Ventilated thimbles are mandated by NFPA-37 where exhaust piping penetrates through combustible walls or combustible roofs.  Even in seemingly "non-combustible" walls/roofs, it may be a good idea to provide an insulated thimble to protect against burning of roof sealers, coatings, stucco, etc.  Wall and roof thimbles are available in carbon steel or stainless steel.