There are many different kinds of deadbolts but for this discussion the distinction to be made is whether or not there is a thumbturn on the non-locking side of the door or not. This has ramifications for security as well as ease of egress in an emergency. There is also a cost difference.
A single cylinder deadbolt is the most common and features a thumbturn with which one can lock or unlock the door without a key, unless it is an abloy protec2 deadbolt with a lockable thumbturn. From the outside of the door they must use a key to lock or unlock the door unless they know how to pick locks. A double cylinder deadbolt replaces the thumbturn with another keyed cylinder. To lock or unlock the door from either side requires a key.
Double deadbolts are usually seen when there is a glass window near or on the door. The idea is that if somebody breaks the window they might reach in and unlock the deadbolt. If the window is man-sized, a two-legged skunk could still walk in with the door locked but would be unable to remove large heavy items using a hand truck or dolly or other wheeled device because the door would still be closed. They could get in but could only remove smaller items that could be fit through the broken window.
If a fire occurs a double cylinder deadbolt is very dangerous. Smoke quickly fills a room during a fire and seconds count when trying to get out alive. During this time finding your keys and then finding the keyhole is a lot of extra time spent during which there is a lot of smoke inhalation that could be deadly. I believe it is illegal to add double cylinder deadbolts to fire exits. I rekey existing ones and am willing to install them in doors that aren’t fire exits. I always advise those with double cylinder deadbolts to leave a key in the deadbolt at all times that they or anybody else is home.
A great application for double cylinder deadbolts are commercial spaces. I installed a double cylinder deadbolt on a maintenance room that kept getting broken into because the walls didn’t connect with the ceiling. People would scale the wall and open the door from the inside. Once the deadbolt was installed and the screw heads rounded off people could still break in but were unable to remove anything but the lightest most portable object. They also later added concertina wire on top of the wall.
Elevator rooms and other parking garage utility rooms are favorite places for drug addicts and the homeless to make themselves feel at home. Sometimes these rooms can be broken into by climbing over the doorframe if the building uses removable ceiling tiles. Once inside the most resourceful of these desperate people may leave the door unlocked and come and go as they please or even worse, remove the deadbolt and replace it with one bought or stolen elsewhere and lock everybody else out of the room.
This scenario couldn’t happen if an Arrow E series double cylinder deadbolt is employed because the mounting bolts can only be exposed by turning the cylinder with a key. You can’t remove this deadbolt unless you have a key or know how to pick locks. This is advantageous not only in the utility room situation which forces trespassers to climb back out if they want to leave, but also makes it difficult for somebody with the intention of taking your deadbolt apart and recovering the bitting for your master key. Note that this scenario is pretty uncommon, but you can see the danger and the high cost of somebody reverse engineering a master key for a large building.
A good product for those with double cylinder deadbolts is a key with a thumbturn head. I have seen them before but am having trouble locating a supplier. I found this product on amazon.com, this one in New Zealand,
