Running radial semi rigid diction in attic space

[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.

 

 

[MJNewton]

There'd only be a risk of condensation if the attic space is cold which, if it is, you'll want to bury the ducting in insulation anyway. Even running perpendicular to the joists I doubt you'd get much sagging and no more than you'd get elsewhere in the property, but it won't be an issue if you do. 

[Nickfromwales]

3 hours ago, tedo said:

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.

 

 

Is this for an MVHR system, continuous airflow, or for an extractor fan, sporadic airflow?