand. Standard sized windows will be easier to find and quicker to get to the job site. If you decide to install a non-standard size window, allow plenty of time for the custom build.

When measuring for your window, measure the opening for the actual window rather than the overall size. This will be the size of the frame opening.

The extra surrounding material on the window is for mounting it to the exterior of the home and is not part of the window’s measurements.

You’ll actually want your window to be ½” smaller than your opening so you have a bit of wiggle room during installation.

If you’re installing a window into a brand new opening in a wall, work with some of the studs that are already in place to make the job easier. If you place the window between two existing vertical studs, you’ll only need to install the horizontal top and bottom supports to complete the framing for the window.

For a home with older, inefficient windows, you’ll be doing a replacement, which requires much less preparation and is a much faster operation. Measure your existing window and buy a new one.

Some newer windows don’t need a complete replacement. Instead, you’ll leave the frame, if it’s in good shape, and replace the insert, which is also known as a pocket window.

Step 2 - Remove Old Windows

Caution—If your windows are older than 1978, they may have lead-based paint and should be removed by a professional equipped to deal with the toxins.

There are many types of windows, and removal can take many forms. Take a look at your system to figure out the best way to dismantle it. If you have storm windows, score the paint with a sharp razor and look for screws along the frame. Remove the screws to remove the sashes.

Other windows will come out as one unit, complete with the frame. To remove this type of window, cut around the entire window opening with a circular, or similar, saw.

Stay close to the window, but you can allow a bit of room to work. The gap will be covered up later in the process, so cutting a few inches away from the window edge is fine.

Use hand and manual saws, depending on your particular situation, until the surrounding material has been removed.

Once you’ve removed the surrounding materials, the window can be removed, complete with the frame. You may need to use a chisel or saw to cut through some of the framing holding the sashes or window frame in place.

Step 3 - Clean up the Frame

If your project does not include new framing, clean up the window opening by scraping away old sealant. Remove nails if needed. Also replace any rotted wood pieces and vacuum up all dust and debris.

Cut out around the frame to a width that equals the trim you plan to use around the window. This is typically around four inches, but have your trim available so you can be precise. Use a straightedge to create a line for the cut. Be as precise as possible, but know you will be filling the surrounding gap with sealant at a later step.

Step 4 - Install Flashing

Leaks are the most common issue with new window installations. Therefore, it’s critical you seal every possible entry point for water. Think of it as putting a rain jacket around your window.

Every layer should overlap from top to bottom so any water that runs down the house runs over the top of the layers, rather than having an opportunity to seep underneath.

The first layer of protection is called flashing. There are many forms of flashing, from preformed corner protectors to rolls of cut-to-size self-stick flashing tape.

Apply the flashing tape along the bottom of the window frame from corner to corner, making sure it covers the entire area. Press the flashing tape into corners and edges for a solid seal. Only apply the flashing tape or corner protectors on the bottom of the window at this time.

Step 5 - Place Shims

Make sure your window will set level inside the window frame. Use shims to lift one or both sides to make it level. Use flashing tape to hold the shims in place.

Then dry fit the window, making sure it sits level. If it doesn’t, install more shims until it sits correctly. Remove the window and set it aside.

Step 6 - Apply Sealant

You’re ready to install your window! Start by using a tube of sealant, applied with a caulk gun. Run a thick bead of sealant all the way around the frame, where the window will press into the window stops. These are the ledges on the inside of the window frame.

Step 7 - Place the Window

Some people ask if windows are installed from indoors or outside. The answer is your window will be installed from the outside. Lift your window into place and press it into the sealant all the way around the window.

This is a good time to ask a friend for help, especially with larger windows. Place shims around the window. Not only do these shims keep the window from moving, but they also provide thickness to screw into when securing the window in place.

Drive screws through the sides of the window frame into the shims. There are commonly predrilled holes in the sides of the frame for this purpose.

Hold the window securely until several nails or screws are in place.

Check for plumb, level, and square and make adjustments as needed. Use additional shims as necessary.

