A few years ago while attending a building trade show in Orlando I attended a lecture and demo on using a mylar radiant heat barrier under the roof deck.  The required air space between the bottom of the roof deck and the mylar was established by draping the mylar across the rafters/trusses and then snapping a plastic/vinyl U-channel over the rafter to press down and tighten the mylar.  The U-channels made the mylar tight enough that foam could be sprayed on the bottom side of the mylar without losing the air space and reflecting ability.  The U-channel was deep enough that the roofing nails did not puncture the mylar.  In fact, the mylar could serve as a second water barrier.  I thought this was a neat idea that eliminated having to use furring strips which would make the roof assembly thicker.   

Other than the demo I have not seen anyone using this system although it was highly promoted for use in hot climates.  Does anyone know if this system was adopted for this use?  I can not remember the name of the system so it will be difficult to find it on the internet.  Are vinyl U-channels readily available that will fit tightly over a rafter?