Prefabricated buildings

Prefabs

New construction: On-Site-Built or Prefab?

09-14-2021

Prefabricated homes are dwelling types that are partially or completely manufactured off-site in advance. The factory-made components or units are transported and assembled at the building site to form the complete building. In standard construction, the materials are shipped to the building site and are assembled and constructed on-site.

Prefab homes are sometimes called factory-built or systems-built homes and are nicknamed prefabs

 

Types of Prefabricated homes:

Different types of prefabricated homes vary by the amount of work that is done at the factory and how much is left for on-site assembly and construction:

Manufactured Homes

Manufactured homes are constructed completely off-site before delivery. No construction happens on the site. These homes are pre-assembled under a type of code called the Housing and Urban Development code (HUD), which is federally regulated. According to the latest survey performed by The U.S Census Bureau, almost 8,000 manufactured homes were shipped in August 2020, which is also roughly the yearly average. The vast majority is for residential use and is less than 1% of the yearly average of all new residential construction in the U.S. 

Perception-wise, manufactured homes are seen as low quality and of poor design for two main reasons. The first is that manufactured homes were mobile homes/trailers that were rightfully considered not to withstand severe storms and earthquakes. Second, manufactured homes were used as a solution for affordable housing due to their relatively low price. Yet over the years, the HUD code has been modified and became more stringent, as a result, the design and construction of modern manufactured homes have become good contesters to standard housing.   
 

Modular Homes

Modular homes usually consist of modules that are made off-site, which may include plumbing, electrical, doors, windows, and closets. The modules are transferred to the site/land where they are assembled.

In most cases, states have adopted the International Residential Code (IRC), which is part of the ICC (International Code Council) family of codes.

  • Panelized Homes: Panelized homes are built in panels, such as a whole wall. The panels are transported to the building site, in which they are constructed and assembled. Typically, panelized homes require more work on-site, such as painting, installing cabinetry, stairs, and flooring.
  • Pre-cut/Kit homes: The materials are cut to size and shipped to the building site/the kit comes with a detailed list of instructions and parts. The kit home is then assembled by the buyer or a general contractor. 
  • Prefabricated 3D Printed Homes: Dwelling structures that are built with 3D technologies and are shipped to the building site where they are assembled. 

Read more about this type in our dedicated blog about 3D printed homes.

The time of construction varies greatly between the different prefab types and within each type. The general rule is that the more work done in the factory, the less time it takes overall to get to the finish line. 

 

Are Prefabs Resilient?

We are interested in the ability of a prefab house design to fit specific climate zone characteristics as well as withstand extreme weather, including future forecasts due to climate change. 

Modular and panelized homes are often considered safer and better-built than traditional construction. In fact, in a report from 2014, the Federal Emergency Management Agency (FEMA) supports using prefabricated structures as safe rooms. 

FEMA mentions that prefabricated safe rooms can be used as shelters from storms, mainly due to their affordability and use of durable materials, such as metal panels, and concrete.

In our blog post about community resilience, we shared the story of a house that survived Hurricane Michael. While most other houses in the area didn’t fare well in that storm, the house that did is a modular home that was prefabricated and assembled on a foundation built to the local rigid building code.

As mentioned, HUD (the code for the manufactured homes), has been adjusted over the years to be more stringent and better withstand winds and fires. 

Inherently, prefabs hold several advantages that may enhance their resilience compared to standard construction:

  • Prefab homes generally go through more inspections and quality controls in the factory than standard on-site constructed homes, to ensure all phases of construction are according to code. 
  • Prefab homes must be able to withstand the shipping process from the factory to the building site. Therefore, they are usually built stronger than standard construction, with the shipping hurdles in mind.
  • Much of the construction of prefab homes is done in a factory, which is protected from and not affected by weather conditions. This facilitates the construction process and reduces the risk of material damage (wet materials).
  • Any additional resilient features depend on the materials and assembly process used by the manufacturer. 

Note that a crucial component of the structure’s resilience is its foundation. The prefab structures need to be assembled properly on a solid foundation at the building site. This stage is extremely important and will highly affect the property’s resilience. 

