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The Thermo Sash window dog door is aethsetically pleasing and build to last. It comes in a variety of sizes to fit many animals.
Thermo Sash 3e Showing "Endura" Flap large sized window pet door.
Thermo Sash 3e Showing "Endura" Flap large sized pet door for window.
Thermo Sash 3e Showing "Endura" Flap large sized pet door for window.
Thermo Sash Small Bronze Frame dog door for window.
Thermo Sash Small Bronze Frame dog door for window.
Thermo Sash 3e Showing "Endura" Flap large sized doggie door for window.
Thermo Sash Small Satin Frame pet door sash.
White Thermo Sash dog door window.

Thermo Sash 3e Cat Doors for Sash Windows

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The Endura Flap Thermo-Sash 3e Sash Window Pet Door 

  • Durable and insulating pet door for windows due to the dual-pane, LoE glass, and sturdy aluminum framing
  • Various colors and adjustment ranges to fit your pet and your window
  • No tools required for installation, perfect for renters and vacation homes

Size Chart

Flap Size Flap Dimension Height To Raise Window
Small 6" w x 11" h 18 1/2"


Adjustment ranges

22"-25", 25"-28, 28"-31", 31"-34", 34"-37", 37"-40", 40"-43"

Important Note

All Extra Large and 22"-25" adjustment range sales are final, no return.‚ Please simulate the opening (open the window, use duct tape
to simulate the opening) to be sure this size is appropriate before ordering.

Our normal 90-day return policy and 15-year warranty apply to Small, Medium, and Large sizes as well as all other adjustment ranges.



Thermo Sash 3e

1 window pet door

1 locking cover

1 center post weather seal draft stopper

1 foam weatherstrip

1 installation instruction booklet


Popular Features Include

  • Highly insulating flap with a complete magnetic seal, designed to stay closed in winds up to 50mph. The flap is made out of a polyolefin-based polymer which can keep your home comfortable in temperatures between -40 and 110 degrees Fahrenheit
  • Dual-pane, LoE glass gives you extra insulation for your home, pairs perfectly with newer energy-efficient windows
  • Fits all vertical sliding sash window widths between 22" - 43"‚ with a track width of at least 1"
  • Three color options including white, satin, and bronze (very dark brown)
  • Includes a draft stopper and weatherstripping for extra insulation and weather resistance
  • Can be installed and removed in seconds due to the spring-loaded design, no tools required
  • 15-year warranty for all pet doors
  • 100% made in the USA

There has been lots of discussion as to why the Endura Flap is such a‚ high-performing pet door, but for engineers or those looking for some more technical information here is a 74-page‚ dog door energy efficiency study‚ showing what makes it so special.


Need help Installing? Take a look at our Thermo Sash Installation page.


Replacement Parts and Accessories


Installing a Thermo Sash 3e


Design of the Endura Flap‚


Variable Magnetic Resistance‚ 


Replacing the Flap in the Sash Pet Door

Switching Flap Sides

Wind Test

Endura Flap® Pet Door Warranty

To the original purchaser, Endura Flap® Pet Doors warrants this product and all of its component parts to be free of defects in materials or workmanship for a period of 15 years from the date of delivery to the customer.

Damage caused by accident, improper installation, abuse or normal wear, including damage by pets or glass breakage is specifically excluded from this warranty. Defective materials returned to Endura Flap® Pet Doors together with proof of purchase, will be repaired or replaced and returned to the customer free of charge.