The interior trim around a window serves both decorative and functional purposes. It covers the rough window framing and creates an aesthetic transition between the wall and the window. The specific names for window trim pieces vary somewhat depending on the style of the window and trim, but some common terms are:
- Window stool – The horizontal piece at the bottom of the window frame on the interior side.
- Apron – A piece directly under the stool, covering the area between the bottom of the window frame and the wall.
- Side casings – The vertical pieces on either side of the window frame.
- Header – The horizontal piece across the top of the window frame on the interior side.
Additional molded trim pieces may be added for decorative effect. Some other names used for trim pieces include mullions (vertical dividers between window sashes), muntins (horizontal dividers within a sash), and stops (removable trim used to hold the window in place).
The purpose of the trim is to cover the rough window framing, create a smooth transition from the window to the wall, and allow flexibility for adjusting and securing the window. Well-trimmed windows have a neat, finished look and blend in with the style of the interior design.
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Types of Window Trim
There are several common types of interior window trim, which vary based on the window style:
Double Hung Windows
Double hung windows have two vertically sliding sashes and typically feature:
- Side casings – Vertical trim pieces on each side.
- Stool – Horizontal trim piece on the bottom.
- Apron – Covers the area under the stool between the window frame and wall.
- Header – Horizontal trim across the top.
Optional decorative pieces include rosettes in the top corners and a pediment trim piece over the header.
Casement Windows
Casement windows have hinged sashes that swing open to the side. Typical trim pieces include:
- Side casings – Surround the left and right sides.
- Header – Across the top.
- Stool – On the bottom.
- Apron – Under the stool.
Some casement windows extend down to the floor and do not have a stool or apron.
Bay and Bow Windows
Bay and bow windows have a series of connected window units in an angled formation that projects outward from the wall. Common trim pieces are:
- Side casings – Run vertically on the far left and far right sides.
- Head casings – Horizontal trim where the projecting windows meet the wall above.
- Stools – Provide a finished edge below each projecting window.
- Panel trim – Covers the wall areas between the individual window units.
Trim for Other Window Styles
Other window styles like awning, hopper, and specialty shapes have their own trim terminology. But in general, they include side casings running vertically, a stool or sill along the bottom, and header/head casing along the top.
The trim forms a finished border around the window to integrate it with the surrounding wall.
Trim Materials and Options
Window trim can be made from various materials, with the most common options being:
- Wood – Versatile and can be painted or stained. Common types used are pine, oak, poplar, and MDF.
- PVC/vinyl – Low maintenance plastic that resists moisture and insects. Offers durability and a variety of styles.
- Polystyrene – Polyurethane foam trim provides an authentic wood look with low maintenance.
- Fiber cement – Made from wood fibers and cement material. Provides a seamless look.
- Aluminum – Lightweight, low maintenance, and moisture resistant. Available in custom colors.
In addition to the trim pieces themselves, other options include:
- Backband molding – Decorative piece added for aesthetic appeal.
- Sill nose – Rounded front edge of stool for a finished look.
- Mitered joints – Angled trim cuts for a seamless corner.
- Plinth blocks – Decorative blocks placed under the stool at the lower corners.
The trim style and materials should complement the room’s decor and window design. Wood trim offers a traditional look, while PVC and fiber cement are great low maintenance options.
Trim Installation Process
Installing interior window trim is typically a six-step process:
1. Cut the Trim
The first step is taking precise measurements and cutting the trim pieces to size with a miter saw. The side casings, stool, apron, and header must be cut to fit tightly around the window frame. The joints should be mitered for a clean look.
2. Attach the Side Casings
Starting with one side, attach the side casings vertically with finishing nails. Use a level to ensure they are plumb. Repeat for the opposite side casing.
3. Install the Head Casing
Measure the distance across the top of the window from side trim to side trim. Cut the head casing to this length with angled end cuts to match up with the corners. Nail the head casing securely in place.
4. Add the Stool and Apron
Cut these pieces to fit snugly along the bottom of the window. The apron tucks under the stool for a finished look. Attach with adhesive and nails.
5. Caulk the Trim
Caulk all joints and seams between the trim pieces and the window frame with a clear sealant. This helps create an airtight seal.
6. Finish the Trim
Finish the trim to match the desired look, either with paint or stain. Add any other decorative elements like rosettes, plinth blocks, or pediment overlays. Let the finish fully cure before exposure to the elements.
Proper installation ensures long-lasting, secure trim that complements the home’s style. Take careful measurements, cut mitered joints, attach securely, caulk gaps, and apply a protective finish.
Trim Replacement and Repair
Over time, interior window trim can become damaged due to moisture, pests, accidents, and normal wear and tear. Some common issues requiring repair include:
- Rotting or cracking wood
- Warping or detachment
- Leaking joints allowing air infiltration
- Infestation from termites or carpenter ants
- Layers of chipped, peeling paint
- Dents, holes, or other physical damage
Replacing damaged sections or entire trim assemblies can renew the look and prevent further issues. For minor repairs, use wood filler, caulk, or spackle to patch small holes or gaps. Severely damaged trim will need removal and replacement.
The process involves:
- Prying off the damaged trim with a pry bar.
- Scraping off old caulk and nails.
- Measuring and cutting new trim pieces.
- Reinstalling with adhesive, nails, and caulk.
- Filling nail holes, repriming, and refinishing.
Professional installation or repairs ensure the job is done correctly and securely. But DIYers can handle basic trim replacement and patching with the right tools and attention to detail.
Cost of Installing Window Trim
For new construction or full window replacements, basic interior trim costs $2 to $6 per linear foot on average. More complex projects or premium materials can range from $6 to $15 per linear foot installed.
The total cost depends on factors like:
- Type of material – Wood is most expensive, followed by fiber cement and PVC.
- Trim style and ornamentation – Simple flat casing vs. molded trim with corners and layers.
- Labor rates in your region.
- Accessibility – Second floors or high ceilings increase installation cost.
- Prep work needed – Refinishing old trim adds cost.
- Existing wall material – Masonry requires specialty fasteners.
Many homeowners spend $200 to $600 per window on interior trim installation. Get estimates from at least 3 contractors to find the best value.
Conclusion
Interior window trim serves both form and function. It covers rough window framing for a neat appearance while allowing adjustments to the window. Typical trim pieces include side casings, head casing, stool, and apron, along with decorative elements like rosettes or pediments. Trim materials like wood, PVC, and fiber cement add aesthetic appeal and durability. Proper installation and finishing result in secure, long-lasting trim that enhances your windows and interior decor. With some basic DIY skills, minor trim repairs can keep your windows looking their best.