Well I have just completed insulating my attic after a two year period of researching and heming and hawing. Here is the bottom line of what I have found. Sheep wool: The greenest insulation without a doubt is sheep wool insulation. It is not subject to pests as it is treated with the natural boron. It is naturally flame retardant and has been used for centuries as hearth rugs to prevent cinders from burning flooring. It also keeps its R value when wet. It is very luxurious stuff and without a doubt will give you the warmest house. However it is expensive about 4 times the cost of fiberglass. I wish I could have afforded to use it. Blown in cellulose. It sounds like a good idea but there is a problem with it. It causes a lot of dust in the homes of people who have used it. Having to be blown in it can create quite a mess in the attic. What if you have any new wiring or cabling projects in the future? How are you going to wade through all that mess? It may be a good idea if it is sprayed in wet into wall cavities as that way it can fill up cramped spaces and will stay put. It costs a little less than the generic pink fiberglass. Recycled denim: This would be good if it was easy to obtain. It is also one of the more expensive options. Not as expensive as wool but about half as so. The insulation I went with: I ended up going with Formaldehyde free fiberglass insulation. Aka the white stuff. Here were my reasons. 1. It has no formaldehyde so it is not toxic. 2. It is made of long strand fibers and so is bonded very tightly, is far more resilient than the pink crap and is much less likely to release fiber particles into the air. It was very easy to handle and doesn't collapse like the pink stuff. In fact it was very tightly packed and when opened it expanded rapidly and volumnously. Although, I still used gloves even though it was soft to the touch compared to the formaldehyde stuff. 3. It was made from 25% post consumer recycled glass. 4. And the final but certainly not the least reason I bought it is that it was CHEAPER, yes you read that right, CHEAPER than the pink toxic stuff they have at the home improvement stores by a fair amount. So the best green insulation for those on a budget is formaldehyde free fiberglass insulation. The best for those who can afford it is wool. The problem with polyeurethane like most manmade plastics is they contain pthalates, and those are dangerous to the male hormone system.