When it comes time to insulate, frequently the choice comes down to some sort of foam insulation (like Icynene) or dense pack cellulose, between the two they each have their advantages and disadvantages.
The plus of foam is that it is a great air sealer it keeps out all air infiltration. Cellulose is a moderate air sealer, so you have to air seal (caulk or weatherize) your walls/attic/basement before you put in the cellulose.
Foam: one step process.
Cellulose: more than one step.
The advantage of cellulose is that it generally a lot cheaper than foam. Also, certain rebate programs only pay for cellulose.
Another point: cellulose is the “greener”, more environmentally friendly process. Cellulose is made from recycled newspaper treated with Boric Acid; foam is a petroleum based product that takes 10X the amount of energy to create than cellulose.
It practical terms, this means that if the surface you are insulating has a lot of penetrations in it (like a roof which has shingle nails, stack pipes, vent pipes) you are better off foaming as trying to fill all cracks and holes would be tedious and expensive. If, however, you have tight construction, go with the cellulose and save some money.
For more information on the subject, go to our insulation pages or watch our Whiteboard Video on the subject: