The Warriors escaped the Arms Park with a 20-19 win on Saturday but the result could have gone either way.
After the game Rennie was disappointed with his side's use of the ball and wants them to keep it in hand for longer spells in future matches.
"We created a lot of opportunities but didn't hang on to the ball long enough. Too many little kicks in behind that ended up giving away possession and relieving pressure," he told glasgowwarriors.org.
"It was a tough night but we're pleased to come out on the right side of it."
Looking ahead to their next game, at home to Munster, Rennie said: "We're always going to strive to be better. There's no doubt that when we held on to the ball and went through a number of phases we put Cardiff under pressure.
"That was our mindset, to try and play an up-tempo game but there were too many handling errors and turnovers of possession relieving pressure."