I'm sorry to contadict previous answers, but it completely wrong that to make your house greener you have to spend a lot of money in it. This is just a small share of the whole picture, even if it's the one that people giving LEED certificates prefer. Greener houses correspond mainly to SAVING MONEY.
Let me give you just some examples.
WINDOWS: When you see an "environmental friendly house", you usually see large windows, entire walls made of glass. And LEED certificates, for example, give you extra points if you make these windows "energy efficient" with double layers, fancy gases in the between, etc...
However, this idea is wrong, and comes from the fact that green OFFICES are designed to have large windows because they save money in this way, because people stay in the office during the day when the sun shine, so you need less lighting in the interior if windows are large. However, the main energy consumption in houses is HEATING. And it is not even comparable the thermal insulation of a brick wall with the thermal insulation of fancy expensive argon filled windows!
For this reason, apartments consume way less energy in heating than environmental friendly houses in the middle of a wood! Do you know that the energy consumption of Manhattans is the lowest in the whole US? Both for households and for transportation! The gas that every Manhattan uses for transportation corresponds to what an AVERAGE American used to use in the 30s!
Another quick example. In Europe nobody has a dryer at home. We all use "solar powered dryers" :) That is, we hang our clothes outside! However, for some reason, in Southern California, where the sun shine 360 days per year, everybody has a dryer. Stop using your dryer and you will save both money and the environment.
When tall skyscrapers in the financial district are defined as "green" because they have solar panel on their walls, this is just advertising. They are indeed "green" because:
- 95% percent of the people going there are forced to move with public transportation and elevators -- no cars
- the shape of the building (small room/volume ratio compare for example to supermarkets) make their heating/cooling system highly energy efficient.
I strongly suggest you the following book about this.