Placing notoriously leaky HVAC ductwork into an unconditioned space will not make for an energy efficient heating or cooling system. Luckily, there are a few different options which locate the ductwork within the thermal enclosure of the home. Options include moving the ceiling plane to the underside of the roof with an unvented attic or placing the ductwork in a conditioned basement. A third option is the installation of plenum trusses which allow for a compact ducting system to be placed in a conditioned bulkhead.
In a recent study by the Building America Program partner IBACOS, one story slab-on-grade homes were fitted with plenum trusses and compact HVAC systems were installed in reverse bulkheads to see if they could meet 2012 International Convention Energy Code (ICEC). Plenum trusses create a reverse bulkhead in the center of the attic space where ducting can be housed in an insulated area; “The plenum truss bulkhead adds only the amount of “inside conditioned space” required to house the ductwork by modifying the roof framing to create a boxed-in area in the otherwise unconditioned attic.”
First they moved the air handler into the home in a mechanical closet. While this did take up some space, it provided a convenient, accessible location in a conditioned space. It also reduced the amount of ducting required as ducts don’t need to go around the house perimeter.
Installing and sealing an air barrier over the top and sides of the reverse bulkhead means the pressure and thermal boundaries now include the bulkhead interior. The bulkhead is also insulated and all penetrations are properly sealed.
The cost of plenum trusses is higher than other truss options but some of this cost may be offset with the use of a smaller HVAC system since the ducting is in a conditioned space. There is also a reduction in the amount of ducting required and a saving in energy consumption.
The integrity of the bulkhead must be maintained at all times during the lifetime of the system. This means ensuring that the bulkhead is effectively insulated and sealed and that other trades take care when moving through or working in the attic space not to damage the bulkhead in any way.
Another advantage to this method is the option to have raised heels on the trusses. This would allow more depth of attic insulation right to the outside edge of the top plate and using longer length sheathing like TallWall would remove joints at the top plate, the hinge point and likely the metal hurricane clips.