I have a dog, who is used to staying at my house while I have gone for work. I leave the door open so that he can go out for hygienic reasons.

During summer, this is not a problem because I stop the heater, and the radiator next to the open door does not lose its warmth.

Now it is becoming more colder, so the radiator next to the open door is losing heat. Currently, I manage by opening the door (it's a sliding door) just enough for the dog to pass through and along with I am lowering the blinds too but my energy bill is going through the roof.

As you can suggest the easiest solution as shutting my dog out of my house, I think it's too silly to have a guardian dog who can't stop burglars because he can't get into the house to catch them :-)

At this moment the best thing I can think of is a curtain before the door, but I'm curious if somebody can come up with other hacks.

Does anybody have an idea?
Thanks in advance

  • Fyi, an open door in a colder outdoor environment WILL lose you energy... The radiator may just be overdimensioned enough that it can cope (especially if it's an older high temperature one), but your heating bill will be a lot higher than with a closed door.
    – MiG
    Commented Feb 10, 2022 at 16:02
  • Am I missing something? Why have you not considered an actual doggie door? The kind where you make a little hole in the bottom of the door with a flap?
    – goodguy5
    Commented Feb 24, 2022 at 20:49
  • It's a glass door, and especially while working with double-layered glass, it's not possible to make a hole.
    – Dominique
    Commented Feb 25, 2022 at 6:59
  • You can simply make an insert with a large wooden board and mount an actual doggie door in there. Anything that stays open will cause you an increased heating bill, something best to avoid with rising prices...
    – MiG
    Commented Feb 25, 2022 at 9:05

4 Answers 4


You could make a homemade doggie door.

  • Open the door enough for your dog.
  • Get corrugated cardboard (maybe from a leftover packing box) that is the same size and shape as the opening, but can also be bent.
  • Tape the cardboard in the opening to seal in some of the heat - but don't tape the several feet at the bottom of it. That's where the "door" will be.
  • Measure how tall your dog is. Mark a spot on the cardboard that is 6-12" above your dog's head. Bend the cardboard back and forth at that spot several times.
  • Teach your dog how to walk through the door you created.

You should be able to take down the "door" at night and put it back up in the morning. You may need to make another one after a few weeks. There may also be a way to prevent the tape adhesive from getting on the door frame and door over time, but I'm not sure what that solution is yet.

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  • Excellent idea. I think I would probably CUT the cardboard rather than bending it, and use packing tape or gaffer (duct) tape to form the "hinge".
    – Lefty
    Commented Dec 5, 2017 at 7:34
  • @Lefty Thanks! I was thinking of the same idea about cutting the cardboard and creating a "hinge", but I decided to go with the simplified design for my answer. Commented Dec 5, 2017 at 17:17
  • parroted from the question: Am I missing something? Why have you not considered an actual doggie door? The kind where you make a little hole in the bottom of the door with a flap?
    – goodguy5
    Commented Feb 24, 2022 at 20:50

@BrettFromLA's idea is exactly what I would do. But once you establish that the dog is able to use the "dog-flap", you could improve it.

The whole thing could be made from plywood with a thin timber frame just on the sides. You will then probably find that the frame of the sliding door will have a recess and you can wedge your new door between the sliding door and the frame. This will make it much easier to just remove the arrangment at night and lock your door normally.

Another improvement might be to create a "tunnel" outside the door from plywood, this will greatly reduce draughts that would otherwise find their way around the flap. You could even add a second door at the end of the tunnel.

  • 3
    For the less DIY ambitious, there are products on the market to fill this need. Search for "Glass sliding dog door" and choose the one which matches best the dog(s) owned and the decor of the home.
    – anon
    Commented Dec 5, 2017 at 19:40

To build on the previous answers: Go to the local craft store and look at foam core boards. They are probably a better solution than cardboard because they can get wet and not warp or fall apart. You can cut the doorway as shown above, and then use plastic tape or duct tape to make a sort of hinge. The foam boards might even be a little bit lighter than cardboard. And, you can get them in white which may be better than the brown.

Another suggestion instead of a one-piece flap/door is to use vertically hanging strips of clear thick plastic like you see in walk-in freezers. The dog may be more likely to pass through them instead of bumping open a flap. In fact, after doing some quick searching, they actually make pet flaps:



You can use a standard fan to keep the air outside, that works well if you place it correctly.

  • 1
    A fan will increase thermal energy loss, no matter how you place it. If you point it outwards, it accelerates the exhaust of warm air (which because of the pressure drop sucks replacement cold air in). If you point it inwards, it actually actively pushes in cold air (and the pressure increase will additionally push hot air out). If you for some reason point it sideways... best case scenario it does little, worst case the increased circulation near the opening will also accelerate the exchange between hot indoor and cold outdoor air.
    – MiG
    Commented Feb 15, 2022 at 15:30

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