Then operate the window to make sure it opens and closes easily. If everything is in working order, cut the shims back to the surface of the window using a hacksaw, or place a chisel in the proper location and hit it firmly with a hammer to drive it into the shim. You can then peel away and discard the extra sections of shim material.

Step 8 - Secure the Window

While some windows are mounted with screws from the sides as described above, most have the mounting brackets on the outside, around the window.

There are a few options for the hardware you’ll use to install this type of window. Standard frames are made to handle nails. The nails hold the window in place and the nailhead sits flush on the outside of the window mounting bracket.

If you aren’t comfortable hammering so close to your expensive window, or you don’t have a hammer and nails, you can use screws instead.

The most important thing to remember when using screws is they need to have a large, flat head that sits flush with the mounting bracket. Otherwise, you won’t be able to get your trim to sit flush against the surface.

Whether you’re using screws or nails, install the hardware in every other hole all the way around the frame of the window.

Step 9 - Install More Flashing

For windows mounted on the side as described above, apply a spray foam insulation suitable for the project. Fill in the gaps along the sides and top of the window. Follow the directions on the can. Cut away excess insulation foam once it is set.

For windows that mount on the outside of the house, apply flashing tape up both sides. Press it firmly into place from the bottom to the top. Following that, apply flashing tape across the top. Apply them in this order so the top tape overlaps the sides for water protection. Otherwise, water may roll beneath the tape on the sides.

Step 10 - Install Trim

To cover the mounting portion of the exterior of the window, as well as the siding you’ve cut away during installation, you’ll now install trim around the window.

Precisely measure each side of the window and cut each of the two trim pieces exactly to size with 45-degree angles at each corner.

Remember, if you use the measurement of the window, make your cut so the inside of the angle ends at that length. In other words, that measurement is the inside corner of your miter cut.

Next, cut the bottom piece to meet into the sides. When measuring the bottom you can use measurements to the inside corner or the outside corner. Just remember which one you’re cutting when you step up to the miter saw.

Finally, measure and cut the top piece to fit into the space above the window. With all four pieces cut, install them using nails or screws.

There are special kits of window hardware that provide plugs in order to fill the hole created by your hardware. Seek one out for a neatly finished look. Otherwise, cover nail holes with putty. Make sure the putty is paintable.

You could also include a drip edge on the top of the window. This is a product that protects the window by funneling water away so it doesn’t seep underneath the trim.

Step 11 - Caulk

As a final layer of water protection, caulk around the trim.

Make sure your caulk is waterproof and intended for outdoor use. Also check the color. You don’t want to end up with a dark color against a light house. While you will paint over the caulk on the front of your trim, the sides may not get painted until you tackle repainting the house.

Choose a caulk that is paintable or it will repel paint at the next step. Caulk the corners where the trim pieces come together. Also caulk around the top and both sides where the trim meets into the house.

Apply caulk as a bead across each seam. Using a gloved finger, run your finger along the beam, pressing it into the space in an even and consistent way. Then wipe away excess caulk with a wet rag.

To make the job less messy, you can tape off the area you plan to caulk. After placing the caulk, remove the tape and wipe up any excess that remains.

Step 12 - Paint

The final step once your window is installed is to paint the trim. Protect your new window by taping it off before beginning with the paint. Be sure to use exterior paint. Most people choose to paint the face of the trim, but not the sides. If you prefer the sides to be painted, tape off the house for protection.

You could also paint the trim before installation. However, after making cuts and installing the trim with nails or screws, you’ll have touchup to do either way.

Installing windows requires an eye for detail, but is not particularly challenging or technical. The most important steps are to accurately measure for the window before ordering or selecting at the store, and double checking each step of the waterproofing from the flashing tape to the caulking around the trim.

This will ensure a long-lasting installation without problems down the road.

Find more information in our related article Easy Replacement Window Installation. You may also be interested in How to Replace a Broken Window Pane and How to Repair a Stuck, Broken, or Rotted Window.