Martin Holladay (Senior Editor) wrote a article in Jun/Jul 2012 FHB about insulating Cathedral Ceilings.  In the article he talked about Vented and Unvented roofs.  I have a vented roof that I added 2 x 6’s to the existing 2 x 6’s  to make a cathederal ceiling on the outside wall/ roof rafter over a bathroom and plan on following his guidelines from his article.  I am going to build a 1 1/2″ vent channel below the sheathing and then use rigid insulation foam (2 layers of 2 inch) and below this add fiberglass batts.  I have IC cans (with LED lighting) in the space of the new 2 x 6’s.  Drywall will be used on the interior of the structure for the ceiling and then I plan on finishing the ceiling with white washed 3/4″ pine. 

Do I need to add a vapor barrier (plastic?) to the ceiling above the drywall?  Spray the IC cans with foam?  Anything I am doing wrong.  Since this is a bathroom and moisture will be present I just want to make sure that I am allowing for the removal of the moisture and preventing any problems from such. 

Thanks to all that answer!