Special circumstances notwithstanding, buildings are mostly too big to transport as a single piece. If you’re building anything larger than several hundred square feet or so, your building will arrive on the jobsite as separate components that will need to be assembled. The question then becomes, what sort of components should you assemble your building from?
There are two dimensions we can consider when deciding on components. One is their physical size. Small components are mostly what we think of when we think of traditional construction - things like a single brick or a single wall stud. Large components are things like precast panels the size of an entire truck, or pre-assembled building volumes used in modular construction.
The other dimension is the level of completion - how many services and functions of the final building your components incorporate. Low level of completion components are simple, basic materials - a piece of steel, a length of wire, a sheet of drywall, etc. High level of completion components include multiple systems, such as a wall panel with wiring, plumbing, and drywall preinstalled in the factory.
One style of component isn’t necessarily better than another - they each have their strengths and weaknesses:
Larger components means faster site time (fewer crane picks, fewer pieces to attach, etc.) but higher transportation costs . Very large components hit steep cost increases when they exceed the dimensions of a standard sized trailer .
Large components are generally less flexible in the sort of building arrangement they can accommodate compared to smaller components. Minimizing transportation costs means maximizing how much you ship per truck, which tends to result in box-shaped, trailer-sized modules.
Large components require some sort of plant or factory to assemble them, but require less on-site labor. Small components require a large amount of on-site labor to do the assembly, but can avoid the overhead of operating a plant.
High level of completion components require less site work, but more upfront coordination. Coordination errors that do occur will be more costly to fix - it’s harder to move an electrical switch if you’ve already installed drywall.
High level of completion components, when assembled in a factory, can more easily be built to higher standards of quality , but also require much tighter tolerances - lots of preinstalled services mean lots of pieces that can fail to align properly in the field.
High level of completion components are also less flexible in how they can be used (if your building module has HVAC preinstalled, it’ll be hard to use it for both single family homes and restaurants).
Because choosing a component style involves making tradeoffs, it’s interesting to compare different building systems by the points they occupy on each axis. The graph below compares about 40 different modern and historic building systems - the horizontal axis is component volume (measured in cubic feet), and the vertical axis is level of completion (click to embiggen):
The graph is color-coded - green indicates wood-based systems, blue indicates steel, and grey indicates concrete. Darker colors are systems currently in production, and light colors are historical systems no longer in use. Arrows indicate a folding system, with the arrow pointing to the size of the component after it’s been unfolded. The dashed line indicates the largest object that can be practically moved via road.
Positions are based on product literature, what can be observed from photos or videos, and my own experience. The labels on level of completion are meant to be examples rather than a set-in-stone progression - you may install exterior finishes before installing electrical systems on an exterior wall panel, for instance. These should be thought of as rough approximations for relative comparison, not hard and fast values.
Let’s take a look at a few different locations on this graph, and see the sorts of building systems we find there.
Conventional construction occupies the bottom left portion of this graph - it mostly uses smaller components with a lower level of completion, and has the vast majority of assembly work done on-site. Components for this style of building consist of things like joists, trusses, concrete blocks, and sheets of sheathing - things that are usually following some standards’ body specification, and can be purchased in bulk from any number of suppliers. The epitome of this style of construction is cast-in-place concrete, which is made of small pieces of rebar and aggregate. Conventional wood and steel construction are a little farther along in both directions - they will use larger, prefabricated components, cut to the proper length with attachment points already installed.
This style of construction is often criticized, but it has many benefits:
It can accommodate almost any type or arrangement of building.
Everyone in the industry already understands it, so it’s implementation cost and variance is low.
Different services and systems are mostly decoupled - the design of the mechanical system is mostly independent of the design of the structure. This makes coordination easier, makes it easier to make changes, and makes mistakes less costly.
The decoupled systems, combined with using a widely understood style, means renovations are easier and less costly to make, potentially extending the lifespan of your building.
It requires relatively low overhead for the builder - it doesn’t require any sort of plant or production facility, and most of the labor will be subcontracted out.
The downside is that construction time for these systems tends to be measured in months, as very little of the work has been done before arriving on-site. And there’s limited room for economies of scale or large-scale production efficiencies.
This area contains a few different prefab systems, such as Bone Structure (prefabbed steel components for single family homes) and Sears Mail Order (pre-cut, pre-marked lumber model ordered from a catalog and delivered by mail). These systems have the potential to accelerate certain aspects of the site-work and lower the labor burden, but they don’t fundamentally change how a building goes together.
At the upper right of the graph, we have “traditional” volumetric modular systems. These are room-sized (or larger) boxes that are delivered to the jobsite 70-90% complete - most of their services and finishes will be preinstalled in a factory. They just need to be lifted into place and connected together. Construction time for these tends to be measured in days or even hours (the fastest ones are done in a single day), though that can be misleading - modules have often been under construction for weeks or months prior to installation . The existing players in this space generally use light framed wood framing, while many of the newer ones are using steel.
I’ve labeled a few of the VC funded or high profile companies in this space, and several systems I’ve covered previously. But there are dozens of players operating in this space, all occupying a similar place on the graph. Despite the VC hype for some of these companies, they’re mostly trodding well-established paths, at least with their basic building systems.
The farthest point to the right is “Ready to Move” (RTM) Homes. These are the largest building modules I’m aware of that are built as part of normal operations. Unlike manufactured homes, where modules top out at about 1000 square feet, RTM homes can be up to 2500 square feet. I believe these essentially are only sold in Canada and parts of the northern US, where the lack of density makes it physically possible to move something so large.
