tedo

I am at the stage of installing the plenum (plenni, plenums?) and running the radial diction under the insulation in the attic space. Due to the universal law of inconvenience most will have to run at right angles to the joists. My question is do they need to be run along boards to keep them flat or is it OK to allow them to run up and down over the joists like a roller coaster? I am not sure if the extraction ducts will end up with little puddles of condensation in the dips or if the constant air flow keeps them dry. Thanks for the advice in advance.