Using Prefabrication In Housing

Summary | How the Strategy Works | Advantages and Issues | Sources | Case Study #1 ]

How the strategy works

Prefabrication in housing may take three different forms, each reflecting a different degree of mechanization in home construction. These are:

  • prefabricated components,
  • modular housing, and
  • manufactured housing.

Prefabricated components

Prefabrication of windows, doors, kitchen cabinets and roof trusses has long had a place in home construction. Recent innovations have resulted in an even wider variety of prefabricated components, which have a great impact on affordability. The following are descriptions of some of these innovations and how they enhance affordability.

Walls and roofing

Prefabricated panels framed with wood or lightweight steel, clad in a range of exterior and interior finishes, are used for exterior walls. The wall assembly usually contains insulation, wiring and precut openings for windows and doors. A “panelized home” uses factory made panels that include the windows and doors as well as siding. Costs are lower as a result of a reduction in site labour time. The components are brought to the site and erected or assembled there.

Structural insulated panels (SIPs) are the most common form of prefabricated building envelope systems. SIPs are composed of two exterior skins (such as oriented strand board, waferboard, plywood or gypsumboard) adhered to a rigid plastic insulating foam core (usually polystyrene). Panels are available in a variety of thicknesses, depending upon application requirements and sizes. Their primary application is for exterior walls and roofs with conventional wood or steel-stud framing for interior partitions. SIPs form a solid thermal envelope around the structure, uninterrupted by studs, sills or headers, which eliminates gaps found in batt insulation and reduces heating and cooling costs by approximately 15 per cent compared to conventional site-built homes. While the cost of a SIP house may be more for materials, the shortened construction period and superior energy performance contribute to cost savings in both the short and long term.

Roof panels, such as those made of oriented strand board (OSB) have a short construction time and are uniformly sound and extra rigid to handle snow and wind loads. Roof panel systems can decrease the amount of labour required to roof a building by 30 to 40 per cent.

Room kits

Bathrooms and kitchens traditionally have the highest per-square-metre costs in residential construction. Room kits are now being produced containing all the components of these rooms, including cabinets and wall flashing, and all the requisite wiring and plumbing ready for hookup. As in the case of modular housing, computers play a large part in providing the dimensional accuracy needed in such “pop-up” modules. These kits may be shipped as a complete unit, which is lowered into place, or in precut sections. While not yet widely used in Canada, this application of prefabrication has been reported to save 15 per cent over in-situ construction of these areas in Australia, at a project time saving of 13 per cent.

Modular housing

Modular housing was first used in Canada in the more isolated regions of the North to improve housing conditions in remote communities. Pre-built housing sections, shipped by water or air, could be quickly assembled by local labour. More recently, it is becoming widely used across the country for single- and multi-storey affordable rental housing, as well as seniors’ and special needs housing.

A modular home is highly engineered. It is constructed in sections and put together by a builder on the site. Modular homes are designed, engineered and built in a factory-controlled environment. Most modular producers use state-of-the-art, computer-aided design programs. Speed of construction and consistent quality are two of the major advantages of modular housing. For example, a home consisting of two sections can be built in the factory in a couple of weeks. Once the manufacturing is completed, the sections are transported to the housing site where they are placed on the foundation. Final completion is handled by a local builder or general contractor, who connects the utilities and completes a short list of finishing work. The home can be finished on site in another two to three weeks.

Several factors make modular housing an increasingly popular approach, such as greater potential for customization given advances in the technology, successful examples of the use of modular construction in multi-unit housing, and increasing integration between the factory-built and site-built sectors.

Modular housing must follow the same building codes as site-built construction and go through the same development procedures.

Baker Gardens in Cranbrook, British Columbia, is a 36 unit project completed in 2011. This large modular housing project brought the total of affordable modular-built housing units in British Columbia to 400 (see CMHC’s Project Profile: Baker Gardens). More recently, the Baker Gardens unit count was surpassed by a three-storey, 52-unit modular housing project in Surrey, British Columbia, completed in March 2012. This project, Timber Grove Apartments, was one of the six projects built from the 156 relocated temporary modular housing units built for the 2010 Olympics.

Manufactured housing

Manufactured housing is completely built in the factory. Manufactured homes today are virtually indistinguishable from their site-built counterparts. The entire house, containing all the same amenities as a site-built home, is shipped to the site and placed on a permanent foundation. Increasingly, manufactured housing is durable and desirable and represents a viable alternative for providing affordable housing.

There are significant cost advantages to a manufactured home. According to the CMHC-funded report Profile and Prospects of the Factory-Built Housing Industry in Canada, the construction costs per square metre of factory-built homes in Canada are 15 to 20 per cent less than for comparable site-built homes.

The use of land leases in manufactured housing communities also impacts affordability. Particularly common in retirement communities, the land lease tenure greatly reduces the cost of manufactured housing because the lots are “rented” from the owner or management firm of the manufactured housing community land. Significantly less start-up capital is required by the household to obtain this type of housing.

The popularity of manufactured housing is strongest in the Maritimes and in some high growth areas in the West. This popularity is a result of the affordability of manufactured housing.

LaHave Heights, one of two manufactured home communities in Bridgewater, Nova Scotia, developed by the Home Centre Group of Companies, consists of 215 affordable manufactured houses on leased sites (see the LaHave Heights case study). The LaHave “mini houses” are typically around 104 m² (1,120 sq. ft.) in size, are made to look like other private market housing, can be fitted with a porch and garage as required and sell for around $100 per square foot.

The Canadian Manufactured Housing Institute reports that just under 14,500 factory-built, single-family homes were started in 2011, accounting for around 12.5 per cent of all single-family home starts. The average size was just under 120 m² (1,300 sq. ft.).

Manufactured housing has always been a more significant part of the housing sector in the United States than in Canada, and is the primary source of unsubsidized affordable housing in that country. While suffering a sharp drop during the housing downturn, manufactured housing has accounted for 21.5 per cent of all new single-family homes sold in the U.S. since 1989 (estimate from the U.S. Manufactured Housing Institute in 2012).

Standards

In addition to provincial/territorial/municipal building code requirements, there are Canadian Standards Association (CSA) standards for modular and manufactured homes.

Modular homes are covered under CSA A277. This standard involves both the manufacturing plant and the buildings. It deals with the concept of “total quality” throughout the manufacturing process and defines the quality-control procedures and staff that a plant must have in place to ensure that the buildings it produces are built properly and comply with the relevant standards and codes.

Manufactured homes are manufactured to CSA Z240. This standard sets out requirements for the construction of manufactured homes related to structure and plumbing, electrical and heating services.

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