Between those two extremes, we have a roughly linear smear of increasingly large components with an increasingly high level of finish, with different companies and systems staking out different spots along it.
Slightly up from conventional construction is the structural panelizers. These folks will pre-build structural panels, then ship them to the jobsite. Wood and CFS panelizers build their panels out of individual studs, and precasters will cast their panels out of concrete. SIPs would fall into this category as well.
Low level panelization generally won’t include any services or finishes - those will still be installed on-site. At the high end, you’ll start to see some pre installation of services or material - panels that include factory-installed windows or doors, for instance. High end panelized precast might include things like plumbing lines preinstalled.
Panelized construction can be significantly faster than conventional - experienced erectors can set a panel every few minutes (this is why you generally see a new parking garage appear weeks or months before the building it will service is done). There’s generally a cost premium associated with panelization, the exception being precast concrete garages.
High Performance Building Envelope
One step up from basic panelization, we find builders that use prefabricated panels to achieve high performance building envelopes. These folks use their factory environment to produce a tightly sealed panel with no air gaps, which gives the buildings very good energy performance. Some of these systems (especially in places like the UK) might have a limited amount of services preinstalled - electrical outlets, gaps for mechanical or plumbing systems, etc.
In the US at least, these systems will be very expensive, and are used mostly on high-end homes.
Kit of Parts
Off to the left of the volumetric modular systems, we have a few high-level of completion panelized systems. These are panel-sized components with nearly everything (systems, finishes, etc.) preinstalled. The hope here is to get the benefits of modular (virtually no site work, higher level of production efficiency), but with much lower transportation cost. Unlike volumetric modular, where you’re shipping mostly empty space, panels can be packed much more tightly - a truck carrying a single 1000 square foot module could perhaps carry the equivalent of 4000 square feet if it were panelized.
The ultimate goal with these sorts of systems is to have a “kit of parts” - a set of basic, standard components that can be combined in different ways to build any number of possible buildings.
High level of completion panel systems are very difficult to implement, more difficult than high LOC volumetric modular. They need to be built to a very tight tolerance to ensure components align, they require a large amount of upfront design investment to design systems that can be installed in pieces, and they tend to have significant added connector expense.
The difficulty is easy to see this from the graph - while there are many, many high level of completion volumetric modular companies (I’ve only listed a small fraction of them), there are very few attempts at high level of completion wall panels. The only companies I’m aware of attempting it are startups, and it’s unclear how successful they’ve been.
We also see a few of the systems that are outliers, and don’t easily fall into any category:
Broad’s original system is sort of a hybrid system - high level of completion panels on top of lower level of completion infill.
The Lustron Home is also a hybrid, and combined high-ish level of completion panels (windows, doors, and electrical pre-installed) with smaller, simpler enameled steel panels for finishing, that could all be packed together on a single truck.
Boxable and Blu Homes are foldable systems, that are sort of halfway between panels and volumetric modular - they are delivered to the site packed together, then unfolded into a finished house.
My theory with these sorts of hybrid approaches is that you get most of the drawbacks of each without necessarily getting the benefits. It’s perhaps worth mentioning that of these, only Boxable is currently in production (and only for ADUs).
Something I’m surprised by when looking at this graph is the lack of “gaps” with respect to component size. Since panels can be packed much more tightly than modules can, I expected this to show up as a space as you transitioned between the two. But progression appears fairly unbroken, moving from very large panels (precast, Broad v1) to very small modules (boxable, Toyota Since, etc.) that are roughly the same volume. Perhaps at the higher end of the panel scale packing efficiency stops becoming a concern, since you’re mostly fitting just 1-2 to a truck.
On the other hand, we DO see a large gap at the level of finish, with little in the space between “building envelope only” and “all services and finishes”. Broad’s initial system seems to be the only one staking out this territory. Presumably once your panels reach a certain level of completion, there’s little marginal cost from adding the rest of the services.
The Future of Construction
Looking at the systems laid out like this, we see lots of blank spaces - lots of spots that haven’t been explored. But it’s not obvious that any of these areas are likely to be especially fruitful.
The bottom right (large, simple components) seems of limited utility, only useful when it’s not practical to make something smaller. Bridge girders, stadium trusses, or anything else that needs to span over a large space would fall into this category.
The center right also seems unpromising, and seems strictly dominated by traditional volumetric modular.
The top middle, the dream of the “kit of parts” is mostly empty - but folks are trying. It wouldn’t surprise me if a seemingly unimportant innovation that makes it easier to achieve tight tolerances, or obviates the need for them, unlocks a lot of progress here.
The top left, highly complex small components - well, it’s unclear what this would actually look like (maybe programmable matter?). Same for the far bottom left (microscopic, simple components), which seems like the zone of nanotechnology. Things that have long term potential, but far from the adjacent possible - not likely to shape the industry in the immediate future.
If you know of an interesting system that doesn’t appear anywhere on this list, or breaks this categorization, I’d love to hear about it.
 - Buildings are mostly empty space, so the larger, more finished your component, the more you’re shipping empty space. Transportation costs thus tend to scale nonlinearly with component size.
 - Systems like Blu Homes or Broad’s new system try to avoid this problem by having modules that can be packed tightly, then unfold once they’re on-site. This can be thought of as trading of transportation costs for additional system complexity and coordination costs.
 - An example of this can be found in the concrete code, which allows you to use lower values for rebar cover for concrete that is manufactured in a controlled, plant environment.
 - This is another difficulty with modular construction - even “large” operations are rarely big enough to have produce more than a few dozen modules a month, so they tend to accumulate inventory if they’re working on large projects, which is expensive to maintain.
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