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Insulation Alternatives: Non-Fiberglass Batts

Non-fiberglass batts made of cotton, sheep's wool, and rock or slag wool

View the ToolBase TechSpecs- Alternative Insulation Materials PDF file for an overview of this technology.

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 mineral (rock or slag) wool .

Cotton batt insulation is manufactured from post-industrial denim and cotton fibers. The product is available in R-13 (3.5 inch thickness) or R-19 (5.5 inch thickness) unfaced batts. Sheep's wool, used as an insulating material for joints in log homes, is now available in batt form. Both products are borate treated for pest and fire resistance.

Mineral wool insulation refers to fiberglass, rock wool (made from basalt, a volcanic rock, and 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. Mineral wool batts are made primarily of rock wool. Unlike fiberglass, batt widths and thicknesses vary among manufacturers.

Rock wool for residential insulation is more common in Europe, Canada, Australia, and New Zealand than in the U.S. Although batts are primarily made for commercial use for thermal, fire, and sound insulation purposes, there is growing interest by manufacturers in the residential market.

Energy Efficiency

Insulation improves the thermal resistance of exterior walls.


All three alternative batt insulation 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. To avoid stress on the gypsum board, they should not be over-compressed into an cavity. Only one company makes mineral wool batts sized for residential construction.

For an R-19 wall cavity, the installed cost of Sheep’s wool batt insulation is approximately $2.40 per s.f.; cotton batt insulation is about $1.20 per s.f. A separate vapor barrier is required in many areas.

Not Applicable

Like cellulose, cotton batt insulation is treated with borates 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. Sheep’s 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.

Not Applicable

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.

Varies with product and manufacturer.

Cotton batts use about 75% post-industrial recycled material (denim waste from blue-jean manufacturing) and is completely recyclable at the end of its useful life. It does not itch when it contacts skin and contains no formaldehyde or other chemical irritants. The installed costs for cotton batt insulation is about $1.20 per s.f.

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. Installed cost of sheep’s wool batt insulation is about $2.40 per s.f.

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 and does not lose its insulating value when wet.

Slag wool is made from a recycled byproduct of blast-furnace steel production (a considerable amount of slag is imported). Prices for mineral wool batts vary considerably around the country, depending on the distance to the manufacturer. However, cost is comparable to fiberglass batts.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.