"By faith Noah, being warned of God of things not seen as yet, moved with fear, prepared an ark to the saving of his house; by the which he condemned the world, and became heir of the righteousness which is by faith" (Heb.11:7).
When Noah believed God he received the same righteousness of which Paul speaks here:
"But now the righteousness of God apart from law is made known, being witnessed by the law and the prophets; Even the righteousness of God which is by the faithfulness of Jesus Christ unto all and upon all them that believe: for there is no difference: For all have sinned, and come short of the glory of God; Being justified freely by his grace through the redemption that is in Christ Jesus" (Ro.3:21-24).
When Noah believed God he received the imputed righteousness spoken of here:
"But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness" (Ro.4:5).