Window trim not only frames a window, but it also plays a practical role. It covers gaps between the window and wall, protects vulnerable edges, and helps define the overall look of a room or exterior.
In this guide, you’ll learn how window trim works, what each component does, and how to choose the right style and materials for your home. We’ll also walk through key installation steps and common mistakes to avoid, along with practical tips and advice from This Old House general contractor Tom Silva.
Types of Window Trim and Costs
Window trim costs vary based on material, profile complexity, and whether the trim is installed indoors or exposed to the elements outside. Simpler profiles and paint-grade materials cost less upfront, while detailed millwork and stain-grade wood increase both material and labor costs.
Interior Window Trim
Interior trim is primarily decorative, but material choice still affects durability and finish quality. Paint-grade options like MDF (medium-density fiberboard) offer a smooth, consistent surface, while hardwoods are better suited for staining.
Common materials for interior trim are shown below, with average costs per linear foot:
- Hardwood: $0.75–$5.50
- Reclaimed wood: $1.25–$12
- Medium-density fiberboard (MDF): $0.60–$7
- Polyurethane: $2–$6
*Note: The cost data featured in this guide is based on estimates from Angi, unless otherwise noted.
Material selection also impacts appearance. Stain-grade woods such as oak or clear pine highlight natural grain, while MDF and polyurethane are designed for paint and won’t achieve the same finish.
Exterior Window Trim
Exterior trim must handle moisture, temperature swings, and UV exposure. That makes durability just as important as appearance.
Popular materials for exterior trim are listed below, with average prices per foot:
- Vinyl: $0.80–$30
- Fiber cement: $0.70–$5.50
- Cellular PVC: $0.50–$5
- Wood: $1–$10
Pro Tip: As This Old House general contractor Tom Silva explains, cellular PVC is a reliable choice for exterior trim because it won’t rot, stands up well to the elements, and holds paint over time—making it especially useful for long-lasting exterior details.
Window Trim Components
What’s referred to as “window trim” consists of several individual parts that work together to close gaps, protect edges, and create a finished look. While each piece has a specific role, some contribute more to the structure while others are primarily decorative. Understanding how they function together helps when choosing a style or planning an installation.
The Crown
The crown assembly adds visual weight to the top of the window and is most often used in traditional interiors where a more detailed, built-up look is desired.
a) Crown molding: Angles out from the wall at the top of the window, adding depth and shadow lines that flat trim lacks. Often used to elevate a simpler casing into a more architectural feature.
b) Mitered return: Finishes the exposed end grain where the molding meets the wall, preventing a raw edge from showing.
c) Frieze board: Acts as a mounting surface for the crown and cap, helping align the entire assembly with the outer edges of the casing.
Style Note: Together, the crown, frieze, and cap form an entablature—a layered detail rooted in classical architecture and commonly found in Georgian, Federal, and Greek Revival homes.
The Top Corner
These components help transition between horizontal and vertical trim elements while maintaining a consistent profile.
d) Crosshead strip: Bridges the head casing and frieze board, creating a visual break and helping proportion the upper trim assembly.
e) Mitered return: Covers exposed end grain at the wall, ensuring a clean finish at the termination point.
The Side Casing
The side casing defines the window’s vertical lines and is the most prominent part of the trim in everyday use.
f) Side casing: Covers the gap between the window frame and wall while establishing the overall style—narrow and simple for modern spaces, or wider and more detailed for traditional interiors.
g) Backband molding: An L-shaped molding applied to the outer edge of the casing to increase thickness and visual depth.
h) Inside stop: Holds the window sash in place. It can be removed to allow access for repairs or sash replacement.
The Bottom Corner
These details help transition from vertical casing to the horizontal sill while adding balance to the overall composition.
i) Horn: Extends the stool past the casing and apron, typically by 1 to 2 inches, creating a more pronounced, finished look at the base of the window.
j) Mitered return: Finishes exposed edges where trim meets the wall.
The Windowsill
This portion of the trim serves both functional and visual purposes, especially in covering gaps and completing the lower edge of the window.
k) Stool: The flat interior ledge at the base of the window. It hides the gap between the lower sash and the exterior sill while providing a finished surface.
l) Apron: Installed beneath the stool to visually support it and tie the lower trim into the rest of the casing. Its width typically aligns with the outer edges of the casing.
