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Manufactured Housing in the North

Technical Series 94-215
Key Messages
  • Manufactured housing refers to three levels of fabrication:
      - precut and coded materials;
      - prefab components, e.g., floor, wall and roof trusses or wall panel systems, either wood frame or stress skin type.
  • Choice of type of construction will depend on available site labour, materials and on transportation costs.
  • To be transported, modules must be more rigid than would be required on site.
    This additional rigidity allows for: `
      - supporting the structure on 3 or 4 points; - cheaper foundations; and
      - easier levelling.
  • Special care is required with surface foundations to ensure that:
      - the building can move without exposing opened joints;
      - the building will bridge across expected depressions;
      - the building is adjustable to level; and/or
      - uses a 3 point support to avoid racking.
  • Introduction

    Building in a northern or remote region is a challenging and difficult endeavour. The further north and more remote the site, the more challenging and difficult it becomes.

    Manufactured housing systems offer significant benefits in remote and northern regions, given conditions such as:

  • a short building season;
  • difficult and costly modes of transportation;
  • lack of skilled labour and tradespeople;
  • high costs to transport and maintain workers on site;
  • difficulties handling and protecting materials on site; and
  • the inherent challenges and problems encountered when attempting to construct high-quality, climate-resistant buildings in remote locations.

    To lessen the impact of these factors and to facilitate and hasten installation and finishing, companies, agencies or contractors building in the North often use various forms of manufactured (prefabricated) building components.

    Building components or structures can be manufactured to various levels of completeness. The simplest level is pre-cutting, basic preparation and coding or marking of the materials that go into a structure. The manufacturing process can also involve increased levels of component assembly, from truss and panel assemblies, through to completion of full sections of dwelling units, such as sectional or modular housing or mobile homes.

    The level of manufacturing that is appropriate for a particular situation or location is highly influenced by the factors outlined above. Determining the most cost-effective approach requires an economic analysis that will include the availability of equipment and material, labour and transportation costs.

    Prefabricated Components

    Wood-Frame Systems

    A typical prefabricated wood-frame system for northern use consists of stud-framed wall sections. These have insulation installed and protected with exterior grade plywood sheathing or siding on the outside, and plywood, fibreboard or other sheathing material on the inside, but without the final interior cladding. Hand holes at the sides of each panel allow the wall sections to be bolted together. The floor may be precut beams supporting stressed-skin plywood panels or conventional construction with precut joists and sheathing. Precut roof trusses or prefabricated box beams are used to support site-applied roof and ceiling cladding. Much of the work of applying interior finish is done on the site, so careful on-site supervision is required.

    Stressed-Skin Panel Systems

    Stressed-skin designs use light, strong sheet materials as structural skins which are bonded to stabilizing webs to form an enclosed panel. The panels act as groups of I-beams or H-columns. The framed stressed-skin panel offers high rigidity and a strength-to-weight ratio which can only be surpassed by the structural sandwich type of stressed-skin design.

    Structural Sandwich Systems

    Structural sandwich panel design represents a further advance from the framed stressed-skin concept. The structural skins are bonded over their full area to a light core sandwiched between them. The core acts both as the stabilizing web and as insulation. True stressed-skin action is achieved over the whole panel and the structural sandwich allows the highest possible strength/weight ratio with given materials. Skins may be of plywood, hardboard, wood-cement board or metal; honeycomb or plastic foam is normally used as core material.

    Prefabricated Modules and Sectional Units
    FIGURE 1. Prefabricated Modules and Sectional Units Enlarged Image

    Prefabricated Modules, Sectional Units, and Mobile Homes

    Modular building entails factory production of the largest and most complete section of the house that the transportation route and system will allow. This maximizes productivity, minimizes on-site labour, and reduces the need for skilled labour. Freedom of design is somewhat limited, and because modules are bulky and hard to handle, they usually cost more to transport. However, the high factory content allows better quality control. Given careful handling, the better modular units, designed for the North, can allow excellent, permanent housing to be quickly installed in any accessible northern area. Modular units can be shipped by suitable road, or by sea to a port with suitable off-loading facilities.

    Because of the stresses imposed during transportation, the rigidity of a modular unit must exceed its on-site requirements. However, it is easy to make the whole unit into a rigid box by using either a stressed-skin or monocoque (similar to unit-body automobile design) structure.

    The rigid box also simplifies foundation design. The number of support points for a dwelling is usually dictated by the span of the structure, not the loading capacity of the support. When units are made rigid in order to survive transportation, it is possible to support the structure on three or four rather than a dozen points. This has several advantages:

  • greater loading of points which results in less heaving;
  • cheaper foundations; and
  • easier levelling.

    Transportation

    When considering any type of prefabrication for the North, transportation of the components or units is one of the more restrictive considerations. The transportation requirement will affect the weight, size, materials, finish, and strength of the units or components, and obviously the cost.

    Things to consider when transporting components or units to remote and northern regions include the following.

    • Usually only one delivery is possible each year.
    • Cost is often based on volume as well as weight.
    • Lightweight panel systems are usually easier to handle and therefore cheaper.
    • Well designed panels are often used as their own crating to allow compact shipping.
    • Modular or mobile units, while very bulky, can be used as containers for light goods packed inside.
    • Protruding components should be avoided.
    • Careful planning, packaging and labelling are especially important with prefabricated units since it is difficult to improvise substitutes if sections are missing or damaged.
    • In many instances the components must meet handling requirements more rigorous than their building strength requirements.
    • Air transport is much more expensive than shipping and should only be considered when other modes of transportation are not available.

      Siting

      Although various foundation options are available, prefabricated buildings often use surface foundations. These may be treated sills placed directly on the undisturbed ground or on gravel pads. Where the frozen ground is fine grained with a high ice content, adequate drainage away from the site and protection of the integrity of permafrost with ample floor insulation and underfloor venting are especially important. As some differential movement is almost inevitable, the building should be designed so that it will:

    • accommodate movement without exposing opened joints;
    • bridge across expected depressions as a rigid raft;
    • be adjustable for levelling; and/or
    • avoid racking, by using three-point support.

      Condensation

      As with any building in the North, condensation in prefabricated buildings, either within the building envelope (walls, roof and floor) or as surface condensation, can be a major concern. Prefabrication allows the problem points to be addressed consistently and under controlled conditions at the design and production stages, e.g. elimination of thermal bridges, air sealing of the envelope, and provision of proper ventilation/heat recovery. Where this is not done, any building can have condensation problems reduced or eliminated by modest and cost-effective interventions. These interventions can be grouped under two categories:

    • Lifestyle and maintenance activities to reduce moisture generation and improve its venting (e.g., covering pots when cooking, drying clothing and firewood outside).
    • Remedial construction activities or adjustments to mechanical equipment which will improve envelope performance or increase the exhausting of moist air (e.g., a vapour barrier type paint, use of kitchen and bathroom exhaust fans).

      Resources

      Building in the North - Responding to the Environment, Volume 1, prepared by Van Ginkel Associates Ltd., prepared for Canadian Arctic Gas Study Limited, Gulf Oil Canada Limited, Imperial Oil Limited and Shell Canada Limited, March, 1976.

      Energy Conservation Options for Alaskan Mobile Homes, Alaska Residential Energy Conservation Program Technical Assistance Report, Department of Commerce & Economic Development, Division of Energy & Power Development, 1982.

      Condensation in Manufactured Housing, prepared by Tang G. Lee, prepared for Alberta Municipal Affairs, Calgary, Alberta, February, 1987.

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