Modular housing, in its most cost-effective form, compromises good design for economy. Trailer-shaped boxes are bolted to a foundation, hooked up to services, and moved into-not your typical buyer's most romantic idea of home. This is part of the reason that, despite its advantages, acceptance of modular construction has been gradual. Also, if factory-built housing could be made to look more like traditional housing, builders could have a more competitive option, perhaps making it worth the change from tried-and-true methods. One can imagine house modules with site-specific embellishments, such as bays, sunrooms, sheds, porches, garages, and trellises. These components could be produced by the same factory and shipped together with the boxes. This is the idea behind hybrid modular construction.
Modular homebuilding has always relied on some degree of site work. Much of the siding is installed after the units are delivered, and steep roofs are tilted up at the site and supported with panelized gable ends. But most modular housing is limited by the dimensions of delivery trailers, which most efficiently accommodate a rectangular box of 14' wide, 66' long, and 13'-6" high. In addition, the units must be able to withstand the dynamic stresses during transport of braking, acceleration, and turning. Many completed designs reflect this, visibly conforming to the boxy proportions of the delivery concept. Designs with projections, cantilevers, and jogged plans may become less economical to deliver than a site built or strictly panelized approach.
Customized Structures, as featured in PATH's "Dove Street," regularly combines panelized additions with the modular units in its standard line. The site-installed components may consist of porches, garages, and projections in the floor plan with jogged roof sections. Tilt-up roofs, used for steep pitches, are folded down to clear overpasses during transport and hinged up on site.
Apex Homes of Pennsylvania builds 750 homes a year and, according to Apex, "never the same home twice." Although its hybrid modular/panelized approach is adapted to the many expensive homes with complex floor plans that it builds, the company produces the affordable "Alpine" series. These 800 sf to 1,600 sf ranch house sections with a 10-year warranty can be delivered to a foundation for less than $40,000. (Sitework and some finishing work not included.)
Sterling Industries also caters to a pricy market but can substitute materials to build an affordable home. Sterling reports they are considering working with structural insulated panels (SIPs), although they predict a cost premium for the durable and energy efficient panels compared to stick built panels.
The scale of certain design features, such as atriums or two-story living rooms with balconies, do not lend themselves to shipping completed sections, and are often panelized as part of a modular home. Another option in the hybrid modular market is the use of core modular units. In even the most complicated designs it may be worthwhile to modularize at least the bathroom and mechanical core (room with furnace, water heater, electrical closet, and blower). Complex roofs and second stories can be site-built above modular sections. Del Webb Corp., in the early years of its Sun City Grand project near Phoenix, built panelized houses around modular cores over slabs-on-grade. Also, multistory buildings using modular cores with back-to-back plumbing can yield considerable savings.
The modular core concept was combined with a steel-framed, folding envelope by Innovative Building Systems (IBS). By hinging the floor, the shipping size of a section could be reduced to 8'-6" wide by 13'-6" high. The resulting unit erects faster than a strictly panelized structure but takes significantly longer than a complete modular section. The upside is economical delivery of the compact packages.
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Installation of energy efficiency technologies is imperative for proper performance. Factory production of homes systematizes this process and can, therefore, help ensure energy efficiency measures perform as intended.
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Factory construction often reduces material waste. It also decreases the number of site visits and, hence, auto emissions associated with trips to the jobsite.
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Because a large portion of modular and panelized housing is built in a factory, quality control is facilitated.
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Factory panelization of some components offers a compromise between site building and full factory assembly. Site building elements that do not conform to box delivery, such as garages, and some bays and dormers, can add a significant amount of work to a job. By utilizing advanced facilities with two-jig systems, factory panelization is faster and easier than site panelization.
Garages are most economical to build as panels bolted together on site. For all but the largest modules, there is usually enough transporter area to fit framed panels either side-by-side or front-to back with the modular units. To prevent damaging vibration, assemblers use various methods to temporarily affix the panels to the modular boxes. Often they are strapped on. Alternately, for large or oddly-shaped pieces, temporary 2 x 12 joists and brackets are used to hold the units together. Prefabricated dormers are sometimes tucked into a living room or large bedroom to save truck bed space.