Another crucial component is the “stitching”. In some of these forms of construction, units are shipped to the site and are then stitched to one another onsite. The “stitch” becomes a vulnerable point that needs to be properly designed and constructed. 

 

Are Prefabs Green?

First and foremost, new construction has a greater negative impact on the environment than renovating an existing structure, so if you are facing a new construction vs. existing building renovation dilemma, the earth favors the latter option. But prefabrication is mostly a solution for new construction. 

Comparing standard new construction to prefabricated, the prefab homes are inherently greener. The following facts contribute to reduce the impact of prefab homes on the environment:

  • Prefab homes result in less waste because of excess materials, and in many cases, are reused/recycled in the factory, whereas excess materials in standard building construction sites usually go to waste.
  • Building a prefabricated structure in a factory reduces the transportation of subcontractors and materials to different building sites, which ultimately leads to a reduction in carbon emissions.
  • Many manufacturers include energy-efficient and sustainable materials in their selection of modular homes, such as ENERGY STAR appliances, engineered wood, and airtight panels/units. Some prefab manufacturers offer solar panels or energy-efficient water heaters.

 

Cost 

The total cost of a prefab home differs greatly based on many variables, such as the type of prefab, the manufacturer, the size, the amenities included, and the location. 

Generally, prefabricated homes are considered less expensive than standard-built homes. Manufacturing in a factory facilitates production at scale and makes housing marketable, mainly because it reduces labor, materials, and financing costs while shortening the construction schedule. 

Research shows that at a high scale, prefabs can generate 20% savings compared to standard homes. Yet, that is not always the case. Modular homeowners estimate the average basic cost per square foot of a modular home (not including the cost of land, taxes, fees, and exterior additions such as landscaping) is $110. They also provide a price break-down page that shows the total cost of an average 1,780 sqft residential modular home, and the cost of each segment. Modular homes also provide a very similar price breakdown. 

When considering the cost of a prefab, keep in mind that it usually doesn’t include additional expenses, such as the cost of the land, the foundation, connecting utilities, and contractors Choosing a sustainable prefab home, which for us means both resilient and green, usually means it will cost more than $150 per square foot. We aggregated a few cost estimations of prefabs by manufacturers that we consider to be sustainable:

Manufactured housing

 

Other Considerations

Not all prefabs can be built anywhere. Your local codes and permits may allow certain types of structures and forbid others. Make sure the desired structure can be delivered to and accessed at the building site. Consider whether the prefab of your choice requires a specific foundation type, and if so, can that foundation be built on your land?

Some prefab manufacturers provide in-house contractors and installers, but most times, you will have to make sure your contractor is familiar with prefab structures. 

 

Prefabs manufacturers we like 

Modular

 

Panelized

 

Kit Homes

 

Final Thoughts

  • Choose the renovation of an existing building over new construction if feasible.
  • For new construction, prefabricated homes are a solid option that can be as sustainable (resilient and green) as standard construction, and when done right, even more so (and sometimes at a better cost). Do your research. 
  • Different types of prefabs vary by the amount of work left to be done on the building site.
  • Each prefab type has many variables among the different manufacturers. Choosing requires good planning and understanding of what each manufacturer offers. 
  • There needs to be a good fit between the location, the land of the to-be property, the access to it, and the foundation on which the property will be assembled based on what local codes allow.
 

STAY COOL. BUILD RESILIENCE. EAMPACT

 

References

MHS Latest Data

Characteristics of New Housing > Highlights 

Taking Shelter from the Storm Building a Safe Room for your Home or Small Business 

Modular Home Survives Hurricane 

Factory-Built Housing for Affordability, Efficiency, and Resilience 

What Type of Inspection Does a Modular Home Have to Pass? 

Construction Matters: Comparing Environmental Impacts of Building Modular and Conventional Homes in the United States 

Modular construction: From projects to products | McKinsey

ModularHomeowners.com: Buyer's Guide to Prefab and Modular Homes 

Price Breakdown For Modular Homes — ModularHomeowners.com 

Custom Modular Homes and Manufactured Homes - ModularHomes.com 

Modular Home Price Breakdown - ModularHomes.com

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