For the middle size range, from about 20 to 50 feet (7 to 16 meters) in width, bi-fold doors probably have the best efficiency potential. They can be made to seal tightly, and they can be well insulated. Bi-fold doors have other advantages. They fold outward, so they require no internal space, and they require no internal clearance to operate. This type initially came into widespread use for aircraft hangars, and they are gaining in popularity for other applications.The main disadvantage of bi-fold doors is their relatively high cost. The door assembly itself is expensive, because it requires a lot of structure and an operator. And, it is custom-built at the site. Prefabricated bi-fold doors have not yet made an appearance on the market. The building structure must be braced to withstand the stress that the open door exerts on the top of the door opening.