Few things are more important to a building's energy-efficiency than attic insulation. Insulation comes in many materials and R-values. The recommended R-value for an attic is R-30 to R-60, depending on the climate.
An R-value is a measurement of a material's ability to impede heat transfer. Different types of materials have different R-values. The higher the R-value, the more efficient the material. Higher R-values are recommended for climates that experience more extremes in temperature. Generally, the higher R-value materials are more expensive.
Fiberglass insulation comes in rolls, batts, and loose fill. Fiberglass batts and rolls come in R-values ranging from R-11 to R-38. The fill can be blown into an attic to nearly any R-value. Fiberglass is non-combustible. There have been concerns about the health risks associated with fiberglass, but it is considered safe after it has been properly installed and covered or sealed into the attic.
Cellulose insulation is made from shredded recycled newspaper. It contains at least 70% recycled materials. Different types vary in their combustibility and resistance to mold, vermin, and corrosion depending on the combination of chemicals used to treat them. Cellulose insulation is only available in loose-fill form, and can fit easily into small spaces, such as around nails and wires. The main drawback is that that it can settle over time and lose some of its R-value if not installed properly.
Mineral wool, also known as rock and slag wool, is made from molten glass, stone, or mineral compound that is spun into a fiber. Its main selling point is its resistance to fire. It contains an average of 75% post-industrial recycled content. It comes in rolls, batts, and loose fill.
Rigid foam insulation meets in sheets and is installed directly to framing. It comes in "expanded" and "extruded" types. Extruded types are more efficient, but were originally made with chlorofluorocarbon (CFC) gas. Manufacturers have now switched to using the more eco-friendly hydrochlorofluorocarbons (HCFC), and are in the process of developing even more eco-friendly forms.
Vermiculite insulation is made from a mineral that has been heated and expanded. The untreated variety absorbs water when wet and takes a long time to dry. The treated variety is coated to make it water-repellant. It is non-combustible and poses no airborne health-risks, but the R-value is less than other types of insulation.
For the extreme end of the eco-friendly and low-allergen spectrum, cotton and sheep's wool are also available. They are treated with the same chemicals as cellulose for fire and pest resistance.