There will be many different opinions on this and every builder has his (or her) preference.
Personally, I think cellulose insulation is the "greenest" choice.
- It is made of recycled material.
- R-Value per inch is comparable with all other insulations for the most part.
- Typically is only treated with borate so very safe, fire resistant, insect resistent with very natural ingredients.
- Fairly easy to install and pretty easy for the builder to do quality control.
- Has been around for a very long time and has a proven track record.
- When installed properly can really reduce air infiltration/exfiltration as well.
There are many types for all different installations: dry dense packed, damp spray, etc.
As far as price goes it is towards the bottom of the range so also very affordable.It has the best value and best ROI of all other products.
For more information:
Read our insulation backgrounder "Choosing the Best Insulation Delivers Energy Savings" for more details on cellulose, fiberglass, etc.