With the number of prefabricated homes produced yearly in the past twenty years hovering around the 200,000 unit mark [Figure 44], there is a stable business to be attended to. However these numbers for the prefabricated housing industry in Japan are largely monopolised by the ten largest housing manufacturers, producing around 97.2% of all new prefab homes in 1995.
Five major companies consist of Sekisui House, Daiwa House, Misawa Homes, Sekisui Homes and PanaHome, who dominate 80% of the market. Of these companies Misawa is the only housing company which didn’t evolve from a conglomerate. According to Noguchi, the reasons for this are that housing projects require huge resources for factory production and R&D. Clients also rely heavily on the reputation of established companies and distrust newcomers.
Another smaller but important manufacturer of modular housing is Toyota Homes. Although the smallest producer of homes from the top 6 house makers [Figure 44], the application of Toyota’s automobile production line to housing production has proven very effective in creating quality, reliable modules to a target market with a slightly lower salary than that Sekisui Heim targets. In some way the same philosophy of economical, quality and reliable products in the automobile industry is passed onto to their homes. There are three housing factories, with the largest producing 64 modules per day, averaging 5 houses per day. (Volger 2015) The Toyota LEAN manufacturing process is famous throughout the globe and has been implemented in numerous factories.
Prefabrication with engineered wood
Mass customization applied to prefabricated homes is often associated with the concept of creating a number of housing modules with a limited number of arrangements. However, when applied to solid wood the customization capabilities are greatly expanded through the ability to cut and machine pieces rather than just selecting from stock. Below is a list of the main types of prefabrication applied to house creation:
This method of manufacture is the simplest and most widely used in the wood construction industry, particularly in Japanese homes. This method involves cutting all the structural pieces of wood to the required length before delivery on site. This en
ables precise cutting and time-saving on-site. Through the process of continuous rationalisation of the pre-cut system, it remains competitive and applicable to any type of design.
A modern example of this efficient pre-cut system is the one utilised in the construction and design of the Maggie’s centre in Oxford by Wilkinson Eyre architects [Figure 47]. A complex asymmetrical geometry forms the basis of the design and structure. The building itself is elevated above the ground in a forested area on a series of splayed 300mm glulam columns, which are clustered in 3 column sets. Each of the column is angled to follow the supporting beam above, while varying in length to match the terrain and positioned to avoid the trees. This led to each column being of varying heights and angles. Specialist software allowed the input of bepoke components directly into CNC cutting machines to prefabricate all the unique pieces. This cutting system provided minimal disruption to the forested right while increasing assembly time..
Panellised construction involves the entire wall or floor components being cut and assembled in factory before being shipped to site. These can be load-bearing and non-load bearing with the detail placed into each panel, varying to the specification. For example, panels can incorporate insulation, openings, weather barriers, services and cladding. Highly energy efficient panels can be produced. The main weakness for this construction are the numerous joints and connections of these panels on site. The quality of these connections affect air and water infiltration. Advantages include minimal finishing to the interior and exterior of walls because it can all be incorporated in the factory.
An interesting example of prefabricated CLT floor and wall panels combinations the H4 apartment complex by Schnkula Architekten [Figure 48]. The H4 does not use CLT in the facades but rather ‘uses vertical timber posts 24cm2 in section,’
compressed against gypsum board, insulation and timber beams. This very simple wall solution creates 12m by 3.2m panels which eliminate the need for comparatively expensive CLT and adhesive use. Being fully clad on both sides and fitted with windows and doors reduced the on-site work greatly with it being erected in only four days. This is a great example displaying how different wooden products can be used in prefabrication to adjust to the brief given.
The most complex form of prefabrication is the creation of complete boxes or ‘modules’ which can be stacked together on site. These are often used for high repetition housing units such as apartments and hotels due to scale and on-site efficiency. These units can be made air-tight and fully fitted with services and cladding. The main issue with this form of construction is the transportation costs, sequencing complexity, structural doubling and the balance between customization and repetitive design. In relation to airtight, mass customizable wood homes and apartments, CLT modular construction seems an ideal strategy to employ in an urban setting.
The Alpenhotel Ammerwald by Oskar Leo kaufmann and Albert Ruf ZT gmbH sets a precedence for ‘growing’ neighbourhoods in the sense that the standard 174m2 module can extend horizontally and vertically [Figure 49]. The 16 meter tall building is stacked with 96 units all constructed from CLT. The interior of these units expose the raw CLT panels of Austrian pine. It provides a warm and domestic ambience without the complication of fake ceilings and additional gypsum board. The voids between the modules are filled with insulation and the services are hidden in a chase between the bathroom and closet. Only the bathroom is coated in a transparent waterproof layer for moisture protection.