The Top
These elements complete the upper frame and tie into both the casing and any decorative crown assembly.
m) Cap: Covers and protects the top of the crown, adding a final finished layer.
n) Head casing: Covers the gap between the top of the window frame and the wall, forming the horizontal portion of the main trim frame.
The Square Cut Miter
This approach simplifies joinery and is commonly used in Craftsman, Victorian, and Colonial Revival homes.
o) Rosette: A decorative corner block that allows casing to meet without angled cuts, often slightly thicker and wider than the casing itself.
p) Square-cut joint: A butt joint where casing ends meet at 90 degrees instead of being mitered. This method is easier to install and more forgiving on uneven walls.
Pro Tip: Silva says using rosettes lets you skip mitered corners and rely on simple square cuts, which can make installation faster and easier. However, this approach typically uses wider, more decorative trim, which can increase material costs compared to simpler casing profiles.
How To Choose the Right Window Trim
The best window trim for your home depends on more than just appearance. Architectural style, existing trim, installation complexity, and budget all play a role in determining the right approach.
Match Your Home’s Architecture
Start by looking at your home’s overall style. Window trim should feel consistent with the rest of the architecture—not like an afterthought.
- Classical homes (Georgian, Federal, Greek Revival): Often feature wider casings, layered crown details, and symmetrical proportions. These built-up assemblies create a more formal, architectural look.
- Victorian homes: Tend to use more ornate profiles, sometimes with contrasting paint colors and decorative corner elements.
- Craftsman homes: Favor simpler, squared profiles with thicker stock and visible joinery.
- Modern and contemporary homes: Typically use flat, minimal trim—or eliminate casing altogether—for a clean, streamlined appearance.
Coordinate With Existing Trim
Window trim should work with other elements in the room, especially baseboards and door casing.
- Matching widths and profiles creates a cohesive look
- Mixing styles can work, but differences should feel intentional—not mismatched
- Inconsistent trim thickness or proportions can make windows look out of place
If you’re updating one room at a time, it’s often best to match the existing trim rather than introduce a new style.
Balance Detail and Visual Weight
Trim size and layering affect how prominent a window feels in the space.
- Wider casings draw more attention and can make smaller windows feel larger
- Backband or crown details add depth and work well in traditional interiors
- Narrow, flat trim keeps the focus on the wall and fits modern designs
Adding decorative elements like backband molding is one of the simplest ways to upgrade standard casing without fully rebuilding the trim.
Consider Installation Complexity
Some trim styles are easier to install than others, which can affect both labor cost and DIY difficulty.
- Mitered corners create a seamless look but require precise cuts
- Square-cut joints with rosettes simplify installation and are more forgiving on uneven walls
Think About Maintenance and Longevity
Material choice matters just as much as style—especially for exterior trim.
- Painted wood requires ongoing maintenance but offers a traditional look
- Engineered materials like MDF work well indoors, but can’t handle moisture
- Cellular PVC and fiber cement are better suited for exterior use due to their durability
Choosing the right material for the location helps prevent issues like rot, warping, or premature wear.
How To Install Window Trim
Installing window trim requires more than just cutting and nailing boards in place. Clean results depend on consistent spacing, tight joints, and proper preparation.
Tools and Materials Needed
To install window trim, you’ll need:
- Miter saw
- Tape measure
- Level
- Nail gun or hammer
- Caulk gun
- Wood filler
- Sandpaper
- Paint or stain
Key Installation Principles
Before starting, it’s important to understand two fundamentals:
- Reveal: The consistent gap (typically a small fraction of an inch) between the edge of the window jamb and the trim. A consistent reveal line is what makes trim look professional.
- Plumb and level: Trim must follow straight vertical (plumb) and horizontal (level) lines, even if walls are slightly uneven.
Installation Steps
- Prep the opening: Check for gaps between the window frame and rough opening. Pro Tip: Before installing trim, add insulation in this gap to prevent air leaks.
- Install the stool and apron: Set the stool first, ensuring it’s level. Attach the apron beneath it to anchor the bottom of the assembly.
- Install side casings: Cut each piece to align with your reveal line, then fasten to both the jamb and wall framing. Consistency here is more important than perfect measurements.
- Attach the head casing: Install the top piece last, aligning it with the side casings and maintaining a consistent reveal.
