The bigger the R-value number, the thicker and heavier it will be. Also, your climate and geographical location will help determine which R-value to use. Different r-values provide more or less insulation. So, choose the best attic insulation for your specific needs.
Once you have figured the number, the next step is to decide which type is the best attic insulation for your project. Your basic styles are blankets, loose-fill spray, rigid, and reflective insulation. All of these make great attic insulators but some may not be available in the R-value that you need.
My personal best attic insulation is the Loose-fill (blown-in) or spray application. It comes in cellulose, polyurethane foam, rock wool and fiberglass. Not only is it great for unfinished attic floors but also it is good to use for hard to reach places. Another advantage is that you don't have to touch it for application. This type is either sprayed in or blown-in by special equipment. Although if you are more of the do-it yourself type and don't like equipment than you may prefer to use the blankets or reflective insulation as your bet attic insulation choice.
Best Attic Insulation
This is a breakdown of the best attic insulation and the materials from which they are made:
Blankets, which come in batts or rolls, can be made out of fiberglass and rock wool. These can be cut or fitted to lay in your attic or floor. They also are available with or without vapor retardant facings.
Loose-Fill (blown-in) or Spray-applied is made from one of the three: rock wool, fiberglass or cellulose. If applied right it can give extra protection against air access.
Rigid Insulation can be extruded polystyrene foam (XPS), expanded polystyrene foam (EPS), polyurethane foam or polyisocyanurate foam. These though low in weight provide great thermal and acoustical insulation.
Reflective Systems are mainly made from foil-faced paper, foil-faced polyethylene bubbles, foil-faced plastic film or foil-faced cardboard. This type is very good for reducing downward heat flow.