During new construction, batt insulation—thick strips of spun fiberglass or a paper-based product—is cut to fit between wall studs and ceiling joists before the wallboard is put up to increase insulation values. Installing batts in most existing homes, however, is rarely feasible, as drywall would have to be torn down, a messy, expensive, time-consuming proposition. Blown-in insulation can be added to attics and walls without the hassle. What’s more, this type of insulation can also seal small gaps and spaces as it settles, filling these sneaky spots where cold air would otherwise come in. And in addition to creating an insulating blanket, blown insulation helps reduce sound transfer between the outdoors and the indoors, so unwanted street noise will also be softened.

 There are three main types of blown-in insulation.

The three most common types of blown-in insulation are loose-fill fiberglass, cellulose, and rock wool—each with its own pros and cons. Minimum suggested insulation values vary by geographic zones, and you can find the recommended values for your region on this Energy Star map. Not all types of blown-in insulation offer the same thermal value, but in most cases, even adding a little insulation is better than not adding any at all.

• Loose-fill fiberglass: This light-as-air insulation is manufactured from glass that is heated to a liquid and then spun into thin fibers. When blown into attics and wall spaces, loose-fill fiberglass offers an average R-2.5 thermal value per inch (the higher the number, the greater the insulating effect). You’d need a thickness of about 7.5 inches of fiber glass blown-in insulation to match the insulating value of a batt of R-19 insulation (R-19 is a common batt value). One bag of loose-fill fiberglass will provide a thermal value of R-19 over a 106.6-square-foot area.
• Cellulose: For eco-minded homeowners, cellulose is often the insulation of choice, because it’s made from finely shredded recycled cardboard or newspaper. This is the most common type of blown-in insulation on the market, and it’s chemically treated to resist mold and fire. A downside to cellulose is that if it gets wet (from a leaky roof or pipe), it can lose its fluffiness and become soggy and compacted, which reduces its R-value. 
• Rock wool: Also called “mineral wool,” this type of blown-in insulation is made from blast furnace slag. The slag is heated, combined with other minerals, and then spun into an airy product that resembles the texture of raw sheep’s wool. Rock wool insulation has a thermal value of R-3.3 per inch, but a single bag will only cover 60 square feet at a thermal value equal to R-19. Due to its excellent fire resistance, rock wool is often called for in areas subject to fire codes, such as a connecting wall between a house and an attached garage, or in the floor between a garage and a finished room over garage.