Putting heating and air conditioning ducts in an unconditioned attic is stupid. Don’t be stupid. Make a better choice. Choose a smarter truss alternative. Consider the modified roof truss. The modified truss differs from a standard truss in that allows for an open rectangular area in the middle, creating a reverse bulkhead where the ducts will be installed. The primary benefits of this type of truss is that the ductwork will lie conditioned space and that this may be accomplished without placing bulkheads below the main ceiling plane or insulation of the entire attic deck.
Read the full article here: Use Plenum Trusses To Keep Ducts Out of Your Attic