The concept of prefabricated buildings is hundreds of years old, stretching back to the 15th century when William the Conqueror invaded England. Over the centuries, prefabricated buildings have been improved, adapted, changed, and altered to fit the needs of society, with prefabricated farm buildings being used during the Gold Rush of 1848, and portable cottages appearing in Great Britain during the same period.

At the beginning of the 20th century, prefabricated home kits were sold by catalog with precast concrete materials available as foundations. The rapid growth of prefabricated buildings began after World War Two, which had brought the introduction of Quonset huts as temporary barracks for soldiers during the war. From the simple design of Quonset huts has developed an ever-growing industry that provides several types of prefab buildings to serve the needs of manufacturers, farmers, and homeowners.

The two main types of prefabrication are volumetric or modular and panelized. Both types are constructed using timber, steel, concrete, and fiberglass, with certain types being a mixture of the various materials.

• Steel types are made of light gauge galvanized steel.
• Timber prefabricated buildings are the oldest and traditional type and have timber studs and sheathing.
• Panel buildings have fewer studs and are bonded by a rigid insulation core and outer sheathing to enhance their strength.
• Fiberglass prefab buildings have foam core fiberglass walls and roofs.

Components of Prefabricated Buildings

The various components of prefabricated buildings are engineered at a factory and delivered to a location where they are assembled. Prefabricated buildings are a viable and reliable alternative for creating work space, storage area, and flexible manufacturing facilities. The components of prefabricated buildings include framing, secondary supports, wall and roof panels, door and window frames, fasteners, and sheeting and insulation.

Although the larger components may seem to be the foundational and necessary parts of the assembly of prefabricated buildings, the most essential and crucial items are the screws, bolts, nails, and fasteners required to hold the various elements together. A wide selection of fasteners is carefully engineered and designed for long use and easy installation.

Components include construction elements such as windows, doors, and trusses, which by themselves are not complete panels. Among the prefabricated materials, these have the least amount of off-site assembly, but usage is more flexible since they can be placed and installed according to actual site conditions.

Prefabricated Panels

Prefabricated panels are two-dimensional components that are put together on-site to form a building. They require more on-site work than modular prefabricated buildings. These components are commonly available as sub-assemblies with complete finishing and installed features such as windows, doors, and insulation. Panels can also be supplied as bare structural frameworks in which the additional components can be later added on-site. Different types of prefabricated panels vary according to material and form.

Structural Insulated Panels (SIPs)

This type of panel consists of two structural facings with a layer of insulating material sandwiched in-between. The two structural facings or boards can be metal sheets, plywood, and cement. The insulating material can be polymer foams such as expanded polyurethane and polystyrene foams. These are manufactured by gluing the three pieces with strong adhesives or by letting the foam expand and cure whie forming between the facings. In both processes, the facings are clamped together. Pressure and temperature are applied until the adhesive or foam has cured.

Insulated Precast Concrete Panels

The construction of insulated precast concrete panels is similar to SIPs, where two structural facings bound an insulating material. In this type, the facings are concrete layers, called wythes. These wythes are usually pre-stressed to achieve higher structural performance. The insulation is a rigid material with proprietary designs. Since all three components are rigid and can act as load-bearing members, they can be combined and separated depending on the intended function. The connection between the wythes can be stiff, sliding, or deflecting. Fully composite panels are rigidly connected, which makes them able to resist higher loads. Non-composite connectors can slide or deflect and are strong in tension but weak in shear. They enable the wythes to act independently with each other. Non-composites are mostly used for applications such as refrigeration and cooling where high insulation is required.

Insulated Concrete Forms (ICFs)

This type of panel uses rigid insulating materials as permanent formworks for creating reinforced concrete walls. The formworks, ties, and other supporting elements are prefabricated and installed on-site. They can be made as modular units that can interlock together to form a building. Ready-mix concrete is poured on-site, creating a permanent wall. Afterward, finishes and cladding systems can be directly applied to the insulating material. Though the structure is not completely prefabricated, the time and labor spent are mostly off-site. Most ICFs offer better performance than other panels since the main load-bearing structure is steel-reinforced concrete. Due to their monolithic construction, they are stronger and more resistant to moisture penetration.