- Add decorative elements: Install a backband, a crown, or other details, depending on your chosen style.
- Finish the surface: Fill nail holes, sand smooth, caulk gaps, and apply paint or stain.
For a more detailed guide on trimming out a window, you can refer to our step-by-step window trim installation guide.
Common Window Trim Mistakes To Avoid
Even well-cut trim can look off if a few key details are overlooked. Avoiding these common mistakes will help ensure a clean, professional result and prevent issues down the line.
- Inconsistent reveal lines: The reveal, or the small, consistent gap between the window jamb and the trim, is one of the most noticeable details. Even slight variations can make the entire installation look uneven.
- Poorly cut or misaligned joints: Gaps at mitered corners or uneven square cuts draw attention and can worsen over time as materials expand and contract. Clean, tight joints matter more than overly complex profiles.
- Skipping insulation behind trim: Leaving gaps between the window frame and rough opening can lead to drafts and energy loss. Taking a few minutes to insulate before installing trim helps improve comfort and efficiency.
- Using the wrong material for the location: Interior materials like MDF can swell or break down if exposed to moisture, while untreated wood may not hold up well outside. Always match the material to the environment.
- Overusing caulk to hide gaps: Caulk should seal joints, not compensate for poor cuts or uneven installation. Excess caulk can crack over time and make trim look sloppy.
- Ignoring uneven walls or out-of-square openings: Walls are rarely perfectly straight, so trim may need slight adjustments or shimming to sit properly. Forcing trim into place can create visible gaps or misalignment.
Maintaining and Repairing Window Trim
Window trim is exposed to daily wear indoors and constant environmental stress outdoors. Regular inspection and timely repairs help prevent small issues, like hairline cracks or loose joints, from turning into more expensive problems, including rot or water damage.
Common Issues
The type of problem you’ll encounter often depends on the material and location of the trim.
- Rot or decay: Most common in exterior wood trim, especially near the sill or bottom corners where water can collect
- Peeling or cracked paint: Often a sign that moisture is getting into the material or that the surface wasn’t properly prepped
- Gaps or separation: Can occur as materials expand and contract or if trim wasn’t installed securely
- Warping or cracking: More likely with lower-quality wood or improper sealing
Pay close attention to joints and horizontal surfaces, as these areas tend to fail first.
Repair Techniques
Minor damage can usually be addressed with basic tools and materials, while more severe issues may require partial replacement.
For small repairs:
- Sand and repaint areas with peeling or damaged finish
- Fill small cracks or nail holes with wood filler or caulk
- Re-secure loose trim with finishing nails or adhesive
For more advanced repairs:
- Replace sections of trim that show signs of rot or deep cracking
- Cut out and patch damaged areas before repainting
- For exterior trim, ensure all joints are sealed properly to prevent future moisture intrusion
Window Trim vs. Other Home Trims
Window trim shares similarities with other interior trim elements, but it serves a more complex role, especially when installed on exterior walls.
Comparing Window Trim to Baseboard
Baseboards run along the bottom of interior walls, while window trim frames openings. Both contribute to a room’s overall style, but their functions differ.
- Baseboards: Primarily protect walls from scuffs, furniture, and foot traffic
- Window trim: Covers gaps around the window, helps seal the opening, and can improve energy efficiency when properly installed
Because window trim interacts directly with the window unit, it must be installed with tighter tolerances and better sealing than baseboard.
From a design standpoint, coordinating baseboard and window trim helps create a cohesive look. Matching widths or profiles can unify a space, while intentional contrast can highlight architectural features.
Differences Between Window Casing and Trim
The terms “window casing” and “window trim” are often used interchangeably, but they don’t mean exactly the same thing.
- Window casing: The flat boards that frame the window opening on all sides
- Window trim: A broader term that includes the casing plus any additional elements, such as aprons, stools, backband molding, or crown details
Our Conclusion
Window trim plays both a functional and decorative role, bridging the gap between the window and the surrounding wall while shaping the overall look of the space. The right combination of materials, proportions, and installation techniques can elevate even a basic window into a finished architectural feature.
Whether you’re tackling a DIY project or working with a professional, focusing on details like consistent reveals, proper sealing, and material selection will help ensure long-lasting results. With the right approach, window trim can improve durability, enhance energy performance, and add lasting value to your home.
