How to Compare Rock Wool and Fiberglass Insulation

Fiberglass is made from very fine strands of woven glass, which are formed into rolls or blankets that fit within wall cavities. This material also comes in a loose-fill form, which can be piled on the floor in an attic or blown into wall cavities. Rock wool features a similar form, with both rolled blankets and loose-fill products available, but this material is made using strands of heated rock or minerals instead of glass. While the average fiberglass insulation product contains 20 to 30 percent recycled content, according to the U.S. Department of Energy, rock wool consists of 75 percent recycled content or greater, making it a more eco-friendly choice for those concerned about recycling.

Insulation manufacturers use R-value to represent how well products improve the thermal resistance of a wall or other structure. The higher the R-value, the better the material is able to help the structure resist thermal transfer. Fiberglass offers an R-value of 2.2 to 2.7 per inch, while rock wool has an R-value of 3.0 to 3.3 per inch, making it a slightly better insulator than fiberglass, according to the U.S. Department of Energy. While recommended insulation levels vary by region, much of California falls within the DOE's Zone 3 classification. The DOE recommends an R-value of 13 to 15 for walls in Zone 3, with an R-value of 30 to 60 in the attic.

Rock wool offers a higher density than fiberglass, which helps to improve fire resistance. Rock wool can withstand temperatures up to 1,800 degrees Fahrenheit, while fiberglass melts at around 1,100 degrees Fahrenheit, according to "The Solar House -- Passive Heating and Cooling." This higher melting point makes rock wool more effective than fiberglass at slowing the spread of flames during a fire.

Rock wool generally costs more than fiberglass insulation. Fiberglass insulation rated at R-15 to fit a standard 4-inch wall costs around $0.50 per square inch as of 2013, according to Fine Homebuilding. Rock wool rated at R-15 and designed to fit a 4-inch wall costs $0.62 per square foot as of 2013. To insulate a basement measuring 20 feet by 20 feet with 8 foot ceilings, you would need 640 square feet of insulation. At $0.50 per square foot, 640 square feet of insulation costs $320 compared to $397 for rock wool insulation at $0.62 per square foot.

While both fiberglass and rock wool can help block noise, rock wool's greater density and weight make it a slightly better sound blocker than fiberglass. The DOE reports the density of fiberglass insulation at 0.5 to 1.0 pounds per cubic foot, compared to 1.7 pounds per cubic foot for rock wool. This higher density allows rock wool to reduce sound transmission through a wall by around 10 decibels, while fiberglass insulation reduces sound transmission by as few as 4 decibels, or as many as 10, depending on the material and how it is installed.