Materials - What makes it green

To really understand the impact of a material you need to understand Life Cycle Analysis (LCA), which looks at all the impacts of a product from the raw materials that it is made of, through manufacturing, its use during its lifetime and finally its disposal.  Even if manufacturers were upfront about what went into a products, there are so many variables involved with its use and disposal (whether recycling, reusing or throwing it out), that generating any kind of hard numbers is quite difficult.  Of course there are always averages, or reasonable expected outcomes, and it can certainly be argued that making some assumptions is better than have no information at all.  Until better information is available, a good rule of thumb is "buyer beware", because no matter what a manufacturer claims, many products just haven't been around long enough to know how they will perform in the long term.

Further complicating life cycle analysis is how the material is used, in particular whether the materials are assembled in a way that they can be taken apart and reused.  Unfortunately, the general trend in construction is toward more difficult reuse, especially with the use of spray foam for air sealing, sub-floors being glued down etc.

Just selecting a green material doesn't guarantee that overall result could be considered green, because there is an aspect of how much you use also.  In this sense, small home advocates will argue that no matter what material they are using, the impact is less, because they are using less of it (see the design section for more on this).  While the square footage of a home isn't an accurate indicator of how much material is used in it, it still is the case that using less material is often the biggest impact you can have.   Beside designing a home to be just the right size, two other techniques have been used to reduce material consumption: design to minimize waste and using structural materials a finish surface.

Designing to minimize waste can involve a lot of things, some of which are fairly easy to do and others are more difficult. The simplest thing is to plan for the use of materials up front as much as possible: you often can make it so that if you cut a pile of wood a certain way, many of the short pieces are also useful.

In the big picture, the motto is the same as anywhere else: "Reduce, Reuse, Recycle".

What Makes Something Green?
Determining whether a product or material is "green" has been a challenge ever since the first time the question was asked, and continues to be one, but various people and organizations have developed guidelines to help sort it out.  The general rule is that a product has to have a significantly better environmental impact than whatever the commonly used product is, and can't have any really terrible impacts.  Of course these decisions are subjective, and some find them relatively arbitrary, but in most cases there is fairly common agreement as to what is green. The principles used to determine whether a product is green, are generally as follows:

First, the product must be made of materials that at least have the potential to be used sustainably.  This implies that the product is either reusable, recyclable into the same end product (not one of lower value), or can be harvested sustainably.  Products that have a greater longevity are generally preferable, as they are generally closer to sustainable use than shorter lifetime ones, that tend to end up in the landfill.

Second, the manufacture, use and disposal of the product must not release significant toxic byproducts into the environment.

Third, a product which uses less energy in creation is preferable. This energy (called the embodied energy) is a part of the final product as much as the other raw materials that go into it.

There is a wide variety of strategies to satisfy these principles, and any strategy that keeps materials from ending up in the landfill is likely to be at least a very good start.   The old slogan "reduce, reuse, recycle" applies to construction as well as anywhere else.  The first question to ask is whether you need to use the product at all ("reduce") .  The next question is can you reuse a material that have been used before.  The key to reuse in most cases is find products that can be used with minimal processing, although some materials (e.g. an quarter-sawn solid oak door, or other finish woodworking) have high value and are worth the time and money to restore.

Ruling those option out, using a recycled product is an improvement over using new because you are adding another lifetime to a material before it ends up in the landfill.  Unfortunately, recycling often results in a different product than the original, and often that recycled product itself cannot be recycled, for example dimensional lumber ends up as particle board and can never be dimensional lumber again.  Because recycled products are also often a conglomerate of materials, there is an additional barrier to recycling because you typically have to separate out the components to recycle them (in the case of particle board, you'd have to eliminate the glue to recycle the wood particles, or alternatively separate out the glue to use as a raw material). Until there is pressure and/or financial incentive to do so, there is little hope of developing the technology to recycle all products, and construction "waste" will continue to fill landfills everywhere.  Obviously any process that works by separating components while its recycling is likely to be better than one that requires separation as a separate step. 

For a material that is renewable (wood, straw, etc), the strategy is usually to avoid over harvesting it, reuse it however possible (even if its in a lesser material), the let it biodegrade when it is no longer reusable.

Some inorganic materials (cement, metal) are easily recyclable provided you can separate out the material you want from what ever it is bound up with.  In the case of cement, its most often found as concrete which is only about 15% cement, although concrete chunks themselves ("urbanite") can be reused. For metals, its a mater of stripping away whatever wood, plastic or other metals to get at the one of interest.

Other inorganic materials (brick, stone, tile) could be reused as is for a few lifetimes, if  you can separate them from the cement or glue that are set in.  Polished stone slabs can also be reused, but since they usually have holes cut in them (sinks), there is less flexibility in subsequent uses.

As a rule of thumb, the more components a product has, the harder it will be to recycle.