Some prefabs are well designed, but many aren't, especially when it comes to eco-friendliness. And the worst offenders-the houses shoddily built from cheap (and toxic) materials, that offer poor insulation and give prefabs such a bad rap-are particularly problematic. Indeed, a prefab's impact on the environment is only as well-controlled as the designers want it to be.The good news is that many modern (and expensive) designers seem to be making green a priority. Marmol Radziner, for example, uses recycled steel for its structures, "responsible woods" like bamboo, insulation that reduces energy consumption, and offers a slew of additional features like solar panels, radiant heating and low-VOC paint. Michelle Kaufmann, meanwhile, is pushing for a universal sustainability-labeling standard, akin to F.D.A. labeling on food, so that homebuyers can make more informed, sustainable choices. Also, since prefabs use streamlined design, there is automatically less waste and increased efficiency by virtue of repetition. In traditional onsite construction, an estimated 30 percent of materials are either stolen, water damaged or thrown out, producing tons of waste. In a factory environment, most of this is avoided. In fact, sawdust is often reused as insulation, and any surplus can be saved for the next build.All of these discussions are academic absent the context of how many of these structures are actually being built. A look at the number reveals that prefab housing is still very much a niche market. Below is a list of the most popular prefab manufacturers, and how many houses they've built in recent years:LV Homes by Rocio Romero: 64 since 2004MKD Portfolio Series by MKD: 34 since 2002weeHouse by Alchemy Architects: 20 since 2002Ideabox homes: 14 since ~2006The market right now is largely driven by younger buyers on tighter budgets who are interested both in the architect caché (only about 5 percent of all homes built in the United States are architect-designed), and the sustainable implications of prefab housing. But until major developers commit to large scale projects with prefab at the core, the market-and the associated environmental impact-will remain limited.