Photo Credit: Living Homes (via Inhabitat) Bob Ellenberg wrote a good, thought-provoking (and discussion-starting) article at Inhabitat titled 'Prefab Construction: Green or Greenwashing?' and drew comments from Preston Koerner (of Jetson Green) and Lloyd Alter (an architecture writer at Treehugger with whom I had some inter-blog discussion over the past couple of weeks regarding foundations, but more importantly also an entrepreneur in prefab construction with direct experience in the process).
Prefab is a popular concept in green design circles and shows up regularly on a number of blogs. A few of the more prominent examples include: Inhabitat (Pre-Fab Friday); Jetson Green; Treehugger; BldgBlog; MoCo Loco; and even a website devoted to prefabs: FabPrefab. But it's a valid question that is being asked. How "green" is prefab building, and should it be embraced by those who want a greener building? Bob sums his article up this way: "I want to honestly question what is and what isn't 'green' about prefabrication and encourage others to do the same."
Prefab construction can be very green. The LivingHomes prefab illustrating this article is a LEED Platinum building. But, there are very few examples of prefabs that have LEED certificaion. And not every prefab qualifies even as a LEED certified building, let alone a Platinum one.
I'll highlight the points from Bob's article that I want to address, but I recommend reading the whole article.
Material waste/efficient use of materials – Because of the more modular and more engineered nature of prefabs, they often (but not necessarily) use modular dimensions that correspond to standard material sizes (such as using a 2' module to minimize waste from standard 4' wide sheet products like plywood and OSB). Some site-built projects use these approaches as well, but this depends on the designer and the builder, not the construction method.
Over engineering – I think this tag can be hung on site built projects as easily as it can on prefab. There are different ways that materials are used in each kind of construction. Bob's article points out some of the areas where prefabs need to have additional structure for transportation and erection purposes. Once the building is in place, this additional structure serves little useful purpose. On the other hand, for the site-built home, far too often, large, dimensional lumber pieces are used for headers for openings in non-loadbearing conditions where they are completely unnecessary, simply because it is common and traditional practice to do so.
Carbon footprint for transportation (and other uses) – Material delivery from distributed manufacturers to the prefab plant or to the supplier or retailer who then delivers it to the jobsite probably is slightly more efficient on the prefab side. If the prefab manufacture is fairly local to the final instalation site, it's probably beneficial to do the prefab. The greater the distance between prefabricator and final assembly site, the less benefit there is.
The benefits to having workers coming to a consistent work location (the fabrication plant) rather than driving all over the region (as many site-built tradespeople do) is probably a carbon benefit, as Lloyd points out in his comment. The factory, if it is in a cold weater climate, most likely has a heated space that requires additional power. But cold weather construction goes on for site-built homes as well, and 'salamanders' (large propane-fired space heaters) are used to heat these yet-to-be-insulated buildings under construction. I'm not sure ehich way this goes, but it's not as one-sided against prefabs as you might initially think.
A prefab also should benefit from being constructed in a shop environment where tolerances can be much finer. This allows for a building with tighter construction, which should pay benefits in reduced air infiltration, better insulation, more comfort and lower energy use. Exposure to the elements during construction can cause problems with the building later on. Fabricating the building in controlled conditions makes this much less of an issue.
Prefabs may also tend to benefit from a design that needs to take transport of the building into consideration. Though not an explicit requirement of prefabs, I think the general trend for prefabs is to be smaller than corresponding site-built structures. In many cases, the modernist aesthetic of prefabs also contributes to the building being smaller and more efficient. Fine finish is given precedence over raw square footage.
The whole issue of "green-washing" is key. Lots of businesses are seeing their customers asking for green products, whether they are selling soap or shirts or cars or homes. So, whether something is truly green or not becomes an important question consumers are asking. And, with so many competing definitions, and so many tenuous attempts to claim greenness, the question is a valid one.
As with many things that are green, I don't have a clear cut and definitive answer to offer. Instead, I'll say "It depends." (I use this phrase a lot, but it's probably appropriate here as elsewhere.) Pre-fab is a process, not an end-product. Therefore, it can be done well or badly. It's certainly possible to be very efficient and do site built construction. It's also possible to do wasteful and inefficient prefab manufacture.
In and of itself, pre-fab is not automatically "green." When done well, it can be a method that leads to a better constructed home, including one that uses fewer materials and operates more efficiently (meaning less carbon footprint over the building's lifetime, a much larger chunk of its carbon footprint to consider than its construction). In green building, we try to take a look at the larger picture, rather than only focusing on the final building alone. Life cycle issues, and the methods and processes all contribute to making a building green. It's the execution of the concept, and not the idea itself, that makes or breaks a prefab as a greener building.