Insulation Alternatives - Non-Fiberglass Batts Non-fiberglass batts made of cotton, sheep's wool, and rock or slag wool Four entries in the PATH Technology Inventory describe alternatives to conventional fiberglass batts or rolls: - Non-fiberglass batts
- Sprayed foam insulation
- Sprayed fiber insulation
- Blown or Foamed through a membrane
- This document deals with non-fiberglass batts made of cotton, sheep's wool, and rock or slag wool
Cotton batts were introduced in the late 20th Century, and taken off the market. They are now back on the market and are available in 3.5" R-13 or 5.5" R-19 thicknesses. Sheep's wool is used as an insulating material for joints in log homes, and is now available in batt form. Both products are borate treated for pest and fire resistance. The general term "mineral wool" refers to fiberglass, rock wool (made from basalt, a volcanic rock, and from limestone) and slag wool (made from blast furnace slag). In addition to the use of rock and slag wool for insulation of pipes and many other industrial, appliance and transportation uses, it is used in batt and board form in commercial buildings for fire-resistance and acoustical absorption. Batts are made entirely or mostly of rock wool. Unlike fiberglass, batt widths and thicknesses vary among the different manufacturers. Rock wool for residential insulation is more common in Europe, Canada, Australia and New Zealand than in the U.S. It's density ranges from 2.5 to 4.0 pcf, which is comparable to blown-in cellulose. Batts are primarily made for commercial use; for thermal, fire, and sound insulation purposes. However, there is growing interest by manufacturers in the residential market. 
All three products are unfaced. If a vapor retarder is required, it must be added separately. Both cotton and sheep's wool must be protected from water leakage. If the borate treatment fails (washed out by a leak, for example), the material loses its fire and mold resistance (as would cellulose). Also, sheep's wool is attacked by moths if untreated. Mineral wool batts come in 4-foot long sections instead of rolls. As noted above, they should not be over-compressed into an undersized cavity, to avoid stressing the gypsum board. Only one company makes mineral wool batts sized for residential construction.
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Like cellulose, cotton wool is boron treated to resist fire, mold and vermin. It has been ASTM rated for thermal resistance, surface-burning characteristics, water-vapor absorption, mold/fungi resistance and odor emission. Sheeps wool, a niche-market product, has not been thoroughly tested. Mineral wool batts have low flame spread and zero smoke developed rating, and have been thoroughly tested (ASTM 665) for sound, fire resistance, and thermal resistance.
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Cotton and sheep's wool batts are friction fit into cavities, like unfaced fiberglass batts. They stay in place because they are 16" and 24" wide, 1.5" larger than cavity-width. Because neither product contains irritating chemicals or fibers, each can be handled without protective clothing. However, only one manufacturer (Roxul) makes batts that are sized for wood studs (15" and 23" wide), and these products are currently not available in the U.S. It is possible to install mineral wool batts that are wider than the cavity by squeezing in a 16" or 24" wide sound insulating batt with a 2-1/2 pound density. As this may bulge the insulation more than is desirable for installing gypsum board, install a test section before committing to a size. While it is possible to install the next smaller thickness to compensate for any bulging, this practice leaves gaps and is not recommended. Typical batts are 48" long, but 47" batts may be available. Rock wool is less abrasive and therefore more user-friendly than slag wool. Some care is required in installation to work it around wiring and piping. It cuts easily with a sharp tool. Gloves, dust mask, eye protection and loose clothing are recommended when handling mineral wool insulation.
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Cotton batts use 75% post-production recycled material, denim waste from blue-jean manufacturing, and is 100% recyclable at the end of its useful life. The national average cost of the material is $0.50/square foot for R-13 batts, and $0.70/ square foot for R-19 batts. Sheep's wool is a natural product that is sustainable. It is borate treated to resist moths, termites and other pests, and to provide fire and mold resistance. Its ability to hold large quantities of water can be an advantage in walls that can dry out between cycles of water loading, although repeated wetting and drying can leach out the borate treatment. Cost without delivery is about $6.50 a pound, and one pound will insulate about 6 square feet of a nominal 4" thick wall, yielding a square foot cost of $1.06. Because of its high density, mineral wool provides better acoustical absorption than fiberglass and has a higher insulating value: R-3.7/inch for 2-1/2 pcf density, and R-3.9/inch for 4 pcf density. Made of rock, it is a superb fire-resistive material, with a typical smoke developed rating of zero and a flame spread rating of 0 to15. Unlike fiberglass (which contains an organic binder), it can be used in direct contact with flues, stoves and other hot objects. Mineral wool is also water-repellant. Slag wool is made from a recycled byproduct of blast-furnace steel production (a considerable amount of slag is imported). 3" thick mineral wool batts, the industry standard for comparative pricing, has a material cost in New England of under $.30 per square foot. Prices vary considerably around the country, depending on the distance to the manufacturer. | 
