The conclusion we must come to is that evaluation is very difficult technically, and may well have to contain subjective elements. That opinions, as well as facts, matter in evaluation is a point of great importance The environmentalist message would indeed be bleak and unattractive if unassailable logic ruled. It may prevent us from falling into the trap of becoming self-righteous environmental puritans. For simplicity, I will leave these out of the reckoning and concentrate instead on technically quantifiable effects. All energy use is polluting, and most energy available today is non-renewable.
Energy efficiency is all important, giving major savings to the consumer.
Materials may come to the manufacturer already highly processed. Sheet metal has been mined, smelted, alloyed, rolled, finished, packed and transported before it arrives on a production line. We can no longer ignore impacts that have happened elsewhere. Use of fossil fuels will contribute to the greenhouse effect wherever they are used. If a natural product is being used as a raw material, ask if it is being harvested sustainably.