docs: def k_to_R_value(k, SI=True):
      r'''Returns the R-value of a substance given its thermal conductivity,
      Will return R-value in SI units unless SI is false. SI units are
      m^2 K/(W*inch); Imperial units of R-value are ft^2 deg F*h/(BTU*inch).
  
      Parameters
      ----------
      k : float
          Thermal conductivity of a substance [W/m/K]
      SI : bool, optional
          Whether to use the SI conversion or not
  
      Returns
      -------
      R_value : float
          R-value of a substance [m^2 K/(W*inch) or ft^2 deg F*h/(BTU*inch)]
  
      Notes
      -----
      Provides the reverse conversion of R_value_to_k.
  
      Examples
      --------
      >>> k_to_R_value(R_value_to_k(0.12)), k_to_R_value(R_value_to_k(0.71, SI=False), SI=False)
      (0.11999999999999998, 0.7099999999999999)
  
      References
      ----------
      .. [1] Gesellschaft, V. D. I., ed. VDI Heat Atlas. 2nd edition.
         Berlin; New York:: Springer, 2010.
      '''
      r = k_to_thermal_resistivity(k)
      if SI:
          return r*inch
      else:
          return r/(foot**2*degree_Fahrenheit*hour/Btu/inch)