I am resealing and inspecting the ducting on my ranch crawl space house. It has ductboard fiberglass for the trunk and flexduct from the trunk to the registers. This is all new as house is 2 years old...but its all crap stuff. I want to replace the flexduct with 6inch pvc sdr35. Then next year I want to replace the trunk with metal ducting. Currently I am getting good airflow from all registers I just want to upgrade it and do some overkill. My question is can I use pvc rigid pipe and if not why?? I plan to wrap pvc in 1inch insulation. It seems I would have less air restriction and less air leaks with pvc. Thanks for any replies, advice.