Doors by their definition are meant to be opened… and closed. When in use, it is the single largest hole in your home insulation. So make sure when it’s not in use, it is truly closed. Not only will it save you money, it can keep the critters out too. Those cockroaches love to come in from the cold when the mercury starts dropping.
Take a look at the weatherstripping or door sweep and make sure it is not worn. If it’s cracked or torn, or it’s obvious there’s a gap, you should replace it.
An easy test to see if you have gaps in the seal of your door is a light test. After dark, shine a flashlight around the edge of the door and have someone on the other side watch for the light shining through.
If you see a gap, you’ll want to address it.
For the bottom of the door you have a few options. Door sweeps attach to the bottom of the door and have a rubber or plastic botom that seals the gap between the bottom of the door and the threshold. It’s pretty easy to install… but it does require drilling into the door. For
instance, you could try the Macklanburg-Duncan Automatic Door Sweep for doors that open into carpeted areas or over an area rug. The most common size is a 36 inch door sweep but you may need a 42 inch door sweep. If you live in an apartment or don’t want to drill, you can try the Hampton Direct Twin Draft Guard. These attach without modifying the door for easier installation.