In conventional modular, steep roofs are often designed to hinge up on site. However, these are often framed with 2 x 6's and 2 x 4's, limiting the amount of insulation between rafters. These roofs often require additional pieces and labor. Hybrid manufacturers sometimes find it more economical and energy efficient to truck roofs in separately from the boxes, especially cathedral roofs or those framed with 2 x 12's for thicker insulation. Depending on the size and configuration, the roof is delivered in one or several units.
Hinging the floor to compress the shipping size of the unit, as in the IBS system, addresses the issue of just "shipping air" in a cavernous box. In its folded position, the shell still has room for roof trusses and other components, reducing the number of transporters needed. Set by crane, the "roof" of the structure folds down to become a wall, the telescoping floor joists are expanded, and roof trusses are installed. IBS can detail the system with standard infill steel stud walls or to accommodate a SIP envelope. Ray Kavarsky of IBS is looking into licensing the patented folding modular design for inner-city factories. Since the units are delivered without installed roofs, they may be appropriate for multi-story housing, with a steel frame that can meet fire codes. The narrow shipping size is economical also for remote or hard-to-reach areas.
Not all designs are most cost-effectively built using hybrid or modular systems. Few manufacturers can produce highly customized designs more cost-effectively than site builders, and low-end stock designs may not be flexible enough to accommodate real-life program/site conditions. Factors such as local labor and site accessibility may preserve site building as the most viable option for certain projects.
Some manufacturers don't have a design/drafting department. Upfront architectural fees reflecting site/factory coordination issues for modular projects may seem high, though they represent a small fraction of project cost. Also, public perception of factory-built housing and architects'/builders' lack of knowledge of manufacturers' capabilities continue to limit its market. Some of the public confuses modular construction with manufactured or "mobile" homes and may be ill-informed as to its quality and durability.
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Like conventional site-built construction, all modular housing must comply with codes in the region in which they are delivered, unlike manufactured HUD-Code housing, which is built to a separate, federally-administered and -enforced code.
One advantage of the IBS foldable technique is the avoidance of bureaucracy in transporting. By collapsing units to the size of a standard trailer, the builder can sidestep the added expense of permits, police escorts, flag cars and drivers, and bridge crossing restrictions. This is especially useful when crossing state or county lines.
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At $23/sf (excluding foundation, sitework, land, and soft costs, which are typically not included with a modular package), Apex "Alpine" series homes are actually price-competitive with HUD-Code manufactured homes. Part of the savings comes from optimum value engineering 2 x 6 wall studs at 24" o.c. This, together with insulated headers, helps bring the base product to a claimed 25% to 30% higher energy efficiency than most codes require. Customized Structures can deliver its units to your foundations almost 90% finished for $37/sf to $43/sf, though they come closer to $65/sf with higher-end materials. Customized is known for high-quality details and familiarity with durable and energy-efficient construction.
Sterling Building Systems of Colorado serves an exclusive market of mostly ski vacation homes, but can produce a product for $45/sf to $48/sf by using hollow core doors, veneers and painted trim instead of more expensive wood, and similar substitutions. Sterling claims they can build a custom design for 25% to 30% less than stick-built in their resort area. Their past work with Habitat for Humanity got them into the affordable market, producing employee housing for resort areas and working with state grant money to build housing for people earning 30% to 80% of the area's median income.
But the advantages of hybrid modular come more from scheduling than first cost. Because panels and boxes can be built while the site is prepared, the construction cycle can be half that of an entirely site-built house. Savings come in reduced construction loan interest, and land that generates income sooner.
Many builders decry a shortage of skilled labor, especially during booms, and attribute it to reduced enrollment in trade schools; would-be tradespeople are less than thrilled at the prospect of unpaid, lost time due to weather or coordination delays. Because factory work moves on-site time to plant time, construction can occur year-round and more independent from scheduling delays. Building houses in a factory also has the added benefits of a controlled indoor climate, reduced commuting, convenient facilities, and accessible tools.
Much construction litigation results from conflicts with subcontractors on site. Because most subs, hired on a per-job basis, are not committed to other subs, there is less incentive to perform quality preparatory work. In contrast, factory operations encourage accountability and well-defined trade responsibility. All systems are installed by the same extended team as a common effort.
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