When you say flexible ducting, are you talking aluminized plastic or the corrugated aluminum? That would be a fair amount of weight for flexi in either case, you would want to have access along the route to secure it, especially if it is insulated. The aluminized plastic would need more support than aluminum.
Without site inspection, I couldn't say how necessary insulating the duct would be. Using insulated duct would be a good "better safe than sorry" move but will make installation without full access a bit of a bother.