Endura Flap Thermo Sash 3e Pet Doors for Sash Windows

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  • Semi-permanent sash window insert with built-in pet door
  • Withstands extreme climates
  • Dual-pane, Low-E glass installed to reduce harmful UV and infrared light
  • Spring-loaded design for an easy, tool-free installation
  • Free U.S. shipping

The Endura Flap Thermo Sash 3e Pet Door for Windows is an insulating, weatherproof panel that fits snugly into the track of most windows. This semi-permanent door installs completely tools free with a spring-loaded design that safely secures the panel in place without the hassle of permanent installation. The Thermo Sash 3e provides quality insulation and energy efficiency, as the perimeter of the flap contains magnets to keep the flap shut when not in use. This dog/cat door for window sashes will keep your house at your desired temperature, and it will grant your pets the freedom they crave without compromising your time! 


  • Frame color comes in white, dark bronze, and brushed aluminum
  • Pet flaps available in small to extra large
  • Sizes to fit windows from 22" - 43" in width and at least a 1" thick track. Customized sizes available
  • The frame is made from heavy extruded aluminum
  • Blocks UV rays and infrared light with dual-pane Low-E glass
  • Polyolefin polymer flap is designed to remain flexible in temperatures below freezing down to -40 degrees
  • Remains sealed in heavy winds
  • Magnets line the sides of the flap to combat wind gusts and weather
  • One sturdy locking cover included
  • Draft stopper and weather stripping included for enhanced insulation
  • 90-day return policy and a lifetime warranty.


  • Sleek frame and color variety are complementary to many windows
  • The flexible flap is easy for pets to use
  • Polyolefin polymer flap is pet and environmentally friendly
  • Accommodates both dogs and cats
  • Multiple pets can use the door due to its taller and slimmer design
  • Temporary/semi-permanent installation is perfect for renters and vacation homes
  • Keep your pets inside or out with the locking cover
  • Gives pets the ability to come and go when they please

Size Chart


Flap SizeFlap DimensionHeight To Raise Window
Small6" w x 11" h18 1/2"
Medium8" w x 15" h22 1/2"
Large10" w x 19" h26 1/2"
Extra Large12" w x 23" h30 1/2"


For your adjustment range, you need to measure the length of your sash window track from the furthest left to right. The adjustment range of your pet door should ideally have your track length in the middle. The following adjustment ranges are available:

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

Custom orders are subject to special restrictions.


Endura Flap products have a lifetime warranty against defects in materials and workmanship under standard regular use. Damage caused by not using the product as intended, improper installation, and chewing is not covered under this warranty.

Additional Information

YOUR MAXIMUM PET SIZECats, Large Cats, Small Dogs, Medium Dogs, Large Dogs, X-Large Dogs
TYPE OF WINDOWSash (Up & Down)
FRAME MATERIALHeavy Extruded Aluminum
FLAP MATERIALEngage Polyolefin
FLAP DESIGNSingle, One Part , Flexible, Insulated Flap
ABILITY TO LOCK FLAPSeparate Locking Cover-Loads from Top
RETURN PERIOD90 days (except for custom height ranges)

Installation Instructions


1x Window pet door

1x Foam Weather Strip

1x Center Post Weather Seal Draft Stopper


Step 1: Loosen the thumbscrews at the end of the sash. This allows the spring mechanism to move freely side to side.

Step 2: 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 thumbscrews.

If the pet door sits properly, remove the pet door to put on the foam weather-stripping in step 3.

Step 3: Apply the foam weather-stripping by peeling back the paper and exposing the adhesive. Unroll it along the edge of the panel, pressing to ensure it adheres. Install on both the top and the bottom. Trim to fit with scissors.

Step 4: Mount the draft stopper vinyl weatherstrip. This strip prevents air from leaking between the window frame when raised to accommodate the sash. Trim to fit with scissors.


For a permanent lock, install a wood screw in the track immediately above the window on both sides. You will no longer be able to raise the window.

For a temporary lock, place a removable wooden dowel or Charley bar in either track above the window.


