Kitchen Hood - Venting

[uzziel49]

I've got to fit a new extractor hood in my kitchen. The hood will sit on an internal wall. I want to vent the hood rather than recirculate. I intend to run some ducting into the roof space above the kitchen, across the loft area and out through the eaves. Total run, from the fan to the external vent, is around 1800 - 2000mm.

What material do you recommend using for the ducting? Metal concertina-style ducting pipe, or rigid plastic? Also, I assume that I'll need to insulate the section of ducting that's in the loft?

 

[foxhole]

Either will do, insulation may reduce condensing in the pipework during winter, but there should be sufficient air flow to avoid this.