Natural Fiber Insulation Materials
Some natural fibers—including cotton, sheep's wool, straw, and hemp—are used as insulation materials.
Cotton
Cotton insulation consists of 85% recycled cotton and 15% plastic fibers that have been treated with borate—the same flame retardant and insect/rodent repellent as cellulose insulation. One product uses recycled blue jean manufacturing trim waste. As a result of its recycled content, this product uses minimal energy to manufacture. Cotton insulation is available in batts with a thermal resistance or R-value of R-3.4 per inch. Cotton insulation is also nontoxic; you can install it without using respiratory or skin exposure protection. However, cotton insulation costs about 15%–20% more than fiberglass batt insulation.
Sheep's Wool
For use as insulation, sheep's wool is also treated with borate to resist pests, fire, and mold. It can hold large quantities of water, which is an advantage for use in some walls, but repeated wetting and drying can leach out the borate. The thermal resistance or R-value of sheep's wool batts is about R-3.5 per inch, which is similar to other fibrous insulation types.
Straw
Straw bale construction, popular 150 years ago on the Great Plains of the United States, has received renewed interest. Straw bales tested by Oak Ridge National Laboratory yielded R-values of R-2.4 to R-3.0 per inch. But at least one straw bale expert claims R-2.4 per inch is more representative of typical straw bale construction due to the many gaps between the stacked bales.
The process of fusing straw into boards without adhesives was developed in the 1930s. Panels are usually 2–4 inches (51–102 mm) thick and faced with heavyweight kraft paper on each side. Although manufacturers claims vary, R-values realistically range from about R-1.4 to R-2 per inch. The boards also make effective sound-absorbing panels for interior partitions. Some manufacturers have developed structural insulated panels from multiple-layered, compressed-straw panels.
Hemp
Hemp insulation is relatively unknown and not commonly used in the United States. It offers a similar R-value (about R-3.5 per inch of thickness) as other fibrous insulation types.