Installing a waterproof window can protect your home from moisture damage. Proper installation keeps water from seeping into your walls, which could cause rotting and structural issues. This Old House general contractor, Tom Silva, demonstrates the steps to create a watertight seal around a new window installation. The process may change slightly depending on the window type. The installation process for Egress Windows is different than standard windows, but the principles of waterproofing still apply.

Understanding Window Waterproofing

Waterproofing a window involves creating many layers of protection against moisture infiltration. This process helps to maintain your home’s structural integrity and prevents costly water-related damage.

Why Waterproofing Matters

Water intrusion around windows can lead to many problems, including wood rot in framing and sheathing, mold and mildew growth, damage to interior finishes, and structural weakness over time. By properly waterproofing your window installation, you create a barrier against these issues for the longevity of both your window and surrounding structure.

Key Components of Window Waterproofing

Effective window waterproofing relies on several elements, including builder’s felt, waterproof membrane, proper flashing techniques, and precise installation. Builder’s felt is a traditional water-resistant barrier that goes along the bottom of your window. The waterproof membrane is a self-adhered flashing that seals around your window.

Preparing for Waterproof Window Installation

Before starting with your waterproof installation, you need to get the necessary materials together and prepare your window opening.

Tools and Materials Needed

To complete this project, you’ll need:

• 15-pound builder’s felt
• Hammer
• Hammer stapler
• Level
• Nails
• Self-adhering waterproof membrane
• Shims
• Tape measure
• Utility knife

Inspecting the Window Opening

Before installing the waterproofing materials:

1. Make sure the rough opening is square and level.
2. Check for any damage or rot in the framing.
3. Clean the area of debris and dust.
4. Check that the opening is slightly larger than the window unit.

Step-By-Step Waterproofing Process

We’ve assembled all the steps to create a watertight seal around your new window.

Applying the Initial Barrier

1. Staple 15-pound builder’s felt along the bottom of the window opening, extending it past the sides.
2. Apply a strip of self-adhering waterproof membrane over the felt, making sure it overlaps the sides of the opening.
3. Cut the corners of the membrane diagonally and fold the excess material into the opening.

Sealing the Corners

To prevent water infiltration at the vulnerable corner areas:

1. Cut small pieces of waterproof membrane to fit the corners.
2. Apply these pieces to cover any gaps in the corners of the opening.
3. Press firmly for a good seal.

Flashing the Sides

1. Cut two strips of waterproof membrane long enough to extend past the header and overlap the bottom flashing.
2. Apply these strips to each side of the opening, starting at the bottom and working up.
3. Make sure the side flashing overlaps the bottom flashing for proper water drainage and a seamless barrier.

Setting the Window

With the waterproofing in place, it’s time to install the window:

1. Carefully set the window into the opening from the outside.
2. From inside, use shims to level and square the window in the opening.
3. Check that the window is centered and plumb using a level.

Securing and Final Flashing

1. Once the window is positioned, nail through the nailing flange to secure it.
2. Apply a final strip of waterproof membrane across the top of the window, overlapping the side flashing.
3. Check that the top piece extends beyond the sides for water-shedding and a tight seal.

Ensuring Proper Waterproofing Installation

After the waterproofing and window installation is complete, you must ensure everything is correctly set.

Checking for Level and Plumb

Use a level to check the bottom and top sills for level and the vertical sides for plumb. Adjust any shims as necessary to achieve a perfect alignment.

Measuring for Square

To check that the window is square, measure the diagonals of the window frame from corner to corner. Both measurements should be equal, and if they’re not, adjust the window until they match.

Testing for Proper Operation

Before finalizing the installation, open and close the window to check for smooth operation. Make sure that the sashes are aligned and the locking mechanisms are engaged correctly.

Additional Waterproofing Considerations

While the basic waterproofing process is simple, there are more steps you can take to protect against water infiltration.

Caulking and Sealants

Apply a high-quality exterior caulk around the exterior trim where it meets the siding, at the joints of the window frame, and along the bottom of the window sill. Choose a caulk that is compatible with your siding material and can withstand outdoor conditions.

Exterior Trim and Flashing

Consider adding drip cap finishing above the window to direct water away or z-flashing at the bottom of the window for additional protection. You can also install exterior trim to cover gaps between the window and siding.

Interior Insulation

To complete the weatherproofing process, use low-expansion foam insulation around the window frame and install interior trim to cover the gap between the window and the wall. Also, consider adding weatherstripping for better energy efficiency.

Maintenance and Upkeep of Your Waterproofed Window

Regular maintenance helps your waterproofed window stay in good condition and continue to perform effectively.

Regular Inspections

Visual inspections should be conducted regularly to check for signs of water staining around the window. You should also look for gaps in caulking or sealants and examine the exterior flashing for damage or separation.

Cleaning and Resealing

Periodically clean your window tracks and weep holes and touch up paint on the exterior trim to maintain its outdoor protective qualities. Also, reapply caulk or sealant as needed when you notice any deterioration during regular inspections.

Addressing Issues Promptly

If you notice any signs of water infiltration, find the sources immediately and make necessary repairs to prevent further damage. If the issue persists, consider consulting with and hiring a professional.

Enhanced Waterproofing Practices

For the longevity and effectiveness of your waterproof window installation, consider integrating these practices into your process.

Advanced Flashing Techniques

Advanced flashing techniques involve layering and specific placement of waterproofing materials. By overlapping layers, you create a more robust and durable water barrier. This reduces the risk of water infiltration and promotes proper drainage around the window.

Using High-Performance Sealants

High-performance sealants have great adhesion and flexibility compared to standard caulks. These sealants expand and contract with temperature changes, providing a longer-lasting seal around your window. Select sealants designed for exterior use that are compatible with your building materials.

Incorporating a Moisture Barrier

Adding a moisture barrier behind the exterior siding can further protect your home’s structure from water damage. This barrier acts as an additional line of defense, redirecting any water that hits the siding away from your window opening.

Benefits of Professional Installation

While DIY window installation is possible and more cost-effective, professional installation has many benefits, including warranties that cover materials and a quicker, more accurate installation. They can also help with bells and whistles such as installing window shades.

Installing a waterproof window protects your home from moisture damage. You can create a watertight seal by following the steps and maintaining your windows regularly. If you’re unsure about any of the steps in our guide, and don’t think you can follow proper window installation technique, it’s best to consult and hire a professional to take care of the installation.