For maximum strength and thermal efficiency, we recommend the 350 Series. A full-depth layer of polystyrene cushions and insulates each panel, while steel backing adds an extra layer of support.  Together, these provide an R-value of 9.83 for maximum thermal efficiency.  From the solid construction and whisper-quiet operation to the distinctive look of this premium product, the 350 Series was designed to please and perform.  The 351 and 359 Series are constructed and backed with heavy-gauge steel and carry a lifetime limited warranty.