Cellulose

The R-value of insulation materials tends to increase slightly as the temperature difference between the hot side and the cold side of the insulation increases. With cellulose insulation this is exactly what happens. Scientists at Oak Ridge National Laboratory have reported that "R-values [of cellulose insulation] measured under winter conditions increased as the temperature difference across the insulation increased." Based on air permeability, the Oak Ridge scientists have calculated that  cellulose insulation will not lose R-value due to convective heat loss at temperatures as low as 40o F below zero. This means that cellulose insulation maintains its resistance to heat transfer under virtually all weather conditions that occur in North America.