The genius of humanity has devised many ways to cover a large entrance in a building. These are the main types of exterior doors that are used at present:
• sliding panel doors. This is the simplest type of large door. Individual panels hang from an overhead track. The panels overlap when the door opens.This type is used primarily in medium and large sizes, including the largest doors in the world. See Figure 1. Sliding doors are easy to insulate, but somewhat difficult to seal against infiltration.
• horizontally folding doors. This type is made of large panels that are suspended from an overhead track. The sides of the panels are hinged together,so the door folds open in accordion fashion. This type is similar to sliding doors in insulation and infiltration characteristics. It is typically used in medium sizes.
• bi-fold (vertically folding) doors. As the name implies, bi-fold doors consist of two panels. The door folds vertically,usually toward the outside. this type has the potential to be well insulated and to provide good resistance to infiltration. Figures 2 and 4 show typical examples. The structure of bifold doors, shown in Figure 5, makes them expensive. Also, they require structural bracing of the top of the door opening. They are becoming increasingly popular in the medium size range.
• roll-up doors. This type consists of short segments that roll up vertically around a spindle. This provides a compact door installation. Figure 3 shows a typical unit. However, the short segments cannot be insulated effectively, and the design is
difficult to seal against infiltration. It is typically used in sizes that accommodate individual vehicles.
• overhead segmented doors. This type opens by rising in a track that bends to a horizontal position underneath the ceiling. The door is made in hinged segments to accommodate the bend in the track. The number of segments keeps this type from being well insulated, and the track mounting makes it difficult to seal against infiltration. It is typically used in widths that accommodate one or two vehicles.
• overhead single-panel doors. This type consists of a single panel that swings upward to an overhead horizontal position. It can be well insulated, but sealing against infiltration is difficult. It has fallen out of popularity because the mounting hardware is awkward and the door sweeps through interior space when opening. It is typically used in singlevehicle sizes.
• swinging doors. This type is limited to smaller sizes because of the space that is swept when the door opens, the potential for slamming by wind, and the large loads on the hinges. Too bad, because swinging doors can be well insulated and they are easy to seal against infiltration.