Volumetric, panelized or kit-of-parts: which offsite method fits your project

"Offsite construction" covers a lot of ground. A hotel built from finished boxes and a house framed with factory-built wall panels are both offsite projects, but they are very different to buy, ship and build. Picking the right method early shapes almost every later decision.

Volumetric modular

Volumetric modules are three-dimensional boxes, built in a factory with floors, walls and ceilings. Many leave the plant with drywall, finishes, plumbing and even fixtures installed. On site, a crane sets them on a foundation or podium and crews stitch them together.

The appeal is how much work leaves the jobsite. McKinsey's report describes on-site work for 3D modules as essentially assembling 3D modules on site and connecting services, and notes that one five-person team can set up to six modules a day.

The trade-offs:

• Shipping. You are trucking mostly empty space. The same McKinsey report estimated 3D module shipping at roughly five to six times the cost of flat panels per square meter over the same distance.
• Size limits. Module dimensions are set by what can travel on the roads between plant and site. That shapes room widths and floor plans.
• Layout. Volumetric works best for buildings made of repeated, room-sized spaces. Large open floors are harder.

Best fit: hotels, apartments, student housing, dorms, clinics and small commercial buildings with repeating rooms.

Panelized

Panelized systems build flat wall, floor and roof panels in a factory. Panels can be open (framing only, finished on site) or closed (insulation, sheathing, windows and sometimes wiring installed). Structural insulated panels, or SIPs, are a common type: a foam core between two structural skins.

McKinsey points out that panels are much easier to transport than bigger 3D modules, and that components for several rooms can fit in a single standard container. That makes panels a good match for sites far from the plant. The report also notes that panels give more design flexibility, which suits open-plan offices and custom homes.

The trade-off is more on-site work. Someone still has to erect the panels, dry in the building, and finish interiors and MEP systems on site.

Best fit: single-family and townhomes, custom or varied designs, projects far from a module plant, and buildings with large open spaces.

Kit-of-parts and components

A kit-of-parts approach standardizes components rather than whole rooms or walls: framing members cut and labeled by machine, trusses, pre-assembled MEP racks, bathroom pods, stair units and similar pieces. Light-gauge steel roll-forming machines fall here. They turn coil steel into labeled studs and joists sized to the drawing, ready to assemble.

This approach moves less work off site than volumetric, but it is the most flexible and the easiest to ship. It also lets a GC or framer bring some factory precision in house.

Hybrids are common

Many real projects mix methods. McKinsey describes a common hybrid where wet areas are manufactured as bathroom pods, while the remainder of the building is made from 2D panels. Another frequent combination is a site-built podium with volumetric modules above it. The Marriott AC Hotel tower in New York, for example, was announced with factory-built guest rooms and a lobby and restaurant built with traditional methods.

Codes treat them all as buildings

One thing doesn't change with method: the building still has to meet the code adopted where it stands. The ICC and the Modular Building Institute jointly publish standards that apply across methods. ICC/MBI 1200 covers planning, design, fabrication and assembly, and its foreword says it includes componentized, panelized and modularized elements but does not apply to HUD manufactured housing.

Questions that point to the right method

• How repetitive is the floor plan? More repetition favors volumetric.
• How far is the nearest suitable plant, and what roads lie between it and the site?
• Can a crane reach every module location on your site?
• How much do you value finishing on site to adjust details?
• What does your local building department already know how to inspect?

If you are unsure, talk to one supplier from each camp before you finish schematic design. Their questions about your project will tell you a lot about which method suits it.

How each method changes your team

The method you pick also changes who you hire and when.

With volumetric, the manufacturer becomes one of your most important partners, and they need to be involved during design. Your GC's role shifts toward site work, foundations, setting coordination and finishing the joints. You will also need a hauler and crane company early.

With panelized, the framer or GC stays at the center of the project. The panel supplier works more like a major subcontractor. Delivery is simpler, and the site crew does more of the work.

With kit-of-parts, the factory may even belong to your framer or GC. Roll-formed steel members, trusses and pre-assembled MEP racks can be made in a shop close to the site or in a trailer on it. That gives you factory precision without handing the schedule to a distant plant.

A simple way to decide

If most of your building is repeated rooms and a plant is within reasonable trucking distance, start with volumetric. If your design is varied, your site is remote, or you want more control on site, start with panels. If you want to improve an existing site-built process without changing your delivery model, look at components first. Then test the choice by asking two or three suppliers to price it.