To determine the R value of insulating material that comes in sheets of s-in thickness you

To determine the R value of insulating material that comes in sheets of ½ s-in thickness you construct a cubical box of 12 in per side and place a thermometer and a 100-W heater inside the box. After thermal equilibrium has been attained, the temperature inside the box is 90^oC when the external temperature is 20^oC. Determine the R value of this material.