Door sweeps are the small pieces of rubber or vinyl at the bottom of your door that effectively cover the gap where the door meets the bottom of the frame. This area is almost always built with a gap to allow the door to open and close smoothly.
Contrary to its name, the door sweep doesn’t “sweep” anything. It acts as a buffer between the small gap and your home.
So does it save you money on energy?
In short: yes.
By acting as a layer of insulation between the air on the other side of the door (the outside) and the inside controlled temperature air, it is effective at prevent heat transfer or heat loss through air movement. Of course, it is even more effective when combined with a screen or storm door, because then less air is allowed to even come to the door in the first place.
It may also prevent heat loss if it is made of a good insulation material. Heat loss happens when heat is transferred between two materials when there is a difference in surface temperature. If the cold outside air touches the door sweep, and the door sweep is warmer than the air, the heat from the door sweep will be lost. If the door sweep is cooled and the material becomes colder, heat will be lost because the house is warmer. Rubber is a fairly good insulator for this purpose, which is why it is the material most often used in door sweeps.
The “brush” type of weep is good for reducing air flow, but will readily transfer heat if temperatures are different – which is why a lot of modern outer facing doors are longer sold with this type of sweep (though that may be the reason why “door sweep” adopted its name…it looked more like a broom attached to the bottom of the door.)
If your door sweep gets worn and falls off (they all do), you may ask yourself if you need to actually replace it? Do a simple test. If you feel air entering through the bottom of the door, then absolutely – you need a door sweep. If not, and you can not see the light from outside within your home, you may not need one. Though it certainly will never hurt to add one!
It also bears mentioning that your storm door could use a door sweep as well! Considering it is really the first line of defense against cold air entering your house, it should prevent as much heat loss as possible.
So…do you have a door sweep?
If not, or perhaps the one you have on your door is old or worn out, fear not! You can make your own out of rubber, a piece of vinyl (from vinyl molding perhaps?), foam, felt, or even a makeshift less permanent door sweep from a towel or heavy cloth. For instructions and more info, see http://www.utilitybillbusters.com/articles/heating/why-you-need-a-door-sweep.