He continued through adversity. That took strength and faith. That he did not believe. To measure righteous in any age, be it during the time of Noah or now, is a difficult thing. There cannot be only one way to measure.
Righteousness is different for different people.
Noah was righteous, he obeyed God, took his time building the ark to exact specifications and hoped that he would be asked and believe by the people about what was to come. He was not like Abraham who went God was going to destroy Sodom and Gemorah argued with God on behalf of the evil and corrupt people. He was a quiet man who had his own way of warning the people. Intellectually he believed the flood was coming. Maybe in his heart he was hoping this mass destruction would not happen.