The critical radius of insulation for a spherical shell is

$$\frac{thermal conductivity of insulating material}{heat transfer coefficient at the outer surface}$$

$$\frac{2 \times thermal conductivity of insulating material}{heat transfer coefficient at the outer surface}$$

$$\frac{thermal conductivity of insulating material}{2 \times heat transfer coefficient at the outer surface}$$

$$\frac{heat transfer coefficient at the outer surface}{thermal conductivity of insulating material}$$