How to insulate 90mm semi rigid ducting.

[Coanda]

As the title says.

 

How have you or how do you suggest insulating the semi rigid ducting in the roof space?

[Conor]

Wrap in foil backed mineral wool insulation. Tape joins with aluminium tape. Not the nicest job in the world 

[MikeGrahamT21]

Assuming this is in a cold loft, I have my pipes running directly over the joists and then put 200mm wool insulation over the top, in effect burying them