We're designing a second story master suite addition in Los Angeles. It's about 500 square feet for the bedroom, bathroom and walk-in closet.
This will have a flat roof, and routing the HVAC ducting is turning out to be a problem. In a nutshell, we can route ducting through the ceiling in the East-West direction, but not the North-South direction.
The architect is proposing two fake "roof ridges" that would be built on top of the flat roof that run in the north-south direction. Basically two triangles, each one foot high, that run 22 feet from the north end to the south end.
He wants to line these with drywall (or something else that would give a smooth surface) and run air through them. Each of these rooftop "ducts" would have an area of roughly 144 square inches BEFORE insulation.
That's my concern. This is Los Angeles, which gets hot. Running an important part (these would be the main upstairs supply lines) of my HVAC ducting on TOP of the roof seems like it's asking for trouble, since it's directly exposed to the sun (or cold,) and even with insulation, I'm going to lose efficiency compared to running all my ducting through much cooler walls or a ceiling or under the floor.
Or am I wrong? Is it actually only a very slight loss when running ducting on top of a roof?