What Is a Windowsill?

Written by This Old House Reviews Team Updated 08/16/2024

Windowsills add usable space to rooms, protect your home against water, and can even make your home more energy efficient. Our guide below will help you learn about the different types of windowsills, offer various installation tips, and mention how to properly maintain a windowsill.

Importance of Windowsills

Windowsills are more than just ornamental accents or home decor pieces. Instead, they’re important pieces of your window’s structure for a few reasons.

Structural Integrity

The windowsill sits at the base of a window and helps to evenly distribute the window frame’s weight. This reduces stress on the window structure and prevents it from sagging or shifting as the foundation settles.

Water Protection

Windowsills help to divert rainwater away from the interior of a building. This prevents water from entering a structure and also protects against mold growth. Without the sill, your window structure could suffer wood rot or another form of water damage.

Energy Efficiency

Windowsills create an additional layer of insulation between the interior and exterior parts of your home. This reduces heat transfer and air leakage, leading to lower energy bills.

Aesthetic Appeal

Interior windowsills and exterior windowsills can enhance the visual appeal of your property along with window trim, baseboards, and other design elements. You can make decorative statements or complement your home’s design styles with a wide variety of windowsills.

Functionality

Homeowners can use windowsills as surfaces on which to grow plants, place family photos, or even create a nice sleeping area for their cats.

Types of Windowsills

You can choose from several types of windowsills that cater to different decorative preferences and budgets. Some of the most common options are found below.

Concrete

Concrete windowsills are durable, resistant to moisture and damage, and especially useful in areas with harsh weather conditions. Concrete windowsills can be cast to suit many different architectural styles.

Concrete windowsills are heavy, though, and you’ll probably need to hire a professional for installation.

Fiber Cement

Fiber cement combines the durability of cement with the flexibility of fibers. These windowsills are sturdy, weather-resistant, and impervious to rot, insect damage, and fire.

The downsides of fiber cement are its installation difficulty and higher-than-average price tag.

PVC

Polyvinyl chloride (PVC) windowsills are lightweight, affordable, and highly resistant to moisture and insect damage. They’re also available in many different styles and textures but don’t require much maintenance.

The downsides of this versatile vinyl material are that it’s not as attractive as other types of windowsills and lasts a shorter amount of time.

Stone

Natural stone such as marble, granite, or limestone looks elegant as a windowsill and lasts for a long time. It’s also highly resistant to moisture, scratches, and stains.

Stone windowsills are more expensive than other types of windowsills at first, however, and are frequently too heavy to install as part of a do-it-yourself (DIY) project.

Tile

Tile windowsills are aesthetically appealing, but cleaning the grout between tiles can be a laborious chore.

Wood

Wood windowsills have a classic appeal and matching them to other elements inside your home is easy. Popular options include red oak, mahogany, maple, poplar, and chestnut.

The downsides of wood sills are that they’re susceptible to damage, rot, and insects and that they require a lot of maintenance. Upscale wood windowsills have attractive wood grain patterns but may need professional millwork and specific woodworking, increasing their price.

Installing a Windowsill

Many homeowners hire professional contractors to install windowsills, but you can pick up the supplies below if you want to put them in yourself.

Note that adding a windowsill could be far more complicated if there’s a window stool or a window ledge to deal with. Similarly, the process will be trickier if you’ve got decorative molding, window jambs, or intersections with door casing or stair nosing.

Steps to follow

Step 1: Measure and Remove

Measure the size of the window opening to determine the length of the sill and mark the cut lines on the seal materials accordingly. Use a pry bar and putty knife to remove the existing windowsill.

Step 2: Cut the Windowsill

Use a saw to cut the windowsill material according to your measurements. Be especially precise to ensure a perfect fit.

Step 3: Fit the Window

Place the windowsill material you cut into the window opening to make sure it fits well. You may need to make some adjustments to the shape.

Once the material fits perfectly, apply a construction adhesive along the bottom of the window opening and slide the windowsill inside. Press the sill against the adhesive firmly and use a level to make sure that the windowsill is straight across.

Step 4: Secure the Windowsill

Nail or screw the windowsill to the framing, making sure that the fastening tools go through the sill and the frame. Fill any gaps between the sill and the frame with a caulking gun. Allow the adhesive and caulk to dry before using the window.

Maintaining Your Windowsill

Maintenance is key to keeping your windowsills in top shape for decades. The best ways to maintain different windowsill types are included below.

PVC: Clean regularly with a mild detergent and inspect for signs of cracking and warping.
Stone and concrete: Clean regularly with a gentle stone cleaner and a soft brush. Seal the sills once a year so they remain resistant to water and stains.
Wood: Regularly inspect wooden windowsills for wear and tear, moisture damage, and insect infestation. Clean sills at least once a month with a mild detergent and a soft cloth. Apply a fresh coat of paint every five to seven years.

The more diligently you maintain your windowsills, the longer they’ll function for. Lack of proper care could lead to water damage, rot, mold growth, or even structural instability.

Common Windowsill Issues

Most homeowners deal with windowsill issues at some point or another. Below, read about the most common issues you might encounter and how to address them.

Cracks: Repair minor cracks and chips in the windowsill as soon as possible. If damage is substantial, you may have to replace the sill.
Moisture damage: Inspect the sills regularly and repair leaks promptly to stop moisture damage to the windowsill.
Pest infestation: To prevent pest infestation around a windowsill, clear the area of food and debris and seal all entry points around the windows.

You may need to replace the windowsill if you notice extensive rot, structural damage, serious pest infestation, or severe water damage. Consider replacing the windowsill completely if it’s suffered considerable cosmetic damage.

Our Conclusion

Windowsills protect your home from water damage, improve its structural integrity, and expand your house’s functional space. The key to reaping all of these benefits is proper installation and maintenance. Be sure to diligently clean and maintain your windowsills to keep them in good condition and don’t be afraid to replace them if they’re beyond repair.

FAQ About Windowsills

Is it a windowsill or seal?

A windowsill is a part of the window that protrudes from the bottom part of the window frame. A window seal is a material that closes the gaps between the frame, walls, and windows.

What is the trim under a windowsill called?

The trim under a windowsill is called an apron and provides extra insulation for your window.

What is the difference between a windowsill and a window board?

A windowsill and a window board are two names for the same window element. The term window board occasionally refers to the exterior windowsill.

How do I maintain my windowsill?

To maintain your windowsill, regularly inspect it for damages, clean it with a mild detergent, and repair cracks immediately.

Is replacing a windowsill a DIY project?

Replacing a windowsill can sometimes be a DIY home improvement project, though materials such as stone and concrete will likely require professional installation.

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