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.

The window inserts use a spring-loaded design that holds them in place by pressure against your window track — no screws, cutting, or permanent modifications required. If you want the install to be more permanent, you can screw the panel into the track at one or two points, which is still reversible. The window itself then slides shut against the insert and is held in place by your existing window mechanism.

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.

Yes. The insert holds in place using spring tension against your existing window track. No cutting, no drilling, no permanent modifications. Renters install these without putting their security deposit at risk. From one of our owners: "As a Realtor, I love that it is not permanent and won't leave any evidence of ever being in the window."

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.

Yes, very well. The same Endura Flap that goes in our wall and door mounts is used in the window pet doors: thick, two-layer insulated panel with magnets running the entire perimeter for a full-edge seal. The Thermo Sash 3e specifically uses thermal-pane (dual-pane) glass in the panel, which insulates close to the rest of your window glass. From one owner who installed during a windstorm: "I watched the curtain over the window. It never moved. Not even a fraction of an inch." Standard single-pane window panels seal less aggressively but still well in moderate climates.

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.

Usually not. The standard sizes have some adjustment range built in (typically 1-3 inches of spring travel depending on the model), so a measurement that's an inch off is often fine. Check the listed range on the product page first. If your window is genuinely outside the standard range, custom sizing is available, but the standard product often covers more windows than the listed size suggests.

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.

The pet door portion of the Thermo Sash 3e is integrated into the panel during manufacturing, so it can't be swapped between sides after the fact. When you order, pick the side that matches your install location (some windows have furniture or obstacles on one side that determine which side the door needs to be on). If you're unsure, customer service can walk you through it before you order.

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.

Not while the insert is installed in the window opening — the insert takes the place of the open slider, so the screen behind it can't be used without obstruction. Most owners remove the screen for the catio season and reinstall it when they take the pet door insert out.

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.

This is the easiest installation in our entire product line. Most owners are done in 15 minutes with no tools required. Open the window, drop the insert into the track, expand it under spring tension until it holds, close the window down against it. Add the weather stripping and locking hardware that comes in the box. That's the whole process. From one owner: "I had it set up in less than ten minutes."

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.

The flap is held shut by magnets along the edges. If your cat hesitates, the magnets are usually the cause, not the flap weight. You can remove some of the magnets to reduce the force needed to push through. Most cats figure out the door within a few days, especially if you prop it open during their first few exposures or use treats to motivate them through.

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.

Reach out to our customer service team. Send them a photo of the window you're working with and the rough dimensions; they can usually identify the right product in one reply.