Dog Doors for Windows

A sash window opens vertically: you lift the bottom pane up to open it. A horizontal sliding window opens side-to-side: one pane slides along a track past the other. Window pet doors aren't interchangeable between the two — sash inserts sit on the sill with the upper pane coming down to meet them; horizontal sliding inserts go vertically into the track between the panes. Look at how your window opens before you order.

Two locking layers. First, the pet door itself: every Endura Flap window pet door ships with a slide-in ABS plastic locking cover that blocks the flap opening. Second, the window: for horizontal sliding inserts and sash windows, a Charley Bar or wooden dowel in the track behind the insert prevents the window from being forced open. The Charley Bar unlocks in seconds when you want to use the window normally. For sash inserts that you want to lock permanently, you can install a wood screw above the window on each side; this prevents the window from being raised at all.

The Thermo Sash 3e Pet Door for Sash Windows is the right choice if you have a dog — we've had owners successfully use it for labs in the 65 to 87 pound range when installed in a tall enough sash window. The Cat Door variants and the horizontal sliding model are sized for cats only. The constraint isn't the door technology; it's whether your window is tall enough for the dog to comfortably step through. Measure your window opening height and your dog's shoulder height before ordering.

For a sash window, measure the width of the opening (inside the frame, side to side) — that's your number. For a horizontal sliding window, measure the height of the track opening (top to bottom of the track) — that's your number. Measure inside the frame, not over the trim. The most common return reason in this category is a measurement that's off by an inch or two, so it's worth a second pass with the tape.

The Thermo Sash 3e has a minimum adjustment height of 22 inches. If your window is shorter than that, the product won't fit as designed. Some owners build a wood frame into the lower portion of the window to bring the effective opening up to 22 inches, but this is a workaround, not an official install method. If your window is significantly under 22 inches, reach out to customer service to talk through alternatives.

Two reasons. First, most residential windows aren't tall or wide enough to accommodate the larger flaps (XL and XXL) that we make for wall and door mounts. Second, the spring-tension mounting system has practical force limits that work best for smaller flaps. The Thermo Sash 3e Pet Door covers the upper end of what's feasible at this product type. For larger dogs or higher traffic, our wall mount or door mount lines are the right product.

Same window panel technology, different flap size. The Pet Door version comes with the larger flap that fits mid-to-large pets (including dogs up to lab size). The Cat Door version comes with a smaller flap sized for cats and very small dogs. Pick based on the largest pet that will be using the door.

Yes. The insert pops out of the track in seconds, your window returns to normal use, and you can put the insert back when warmer weather returns. Some catio owners do exactly this for snowy climates. Other owners (especially in the South) leave the insert in year-round.

Yes, replacement flaps are available on the Parts and Accessories page. That said, Endura Flaps are designed not to need replacement. Most owners who order replacements do it for cosmetic reasons after years of use, not because the flap has failed.