• Provides window based pet access while helping maintain indoor comfort
  • Preserves energy efficiency with insulated panel and magnetic flap seal
  • Ideal for renters seeking a semi permanent solution without cutting doors
  • Spring loaded compression design secures directly inside the sash track
  • Flexible polyolefin polymer flap remains functional in temperatures down to minus 40 degrees Fahrenheit
  • Locking cover and weather stripping included for added control and insulation
  • Available in multiple flap sizes from small to extra large
    Custom orders are made to your specifications and are non-returnable
  • Made in the USA

Endura Flap Thermo Sash 3e Pet Doors for Sash Windows

Insulated Freedom for Sash Windows

45 Reviews
Made in the USA USA flag icon
Regular price $419.99

Fast & Free Shipping in the USA.

Satisfaction Guarantee -

Lifetime Warranty

Color: White
Flap Size
Width Adjustment Range


Give cats and dogs easy window access without sacrificing comfort using an insulated sash insert with magnetic sealing, spring-loaded compression fit, and built-in locking cover.

The Thermo Sash 3e Pet Door for Sash Windows is built for homeowners who want pet freedom without compromising the performance of their window. By integrating an insulated panel directly into the sash track, it creates a dedicated access point that works with your window rather than replacing it.

Cold Weather Performance and Magnetic Sealing
At the core is a flexible polyolefin polymer flap engineered to remain responsive even in temperatures down to minus 40 degrees Fahrenheit. Magnets line the edges to encourage a consistent seal in windy or cold conditions, while the resistance can be adjusted by adding or removing magnets to suit confident dogs or more cautious cats.

Structured Frame and Integrated Insulation
The extruded aluminum frame offers a clean, structured appearance that pairs naturally with many window styles. Weather stripping and a top loading locking cover are included to help reduce drafts and give you control when pets need to stay inside.

Installation relies on a spring loaded compression system that holds the panel securely within the sash track, making it especially practical for renters or seasonal properties. The window must open high enough to accept the full panel height, which varies by flap size.

Custom and Extra Large Configurations
Extra Large 22 inch to 25 inch width panels are produced with plexiglass and built to order. Custom made panels are crafted to the specified dimensions and are non returnable.


Installation & Fit

  • Fits vertically sliding sash windows with a track width between 22 inches and 43 inches and at least a 1 inch thick track. Here is a guide on how to properly measure your window
  • Works with traditional sash windows that slide up and down.
  • Slides into the existing window track and secures using a spring loaded compression design.
  • Stays in place through pressure within the track rather than permanent fasteners.
  • Includes a top loading locking cover that inserts directly into the pet door frame.
  • Extra Large 22 inch to 25 inch width panels are built with plexiglass and made to order.
  • Height needed to raise window per flap size
    Small - 18 1/2"
    Medium - 22 1/2"
    Large - 26 1/2"
    Extra Large - 30 1/2"

Best Suited For

Ideal for:

  • Renters and homeowners who want pet access without altering doors or cutting into walls.
  • Homes where preserving insulation and minimizing drafts around a window opening is important.
  • Cats and dogs that prefer a lower step through opening within a window location.
  • Vacation homes or seasonal spaces where a semi permanent solution is preferred.
  • Households that want to remove or adjust magnets to fine tune flap resistance for timid pets.
  • Owners who value a locking cover for added security when pets need to stay inside.

Not ideal for:

  • Very narrow windows under 20 inches wide.
  • Horizontal sliding windows that open side to side where a horizontal insert such as Liberty Pet Door for Horizontal Sliding Windows would be more appropriate.
  • Windows that cannot open high enough to accommodate the full outside frame height of the panel which varies by flap size.















Questions

What's in the box?

  • 1 Sash Window Pet Door
  • 1 Foam Weather Strip
  • 1 Center Post Weather Seal Draft Stopper

Do I need tools to install the Thermo Sash Window Pet Door?

No. The panel installs using a spring-loaded compression design that expands to fit tightly within the window track. This allows quick installation without cutting or permanently modifying the window.

What type of windows does the Thermo Sash Pet Door fit?

This pet door is designed for traditional vertical sliding sash windows. It fits window track widths roughly between 22 and 43 inches with at least a 1-inch thick track.

How does the Thermo Sash Window Pet Door work?

The Thermo Sash Window Pet Door installs directly into the track of a vertically sliding sash window. The window closes down against the insulated panel, allowing your pet to use the built-in flap while maintaining a sealed window opening.

Explore all questions
Product Details

Product Details

The Thermo Sash 3e Pet Door for Sash Windows is built for homeowners who want pet freedom without compromising the performance of their window. By integrating an insulated panel directly into the sash track, it creates a dedicated access point that works with your window rather than replacing it.

Cold Weather Performance and Magnetic Sealing
At the core is a flexible polyolefin polymer flap engineered to remain responsive even in temperatures down to minus 40 degrees Fahrenheit. Magnets line the edges to encourage a consistent seal in windy or cold conditions, while the resistance can be adjusted by adding or removing magnets to suit confident dogs or more cautious cats.

