How to Install a Window for Replacement | Step-by-Step Guide

You've measured your opening, ordered your window, and it's arrived at your door. Now comes the satisfying part: the install. A full-frame window installation is the most complete type of replacement, and when done right, it delivers a weathertight, long-lasting result. This guide walks you through every step so you can do it with confidence.

Before You Start

This guide covers a full-frame (tear-out) brick-to-brick installation. Before you begin, confirm the following:

• Install type: Full frame (tear-out). The existing window and frame are fully removed.
• Opening check: The rough opening must be plumb, level, and square
• Window fit: Your window should be 1" overall smaller than the rough opening (½" per side) to allow for shimming and insulation
• Code compliance: Check local building codes, especially for egress requirements in bedrooms and basements

Before you remove the old window: Measure your new window and compare it to the existing window and compare the sizes. You want to confirm the new window fits the opening before the old one comes out. Discovering a sizing issue after the old window is already removed is a situation you want to avoid entirely. Take 60 seconds to do this check first.

What You'll Need

Gather these tools and materials before you start so the install goes smoothly without interruption:

• Peel-and-stick flashing tape (blue skin or equivalent)
• Low-expansion spray foam
• Window-specific installation screws
• Shims
• Level
• Hammer and utility knife
• Vacuum or brush
• Measuring tape
• Safety glasses and gloves

How to Install Your Window

1. Clean and prep the rough opening

A clean opening is the foundation of a good install. Start by removing all old debris, shims, nails, and screws left from the previous window installation. Vacuum or sweep the sill and corners thoroughly to remove dust and particles. Hammer in any protruding fasteners and remove any loose wood or insulation. Finally, clean the sill and frame contact areas to ensure a completely flat surface. Any high spots here will throw off your level later.

2. Apply flashing tape

Flashing tape creates a weathertight seal around the rough opening before the window goes in. Use blue skin or an equivalent peel-and-stick flashing tape and follow this sequence:

1. Sill first: Apply from end to end, extending slightly past the opening on both sides
2. Side jambs next: Apply to both sides of the rough opening, ensuring the tape overlaps onto the sill tape below
3. Head last: Apply across the top of the rough opening, overlapping onto both pieces of side jamb tape

This sequence (sill, sides, head) ensures each layer sheds water onto the layer below it, just like shingles on a roof.

3. Test fit the window

Before committing to the install, place the window into the cleaned and prepared opening to check for a proper fit. Confirm that the brickmould or window frame clears the opening with enough room for shimming on all sides. Using a level, ensure that all four corners sit flush with the drywall on the interior. Do not fasten anything yet. This step is purely to confirm fit and identify any issues before the window is in place.

4. Level the sill

Place a level across the bottom sill of the window opening. If it's not level, add shims under the sill until it reads perfectly level. This is the most important step. Everything else is built on top of it. A sill that's even slightly off will cause the window frame to fall out of plumb, making the sash difficult to operate

5. Shim and fasten the window

Shim locations should match closely to your fastening locations: 6 to 8 inches from the sill and header, then every 24 inches in between. Use window-specific installation screws, not drywall or wood screws, as these are designed to hold under the expansion and contraction of a window frame.

Start by placing shims at the sill corners and driving your first screws partially. Don't fully tighten yet. Use a level vertically along the left and right jambs. If a jamb is out of plumb, apply shims behind it until the level reads plumb, then drive the screw to lock it in. Work your way around all four sides before fully tightening any fasteners.

Open and close the sash after placing each set of shims to confirm smooth operation and even reveal alignment. If the sash binds or drags, adjust your shim placement and recheck before tightening further.

6. Insulate with low-expansion foam

Apply foam in two passes:

1. Exterior bead first: Apply a thin bead of low-expansion foam at the exterior side of the window frame to stop outside air and moisture from entering
2. Interior bead second: Apply a second bead near the interior side, leaving a small gap between the two beads

Use low-expansion foam only. High-expansion foam can bow the frame and prevent the sash from operating correctly. Once cured, cut off any shim material that extends past the window frame so everything sits flush.

8. Finish and trim

Once the foam has fully cured, cut off any excess shim and foam material flush with the window frame. Apply interior trim (casing) to cover the gap between the window frame and the drywall. On the exterior, ensure the exterior brickmould is caulked along all edges where it meets the brick or siding to create a clean, weathertight finish.

Do a final check. Open and close every sash, test any locks or hardware, and inspect the exterior seal before calling the job complete.

Common Mistakes to Avoid

• Skipping the flashing tape. It takes 10 minutes and prevents years of moisture damage. Never skip it.
• Using high-expansion foam. It will bow your frame and bind the sash. Always use low-expansion foam for windows.
• Not testing the sash as you go. Check the operation after every set of shims. Catching a problem early saves you from backing everything out later.
• Overtightening screws before levelling. Get the window plumb, level, and square before fully tightening any fastener.
• Forgetting code compliance. Bedroom and basement windows often have egress requirements. Check local codes before starting.

Frequently Asked Questions

What is a full-frame (tear-out) installation?

A full-frame installation involves removing the existing window and its entire frame down to the rough opening, then installing a new window from scratch. This gives you the cleanest result and is the right approach when the existing frame is damaged, rotted, or when you're changing the window size.

What type of foam should I use?

Always use low-expansion window and door foam. Standard expanding foam generates too much pressure and can bow the window frame inward, causing the sash to bind. We recommend Hilti foam; it's a professional-grade low-expansion foam specifically designed for window and door installations, widely available at most building supply stores. Look for foam labelled for windows and doors, and you'll be in good shape.

How do I know if my window is plumb and level?

Use a 12-24" level on the sill (for level) and along each jamb (for plumb). The bubble should sit perfectly centred between the lines. Check all four sides. A window can be level on the sill but still have a jamb that's out of plumb if the rough opening isn't square.

Haven't measured yet?

Before you install, make sure your measurements are right. Read our window measuring guide first.