Newbie here and before I post my question, let me give thanks in advance to those who will take the time and provide an opinion/suggestion, I appreciate it. Now on to my question, I'm replacing a 6' flex duct (10") and when I try to put on the new flex duct the fit is very tight. I've done some research and applied mastic at the joint, but when I try to put my flex duct over my duct, it just wont go over it easy. My question is, would my system suffer if instead I put a 12" flex duct since that one will give me more room and after applying mastic at the joint areas, followed by a zip line and then sealed with tape I imagine that finish would be pretty air tight. Any suggestions/tips are welcome.