The pilot reported that during takeoff, the forward wind screen began to fog and became progressively worse. As he increased the pitch for rotation, all forward visibility was obscured when the sun glare moved directly into his eyes and combined with the condensation on the wind screen. Shortly after rotation, the airplane impacted a taxiway directional sign on the right side of the runway with the left main landing gear. The pilot returned and landed without incident. A post-accident examination revealed substantial damage to the elevator. The pilot reported that there were no mechanical malfunctions or anomalies that would have precluded normal operation. The operator reported that they updated their operating procedures to formalize the requirement during preflight to remove any debris or condensation from the windscreen, which may include the use of hot water. In addition, the subjects of windscreen cleanliness and sunshade devices were added to their training manual.