How to Measure & Install Your Cat Window Insert (Step-by-Step Guide)

Your cat deserves safe freedom without drafts or damage to your home. This easy, step-by-step guide shows you exactly how to measure and install a cat window insert for a perfect custom fit. You’ll keep your home comfortable, give your cat catio access if you have an outdoor enclosure, and enjoy a durable solution that’s Made in the USA—engineered for energy efficiency for winter months.

Why a Cat Window Insert Is the Purr-fect DIY Project

A window insert lets you add a locking cat flap without cutting doors or walls. It’s renter-friendly, removable, and custom-fit to your window track for a snug, draft-reducing seal. With foam and brush weatherstripping around the flap area and a precision-cut base, you’ll keep your indoor temps stable year-round—great for comfort and utility bills.

Explore our most popular models: Plexiglass Cat Door Window Insert and Solid PVC Base Insert.

What You’ll Need

• Measuring tape (read to the nearest 1/16")
• Pencil or fine marker
• Small level (optional but helpful)
• Your Cat Flap Fever custom cat window insert

Tip: Our PVC base is 5/8" thick. If your tracks are narrower, let us know and we’ll taper the edges for a proper fit.

Step 1 — Measure Your Window Track Accurately

For Horizontal (Up-and-Down) Windows

1. Open the window.
2. Place your tape measure deep into the left side track—as far as it will go.
3. Measure straight across to the edge of the right side (this is the critical “slip-past” distance for installation).
4. Record this width precisely (include decimals), then double-check once more.

See our official Horizontal Measurement Instructions for a quick visual reference.

For Vertical (Side-to-Side/Sliding) Windows

1. Open the slider.
2. Measure from the inside top track all the way down into the bottom of the lower track.
3. Record this height precisely (include decimals), then double-check once more.

See our official Vertical Measurement Instructions for a quick visual reference.

Orientation note: Our vertical inserts are cut to the height of your window and designed so you can close the slider against the insert to hold it in place. The locking cat flap is positioned at the bottom for easy pet access.

Step 2 — Dry-Fit & Prep

Before final placement, do a quick dry-fit to confirm your measurements and alignment:

• Confirm the insert seats squarely in the track with no wobble.
• Use a level (optional) to verify the insert is plumb (vertical) or level (horizontal), depending on orientation.
• Check that the foam weatherstripping and brush seal touch the mating surfaces to reduce drafts and keep out bugs.

Step 3 — Install & Secure the Insert

1. Slide the insert fully into the track (bottom/top for vertical; left/right for horizontal) so it seats flush.
2. Gently close the window or slider against the insert to create a snug, secure fit.
3. If your insert includes a tension or set-screw system, tighten as directed for added stability.

No permanent modification is required—ideal for renters and homeowners who want flexibility and a clean, professional look.

Step 4 — Test the Cat Flap & Locking Options

• Push the flap in both directions to ensure smooth movement.
• Engage and disengage the locking feature to confirm operation.
• Place the latch setting you prefer (locked, unlocked, or one-way entry/exit—if applicable).

Bonus: Quick Training Tips for First-Time Users

Most cats learn quickly, but a little encouragement goes a long way:

• Prop the flap open with a soft cloth and offer treats on the other side.
• Call your cat through the opening; reward immediately.
• Lower the flap gradually over a few trials until your cat pushes it independently.

Connect Your Insert to a Catio for Safe Outdoor Freedom

If you’ve built (or plan to build) a catio, your insert is the simplest way to give catio access—no door cuts, no wall cuts, and a tight seal that supports comfort and energy savings. It’s a sleek, renter-friendly path to fresh air, sunshine, and enrichment.

Keep Your Home Comfortable Year-Round

Our inserts are crafted with durable PVC or Plexiglass and finished with foam weatherstripping around all sides and brush weatherstripping around the swinging door to block drafts and bugs. That means fewer cold spots and better energy efficiency for winter months—without sacrificing your cat’s independence.

Choose the Right Flap & Features

• Locking flaps for security and schedule control.
• Microchip-compatible options to help keep strays out.
• Larger flap sizes for big cats or small dogs.

Not sure which flap is best? Use our measurement guide and reach out—our team can help you select the right size and features for your window style and pet.

See what other pet parents are saying on our Reviews Page.

Common Measuring & Installation Mistakes to Avoid

• Measuring the wrong surfaces: Always measure the inside track (left-to-right for horizontal; top-to-bottom for vertical).
• Rounding too much: Record exact decimals—precision ensures a tight, draft-reducing fit.
• Skipping the dry-fit: A quick test placement catches alignment issues before you secure the insert.
• Ignoring track thickness: Our PVC base is 5/8" thick; tell us if your tracks are narrower so we can taper edges.

Quick FAQs

Does the insert damage my window?

No. It seats in the existing track and is secured by closing the window/slider against it. No cutting required.

Will it help with drafts?

Yes. The foam and brush weatherstripping are designed to reduce drafts and keep out bugs when properly measured and installed.

Is this a good option for renters?

Absolutely—no permanent alterations and easy to remove. It’s a smart, reversible solution.

Ready to Give Your Cat Safe Freedom?

Measure once, order with confidence, and install in minutes. Start with our official measurement instructions, then choose the flap style that fits your home. Your cat gets freedom and enrichment; you get comfort, energy efficiency for winter months, and a clean, modern look—proudly Made in the USA.

Shop the Plexiglass Cat Door Insert Now