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. 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 thumbscrews. Replace the end spacer and screws.


Make sure you place steps or ramps leading to your pet™s door at a height that works! When they go through the flap, the top of the flap should be at least 1" (2,5 cm) higher than the top of their back. Watch your pet and adjust as needed to keep them comfortable.



What things should I measure/consider when choosing a cat flap door for my sash window?

Four things:

  • 1. What size pet door your dog or cat will need
  • 2. How high you can raise the window (will it raise high enough to fit the pet door size you selected)
  • 3. The width of the window (from the deepest part of the left track to the deepest part of the right track)
  • 4. The width of the track (must be at least 1" to accommodate the Thermo Panel)

How do I determine the size of pet door my pet will need?

The door should be wide enough that it does not touch your pet's sides as they go through, and the top of the pet door should clear your pet's back by at least one inch. Take a look at our guide on how to measure your pet for more information. For a large cat door or dog door, make sure that the window you are installing in will accommodate the flap height you choose.

What makes the Thermo Sash 3e better than the other manufacturer's sash window pet doors?

The Thermo Sash includes the patented Endura Flap Pet Door which has a dual-layered flap employing magnets around the edges for superior sealability and insulation. It uses dual-pane, LoE glass for added insulating properties. Both of these are housed in a sturdy aluminum frame. It is made in the USA and backed by a lifetime warranty.

How wide does the window track have to be to accommodate the Thermo Sash 3e?

Your window track must be at least 1 inch wide for thermo sash 3e pet door installation.

Which Thermo Sashes are considered custom and are not returnable?

All Extra Large pet door sashes and any pet door size in a sash width with a 22"-25" adjustment range.

Can I move the flap on the Thermo Sash from the left side to the right?

Yes you can switch the side the pet door is on. You start by removing the screws on the top and bottom, then sliding the flap assembly out. Once it is removed you simply flip it upside down and slide it right back in. Next you put the screws back in, tighten them all down and you're finished. You'll want to make sure the thumb screws and locking cover are on the same side.

Can you make a Thermo Sash smaller than the 22" wide?

You can make it 3/4" inch smaller by trimming the right side (spring loaded piece). If this is not enough, we may be able to make a custom width with less or no glass. Please call us with your specific needs for us to review the possibility.

What tools are needed to install a Thermo Sash?

None. The Thermo Sash is installed without the use of tools. If desired, you may need tools to install a locking device to secure your window. Take a look at the Installations Instructions and Videos tab above.

How high do I have to raise my window to install the Thermo Sash?

It varies based on the pet door size: Small is 19", Medium is 23", Large is 27" and Extra Large is 31" high.

I ordered a 22"-25" adjustment range and mis-measured. My window is 25.5" wide. Is there anything I can do?

We make a width adjustment which extends the width by 3/4". You can order Patio panel height extensions for additional height. You could also cut, paint and use a small piece of wood to extend the width.

Can I adjust the height of the Thermo Sash?

The Thermo Sash does not have an adjustable height. The height of the unit is determined by the height of the pet door.

Which window cat door insert will keep out the desert heat?

The most energy efficient one is the Thermo Sash 3e Pet Doors for Sash Windows.

What energy saving dog door window insert do you have?

The Thermo Sash 3e pet door is the most energy efficient pet door window insert on the market.

I see the Thermo Sash 3e Pet Door for Sash Windows, what side sliding window cat door do you offer?

There is a vertical version of the Thermo Sash called the Thermo Panel. I is available in the same four flap sizes. It has three standard height ranges. If your window is shorter, a custom panel can be made.

Do you make Doggie Doors for Windows?

Yes! The Thermo Sash is designed for windows that lift up and down. The Thermo Panel is designed for windows that slide left to right. Both are available in Custom sizes.

Can I install a dog door in window if the window is not at ground level.

Yes. We've seen many creative ways to help our dog access the window. Pet parents usually install steps or a ramp on both sides of the window to the doggy access the window. This may require training, but can be successful.

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