I would say that all of the heat sources in the front of the car are from radiant sources. Simply blocking off the radiant heat from the radiator to the HVAC duct provided an appreciable drop in vent temps.
I know what the effectiveness of the material CAN be, but all my testing was for a completely different application ...so what I really don't know is how it will translate to this Elise HVAC matter.
In my case I undoubtedly had plenty of radiant sources (primarily the header) but also all the conduction and convection from the rest of the engine bay.
In my case, all the insulation and gap-sealing didn't drop air temps any (but the gap-sealing did drop coolant temps) ... but when I topped the insulated air intake plumbing with the gold foil I got about a 10 degree drop (down from 114 F) when everything was fully heat soaked.
Can you hazard any guess as to how much of what you are trying to combat with the Elise HVAC is radiant in nature? I was specifically curious about the "Radiator Cowl" piece.
(I think I just figured out that the other posters are referring to the photo numbers 1, 2, 3, and I was referring to the numbered paragraphs in the write-up.)