Peepholes and door viewers allow the occupants of a home to see who is outside before opening the door. These devices provide a safe method of seeing through the door, yet are small enough that they do not pose a threat to the door's integrity or security features. If your existing door does not have a peephole, you can install one very easily on your own. These units are available in many different styles and colors to complement your door as well as your existing hardware.
Measure the thickness of your door using a tape measure. Most commercial doors are 1 3/4 inches thick, while most residential doors are 1 3/8 inches thick. It is critical that you choose a peephole that is the right length to fit your door. An incorrectly sized unit may not be functional and will definitely detract from the door's aesthetic appeal.
Choose a peephole of stainless steel or brass to avoid rust and corrosion on exterior doors. Other metals will rust over time and may even stain the surrounding door surface.
Decide where you will place the peephole on the door. The unit should be installed so that every member of the family can see through it. Shorter family members or children should be able to use the peephole without a step stool. Taller members may have to bend down for access. Once you've chosen a mounting height, measure the width of the door to determine the midpoint, and mark the location for the peephole with a pencil.
Drill a hole for the viewer that is the same diameter as the unit itself. For peepholes that are not the same diameter across their entire length, follow the installation instructions to determine how large to drill the hole. You'll need a bimetal hole saw for this job. A bimetal hole saw is used just like a traditional saw but is designed for cutting holes in metal doors. Look for a unit with an attached arbor, or pilot bit, in the center. This pilot bit makes it easier to make smooth, easy cuts with your hole saw. Make sure to wear protective safety glasses when making cuts in a metal door.
File away any rough edges from around the hole using a metal file. This will create a smooth surface for installing your peephole.
Insert the lens portion of the peephole through the front of the door. Add the scope portion by inserting it through the interior side of the door. Twist the two pieces together until they are tightly fastened.
Caulk around the peephole to seal gaps or air spaces as needed. Depending on the size of the hole relative to the peephole, you may not need to add caulk. Caulk can also be helpful if you have an odd-sized peephole and can't find a bit or hole saw of exactly the right diameter.