Share 'To replace old leaky ducts with new flex ducts or to seal the old with mastic?'

A number of home energy pros I'm in discussion with don't think there is much use in keeping old ducts and prefer to replace them with new insulated flex ducts. Can anyone give me a more definitive economic argument, findings or a study? Are there any studies on the effectiveness on one way or the other. I'd love to see which method is more effective at controlling air and making money crawling u…