Second Nature. Sheep’s wool is an insulation product commonly used in Europe and available in the United States through the Internet. A natural insulator, wool has a slightly higher R-value per inch than fiberglass and does not lose its insulating property when wet. It has inherent properties that resist both flame and many insects, but remains susceptible to moths, so it is treated with boron. Thermafleece comes in 2-inch-thick batts cut to friction fit within 16- and 24-inch stud spacing. They carry an R value of 3.8 per inch and can be layered to achieve the desired total R value.