Guide to Planning Your Modular Prefab Home
Is a modular home right for your project? Here we will walk through the feasibility, the design and planning requirements, and the deployment options for modular homes.
A prefab home uses off-site construction to minimize the impact of construction activities on-site. The dream of prefab is for homes to be manufactured on framing tables or utilizing automation, in a clean and dry environment, and erected rapidly on-site before some weather happens; at lower cost, at higher quality, and with better quality control.
A panelized prefab home utilizes wall panels, and possibly even prefab floor or roof trays, to rapidly erect the home. These panels may be structural panels only, consisting of framing and sheathing. Or, they may be structural insulation panels (SIP’s), containing an inner and outer layer of sheathing bonded to an insulated core of solid rigid foam. These are common options. There are even more complete building packages that include pre-installed windows and doors.
A modular prefab home is a box, 15 feet wide and up to 65 feet long, that comes out of the factory as complete as possible, with interior and exterior finishes. A modular home may be designed custom, with custom finishes. Each box is conceived as a component of the overall design, so boxes are engineered to stack or sit side-by-side.
Although historically, American prefab has been largely contained to a 5% market share (in rambler-style developments or manufactured homes with limited options) prefab already dominates in many parts of the world and offers better quality and more customization. I believe it is the future of building construction.
I have designed or permitted over a dozen prefab homes, both modular and panelized, and feel they are an exciting area of design innovation. Prefab homes require more coordination and up-front planning, and they require the designer to respect the natural limitations of the system. But I believe there is untapped beauty and opportunity in prefab.
Once a house is built, it really doesn’t matter how it got there. But prefab has the following potential advantages to the occupant:
After permits have been obtained, prefab construction may proceed simultaneously with site work, saving time on the overall construction schedule. SIPS panels will speed up on-site construction by speeding up framing, sheathing, and insulation. A full modular build can represent the most time saved if the fabricator’s schedule coincides with the site schedule, and the modules can be fabricated in a time-frame equivalent to the site preparation.
Less Site Activity
Construction activity has the potential to disturb wildlife, neighbors, and occupants on site. Consider modular construction when there is value in 90% of construction activities remaining out-of-site, out-of-mind. Consider panelized construction to accelerate the on-site schedule to minimize impact.
If your project will require contractors to travel hours to a remote job site, or pay for temporary living accommodations, consider a full modular build. If local contractors are unavailable or have a long wait before construction can begin, modular construction may accelerate the entire project.
Durability and Air Quality
On-site projects that are not properly scheduled for dry-season construction may be rained on, which will place moisture and microorganisms in the building cavities. These homes are more susceptible to mold growth, although this also depends on the quality of construction and how the builder or designer has prepared for vapor transport through the wall. A factory built home is protected from the elements during construction.
The Fun and Cool Factor
Modular construction can lend itself to design styles that are exciting, architecturally. Although some modular homes intentionally look traditional, many custom modular homes exhibit a style that allows you to see how the parts add up to make the whole.
Having a home delivered, wrapped like a smart phone, is also a unique experience that is exciting to many homeowners. On delivery day, have a neighborhood barbecue.
In my experience, the cost of a custom modular home is not significantly different than that of a custom site-built or panelized home, given the same quality and level of specification. Custom modular fabricators are usually smaller shops that don’t have the economies of scale of a larger production builder, that may focus on standard models for large developments.
The ‘soft’ costs of design and permitting should be only marginally higher due to the need to provide mechanical, electrical, and plumbing drawings, and submit a permit to the state. The fabricator may also choose to develop shop drawings for the project.
Cost savings in the project should center around the accelerated schedule that can affect whether a family continues to pay rent on another property, or whether the unit becomes available to rent out, sooner.
If you are thinking about going prefab, let’s consider some of the feasibility concerns:
A panelized home should be accessible to a boom truck and a small crane or telehandler for setting the panels.
A modular home should be accessible to a large crane, which must also have access to the parked module and an overhead swing radius to the building site. Overhead power are an obstacle. Tree canopy can also interfere. For sites that are far removed from the street, a larger crane may do the trick. I have seen a backyard cottage set by a crane that reached over the top of neighboring homes. It filled me with no small amount of terror.
A modular home may also be roll set, or slide set, or drop set, depending on the layout of the foundation and the site access. These are more feasible for one-story structures, although in rare cases hydrolic jacks may be used to elevate the second-story module.
Is your site in a region with difficult access from major highways? You may need the truck driver to test the route and make recommendations for module length and height clearance.
Are there at least a couple modular fabricators in your area? If not, you may be paying for hundreds of miles of transport fees. The cost may be justified in markets that have very few local contractors, or the cost of building is very high.
prefab design guide
length, width, height
with whom do I contract?
Should I contract with an architect, design-builder, or design-fabricator?
I advise clients to contract directly with an architect. In this relationship, the client has an unbiased advocate. The architect can educate the client and help them to document, in the contract, expectations regarding the schedule or specifications of the project.
On the other hand, no one likes competitive bidding as it requires a tremendous amount of work on the part of the builder, and subtleties may be easily concealed in estimates. The best situation is when a builder is involved from an early point in the design process and can participate in decision making while estimating the project.
Should I contract with a general contractor or a prefabricator?
Some shops will offer full-service prefabrication and site construction. In my experience, these shops are experts in prefabrication, and subcontract the site work to a site general. In my opinion, this is backwards. A client should hold a contract with whoever is the Last Person On Site (LPOS).
Every mistake, omission, or code violation that occurs throughout the life of the project is the responsibility of the LPOS to correct. Fabricators who specialize in one part of the production should be subcontracted to the LPOS, and this helps to maintain the flexible path. Severe schedule violations can result in a project derailing into another means of fabrication, or even a stick-built solution if needed. At the end of the day the LPOS has all options at their disposal for deploying the project, and contracts with prefabricators are business-to-business. This maintains a flexible path through deployment of the project.
What does airmod mean by “flexible path?”
Our approach to prefabrication uses a flexible path. The project is designed from conception to be friendly to prefabrication, often both panelized and modular. When the project is nearing the end of design, a choice can be made based on a number of factors: how the design has evolved, a trusting relationship with a builder or fabricator; or a competitive bid between two options, such as a modular fabricator and a SIPs builder.