Timber Frame Panels

These types of prefabricated panels are timber stud walls with plywood or proprietary facings attached to either side of the walls. Insulating materials are then fitted with insulation. Access for utilities such as cable conduits and piping is easier to install compared to other panels. Timber frame panels are cheaper, but the downsides are their weaker load-bearing strength, poor sound resistance, and susceptibility to biological attacks such as mold and termites. Chemical preservatives, fungicides, and insecticides are added to prevent such biological attacks.

Lightweight Steel Frame Panels

In this type, the main load-bearing members' studs are made of cold-formed steel, usually C-sections. They are assembled by welding, bolting, or other fastening methods. Facings and insulation materials such as gypsum board, stone wool, oriented strand brands (OSB), and expanded polystyrene foams (EPS) are added. The insulation can be placed within the thickness of the steel (cold frame) or outside of the steel framing (warm frame). Lightweight steel frames have a higher strength-to-weight ratio than other panels, but their capacity is limited to resist mostly static loads and some lateral loads such as wind and earthquakes. Another drawback is the high thermal conductivity of steel and the risk of interstitial condensation. Thus, a thicker insulation material is required.

Prefabricated Modules

Prefabricated modules are three-dimensional in construction, usually made up of four shop-assembled panels. Several modules are placed adjacent or on top of each other, forming the whole building. Modules are connected by inter-module connections, which are bolted on-site. The term modular buildings are synonymous with prefabricated buildings because it is the most popular among all types. All advantages of a prefabricated building are seen in a modular building. A single module can be the complete building, with minimal site work required. There are different types of modular buildings according to their form of construction.

Four-sided Modules

This type of module is manufactured with four closed sides creating a cellular space. The panel frames are load-bearing can transfer both vertical and lateral loads. The maximum height for this form is typically 6 to 10 stories, depending on site conditions. Applications for four-sided modules are hotels, small residential buildings, housing compounds, and dormitories.

Partially Open-sided Modules

This module consists of one or more walls made up of an assembly of panels that do not completely span the entirety of the wall. The partitions are open, serving as accessways or corridors connecting adjacent modules. Edges of the partially open sides have corners or intermediate columns or posts that transfer the vertical load as a replacement for the load-bearing panel. The typical height and applications for this form are the same as that of the four-sided modules.

Open-sided Modules

This module consists of one or two sides that are designed to be fully open. The long sides are usually removed so that a larger space can be created by attaching to other adjacent open-sided modules. The loads are transferred to the corner posts, which are connected to the edge beams by gusset plates and bracings. Since open-sided modules have lesser load-bearing members, they are not applicable for creating tall buildings. The typical height of this type of form is about two to three stories. Applications for these modules are hospitals and schools.

Modules Supported by Primary Structure

In this type of module, an external steel structural frame is added to support and transfer loads. The external structure can provide open spaces at or below ground levels while the modules are stacked above. It can also act as full support for the modules while enabling the walls and partitions to be non-load bearing. Typical applications for this type are residential and mixed-retail buildings.

Hybrid Prefab Systems

Hybrid prefab systems utilize both three- and two- dimensional components from modular and panel systems to create a whole or a part of a building. This type can also be referred to as mixed modular and panel systems. Modular units have the advantage when it comes to construction quality and detail but are sometimes limited by assembly and transportation constraints. The three-dimensional modules are used for highly serviced and higher value parts such as kitchens and bathrooms. Panelized components are added to the assembly due to their flat pack or ready-to-assemble construction. Two-dimensional panels are used for floors and walls of more open areas.

Complete Buildings

These types are stand-alone modules or buildings. Complete buildings are delivered and installed at the site with prepared foundations. Complete buildings require the least amount of site work but are limited by hauling capacity limits and road width and height clearances.