How To Install Window Casing for Vinyl Siding

Vinyl siding jobs from the 1970s often removed decorative home features, leading to a flatter, less appealing facade. Many homeowners are now looking for ways to return those original details without sacrificing the protective qualities of vinyl siding. One method is to install molded vinyl window casing on trimless windows, which is a little different than the standard window installation process.

This project requires moderate skill but can be completed in a few hours per window with the right tools and materials. In the video above, This Old House host Kevin O’Connor and vinyl siding expert Richard Scalzi demonstrate how to properly install window casing for a more polished look. Here, we’ll take you step-by-step through the process.

The Problem with Older Vinyl Siding Installations

Many homes with vinyl siding installed in the 1970s lack the architectural details that give windows character and depth. This is often due to a technique called “jump casing,” which involves the following steps:

• Cutting off the window sill at the edge of the storm window
• Covering the cut sill with aluminum
• Removing the bed molding around the window
• Installing J-channel directly against the window
• Covering the window casing with vinyl siding

While functional, this method strips away the home’s original charm and architectural interest. Fortunately, there’s a way to restore these details using vinyl materials. Incorporating a vinyl window sill and casing replicates the traditional look of wood while providing the benefits of lower maintenance.

Additional Considerations for Older Homes

Older homes may present unique challenges like rotting wood or uneven surfaces. Address these issues before installing new casing. Inspect the underlying structure for any damage and repair it as needed. This step ensures that your new casing has a solid and level foundation to adhere to.

Tools and Materials for Window Casing Installation

Here’s what you’ll need for this DIY job.

Materials

Before beginning the project, gather the following materials. Stick to one brand as much as possible. Selecting compatible products helps avoid issues with fitting and adhesion, which can arise from mixing different brands or material types.

• End caps for window sill
• Finish nails
• J-channel
• Replacement vinyl siding pieces (if necessary)
• Vinyl casing cover
• Vinyl colonial molding
• Vinyl siding adhesive
• Vinyl window sill

Tools

You’ll also need these tools:

• Aluminum coping jig
• Aviation snips
• Caulk gun
• Drill/driver
• Hammer
• Level
• Pencil
• Pry bar
• Tape measure
• Tin snips

Step-By-Step Guide To Installing Window Casing

O’Connor and Scalzi worked on a house that had storm windows with jump casing. Some steps of the process may be different or unnecessary if you’re starting with a different type of window or trim. However, here are the steps they followed.

Step 1: Preparing the Window for New Casing

Before installing the new window casing, you’ll need to carefully remove the existing materials. Taking your time with these steps can prevent costly repairs and additional work later on.

1. If there’s a storm window in place, unscrew and remove it from the outside of the window.
2. Carefully pry off the vinyl siding and J-channel from around the window’s perimeter.
3. Use tin snips to trim off any metal flashing covering the exterior windowsill.

Step 2: Installing the Vinyl Windowsill

Now that you’ve stripped the material around the window, you can begin installing the new sill. 

1. Measure and cut the vinyl windowsill to fit the existing wooden sill, confirming it extends past the edge of the window as it originally did.
2. Notch the vinyl windowsill with tin snips to fit snugly against the existing windowsill.
3. Secure the vinyl windowsill to the old wooden sill using finish nails.
4. Glue vinyl end caps onto each end of the windowsill using vinyl siding adhesive.

Step 3: Adding Vinyl Casing and Molding

With the window sill in place, it’s time to add the casing and molding.

1. Hold the vinyl casing against the vertical side of the window and scribe the bottom end to match the angle of the sill.
2. Cut the bottom end of the casing with aviation snips, then install it against the window.
3. Repeat this process for the opposite side and top of the window.
4. Cut two lengths of vinyl colonial molding and install one on each vertical side of the window casing.
5. For the top piece of molding, measure and mark where it intersects with the vertical moldings.
6. Use an aluminum coping jig to mark precise cutting lines on the top molding piece.
7. Cut along the coped lines with aviation snips.
8. Install the top piece of molding, making sure it snaps over the vertical side moldings for a seamless look.

Step 4: Integrating the New Casing with Existing Siding

Fitting the new casing and J-channel into the existing siding is essential for a watertight seal.

1. Cut and install new J-channel along the sides and top of the window, fitting it into the new molding.
2. Take the vinyl siding pieces removed earlier and cut them down to fit around the new window casing.
3. Slide each siding piece into the J-channel and secure it to the house with roofing nails.

Tips for a Professional-Looking Window Casing Finish

To achieve the best results, follow these tips:

• Apply a small bead of caulk where the new casing meets the window frame to prevent water infiltration.
• Ensure all cuts are clean and precise to prevent gaps or misalignments.
• Paint any exposed nail heads to match the vinyl color for a seamless appearance.
• Use a level to check that the casing is straight before securing it in place.

Extra care with details like caulking and painting can significantly improve the casing’s long-term appearance and durability.

Addressing Common Challenges

When installing window casing for vinyl siding, be wary of these pitfalls:

• Failing to properly measure and cut the vinyl pieces, resulting in gaps or ill-fitting components
• Forgetting to properly seal joints and connections, potentially leading to water damage
• Neglecting to account for expansion and contraction of vinyl materials in different temperatures
• Over-driving nails, which can cause the vinyl to buckle or crack

Remember that any penetrations in your siding are a potential weak point for water to get underneath. If you have concerns about gaps or poorly-fitting vinyl, speak with a professional siding contractor.

Cost Estimation for Window Casing Installation

The cost for your project will largely depend on the size of the window (that is, how many linear feet you need to cover with casing) and the price of your existing siding. You can keep costs down by opting for a narrower sill (3–4 feet) and less ornate window casing and molding. Below are the estimated costs for the materials:

• Vinyl window sill: $24–$79
• Vinyl window casing material: $1.73–$2.63 per linear foot
• Fasteners, connectors, adhesives, and other materials: $9–$10
• J-channel: $2.18–$2.28 per linear foot
• Replacement vinyl siding pieces: $3–$12

Pricing data averaged from The Home Depot, Homewyse, and Angi.

Our Conclusion

Jump casing windows are nearly half a century out of fashion, and they’re not terribly interesting to look at. If you want to restore architectural interest to your home’s windows without needing extensive carpentry skills, install vinyl window casing instead. While this project requires some skill and patience, the result is a more polished and attractive facade that maintains the low-maintenance benefits of vinyl siding.