Structured Frame and Integrated Insulation
The extruded aluminum frame offers a clean, structured appearance that pairs naturally with many window styles. Weather stripping and a top loading locking cover are included to help reduce drafts and give you control when pets need to stay inside.

Installation relies on a spring loaded compression system that holds the panel securely within the sash track, making it especially practical for renters or seasonal properties. The window must open high enough to accept the full panel height, which varies by flap size.

Custom and Extra Large Configurations
Extra Large 22 inch to 25 inch width panels are produced with plexiglass and built to order. Custom made panels are crafted to the specified dimensions and are non returnable.


Sizing & Fit

Size Chart

Flap Dimensions:

6"w x 11"h

The entry way your dog goes through.

Small flap
Small frame
Small rough cut

Height To Raise Window:

18 1/2"

How high you must raise your window in order to fit the panel into the track.

Flap Dimensions:

8"w x 15"h

The entry way your dog goes through.

Medium flap
Medium frame
Medium rough cut

Height To Raise Window:

22 1/2"

How high you must raise your window in order to fit the panel into the track.

Flap Dimensions:

10"w x 19"h

The entry way your dog goes through.

Large flap
Large frame
Large rough cut

Height To Raise Window:

26 1/2"

How high you must raise your window in order to fit the panel into the track.

Flap Dimensions:

12"w x 23"h

The entry way your dog goes through.

Extra Large flap
Extra Large frame
Extra Large rough cut

Height To Raise Window:

30 1/2”

How high you must raise your window in order to fit the panel into the track.

Flap Size Flap Dimension Outside Frame Dimension Recommended Pet Height
Small 6"w x 11"h 18 1/2" 9" - 15"
Medium 8"w x 15"h 22 1/2" 9" - 21"
Large 10"w x 19"h 26 1/2" 9" - 27"
Extra Large 12"w x 23"h 30 1/2” 9" - 34"

 

Do not know which size to get? Here is a measurement guide to help you out

Installation Guide

Installation Information

Thermosash Installation Instructions

Installation icon Difficulty Easy
Installation icon Steps 4
Installation icon Time Required 0.5 hours

Parts

  • 1 Sash Window Pet Door
  • 1 Foam Weather Strip
  • 1 Center Post Weather Seal Draft Stopper

Tools

  • 1 Foam Weather Strip

Step 1 Release Spring Mechanism

Release Spring Mechanism
  • Loosen the width adjustment locks at the end of the pet door insert with the screwdriver. This allows the spring mechanism to move freely from side to side.

Step 2 Check Panel Fit

Check Panel Fit
  • Insert the sash into the window track. Push against the spring mechanism and swing the opposite end into the opposite window track. Gently release. Tighten the width adjustment locks.

Step 3 Apply Weatherstripping

Apply Weatherstripping
  • Once you're sure the panel fits, remove it from the frame and lay it on its side on the ground.
  • For foam weatherstripping: Peel the backing off the foam weatherstripping. Press it along both edges of the panel, trimming as needed with scissors to ensure a snug fit.
  • For rubber weatherstripping: Clean the edge of your sliding door or window with rubbing alcohol to help the weatherstripping stick properly. Starting at the top or bottom, peel and press the strip into place with the folded edge facing indoors—align angled cuts if you have two pieces. For best results, apply the seal to the sliding door or window, but it can go on the pet door panel if needed.
  • Foam weatherstripping is applied onto both sides of the panel, but the rubber weather stripping only applies on the side of the slider door. Your panel will come with one type of weather stripping. You do not need both types.

Step 4 Mount Seal Draft Stopper

Mount Seal Draft Stopper
  • Mount the Center Post Weather Seal Draft Stopper. This strip prevents air from leaking between the window frame when raised to accommodate the sash. Trim to fit with scissors.
  • Put the panel back into the window track. Tighten width adjustment locks.

Change the Orientation of the Flap

Installation icon Difficulty Easy
Installation icon Steps 3
Installation icon Time Required

Step 1 Remove the Screws

Remove the Screws
  • Remove the screws on the top and bottom closest to the edge of the pet door. Then slightly loosen the screws closest to the glass. Next, slide off the side piece, then the flap assembly.

Step 2 Rotate flap assembly

Rotate flap assembly
  • Rotate the flap assembly (essentially flipping it upside down) then slide it back in. Make sure the locking cover is on the same side as the width adjustment locks.

Step 3 Replace the end spacer and screws.

  • Place the end spacer and screws back into place
  • Tighten screws (not too tight)
Videos

Videos

Adjusting Your Endura Flap® Magnet Strength


Design of the Endura Flap


The Endura Flap Pet Door


FAQs

FAQ

Reviews

Customer Reviews

Request Freight Quote

Track heights exceeding 106 inches (107" or higher) may require additional shipping charges. Please provide your contact information for a custom freight quote, and our team will follow up during normal business hours.