Rockwool insulation has good R-value: between R-4 and R-4.3 per inch. Its resistance to high temperatures is far above that of other insulation types. Mineral wool won't melt below 2000 degrees Fahrenheit, making it ideal for chimney insulation. Another good thing about mineral wool is its resistance to mold and moisture damage. It absorbs little or no water and won't compress or fall out of place if it gets wet.