The idea of prefab and modular buildings is very appealing. Prefab is defined as factory built components of buildings, while modular is factory built, nearly complete building ‘pods’. Buildings can be put together in a process more akin to an automobile assembly line, in a very precise, controlled setting, with greater quality control. Prefab/modular can take advantage of the new technologies around advanced 3D computer modelling and CNC cutting. It can make the good old days of a crew of grumpy guys pounding nails in the pouring rain look absurd. There has been plenty of positive press on the subject in recent years, but there are a few myths to debunk right away:
Myth #1:–is it better? Maybe. Built in a factory, climate controlled, more precision, safer work environment–that all sounds good. But the person manning the assembly line probably can get by with less skill and experience than an experienced tradesperson found on any job site, as they can be trained to do one task over and over again. So there may be less craftsmanship with prefab. At the other end of the spectrum, a Hundegger can churn out some rather impressive ‘craftmanship’, albeit done with a robot:
Myth #2–faster: buildings go up ‘in just a few days’ according to some promo material. This is completely absurd. There is a huge amount of prep work on any site that happens before prefab shows up, including site clearing, excavation, site services (sewer, water, power). The prefab components themselves take plenty of time to put together in the factory, but most of this is hidden from the building owner. And then depending on the process, there will be some time, and perhaps a lot of time, to complete the building on site. But, there will often be time savings. Even speeding the process up by 10% is very valuable.
Myth #3–cheaper: It isn’t. There is often another layer of people and process to build into the equation. It will likely cost as much or more than conventional construction. This may improve with economies of scale, particularly with highly repetitive building types like multi-family housing.
Myth #4—greener: there is almost certainly less waste in a factory, and more chance of recycling the waste. Prefab is not inherently greener though. A high performance green building can be done prefab or custom built on site. And a ridiculously over the top trophy house can be churned out of a factory.
Here are some of the pitfalls:
Logistics become incredibly important. Rather that delivering construction materials one load at a time to the site, large assembled pieces arrive all at once. Because the elements are so much larger, a crane is often required on site, and crane time is very expensive, especially on remote sites. Coordinating the crane availability with the trucking schedule and erection crew timing can be very tricky. Also, if the site is difficult to access—because of a winding road, or steep grades, or overhanging trees/power lines, getting large loads to the site may not even be possible. Also, the sizes allowed to be transported by road (or ferry) dictate the largest size that can be fabricated. This can put some serious limits on what can and can’t be done.
Modification on the fly is discouraged and can be awkward, expensive, and in some cases nearly impossible. With conventional stick framing it is fairly normal to make modifications as work progresses, since the system is fairly flexible. For example, if you want to move a window rough opening, it can usually be done without too much effort. Moving a window in a modular building would involve tearing out finishes, insulation, siding, etc. Basically it wouldn’t be done, unless absolutely necessary.
At the economical end of the spectrum, modular buildings often look very boxy and clunky and they are unlikely to be winning many design awards. There is still a leftover aesthetic from Atco trailers and work camps that is hard to completely escape from. At the high end, prefabrication can enable spectacular results that would be hard to do on site.
Personally, I think prefab wall and roof panels has the most potential, particularly for high performance buildings like Passive House. One way or another, we will be seeing a lot more prefabrication in coming years.
The links below show a range of companies working in this realm.