Weather

• The primary purpose of a door threshold is to provide a seal underneath the door, as a protection against heat in the summer and cold in the winter. The threshold shields the interior from drafts that would otherwise flow inside and cause you to either turn up the air conditioning during a heat wave or crank up the heat during the colder months. The threshold also protects the interior from rainwater, which would otherwise find a way into the entry area.
  
  

Interior Air Seal

• Along with the purpose of sealing out incoming cold and hot air, as well as rain and snow, a threshold acts as a barrier to prevent the interior air from leaking out. In doing this, it helps cut down on heating and cooling costs, as you avoid the need to continually replenish the hot or cold air that would be lost underneath a door that did not have a threshold.
  
  

Pests

• A threshold also functions as a barrier to keep out unwanted insects and other small pests that would otherwise have an opening to the inside. Without the threshold, there is a noticeably larger gap between the bottom of the door and the floor, through which snakes and other small rodents could travel.
  
  

Nonfunctioning Threshold

• There are additional methods to help block the large amount of air that can slip underneath a door, even with a threshold in position. A popular method to stop the draft is to install a metal door sweep along the bottom of the entry door. Because thresholds withstand a tremendous amount of traffic and pressure, they deteriorate over time. When this happens, replace your threshold with a new one. Remove the screws, cut the threshold with a backsaw and pry it up with a pry bar. Measure the door opening, apply caulk to the back of a replacement threshold and install the new one with screws.