Here's some pictures of the insulation installation. The ductboards are 2' x 4' in dimension and reflective on the side opposite being seen. They are equivalent of about R-8 (2 inches of fiberglass insulation). I first had to attach 2x2s as extensions to the hip rafters. That allowed an air space between the reflective surface of the ductboard and the foil above it. You can't have a reflective surface touching the surface above or it will directly conduct heat and bypass the radiant insulation properties of the reflective surface. The edges where they touch are insignificant area of the total surface. It can be seen here.
I really couldn't use insulation batts because they flop around too much. But even the duct boards can't be screwed directly, thus the wood strips sandwiching the ductboard against the 2x2s and giving some security.
I then covered all the insulation with foil barrier just to look better. The center section with the backflow damper will remain removable and accessible because behind it is the temperature sensors and damper motor. So I just wrapped the foil around the ductboard next to that section so there won't be damage to the adjoining fiberglass when I have to remove the center section.
Yeah, I know this is all way overkill and taking too long. The only reason I'm even paying so much attention to detail is because, to the best of my knowledge, no one has ever tried using an open system -no air recirculation- and still get high enough temperatures to use that attic air directly. If it works then it will be a first. That's not to say someone somewhere hasn't done it before successfully. Just that I haven't seen it documented anywhere.
Here's the proof of the pudding. Today I got the temp rise back up to 34 degrees with a 98 degree high at the roof. When I close the roof vent I am hoping and expecting to keep that high temp even as the days get colder and shorter. In my climate a 34 degree temperature rise is usable to heat the house. I can't say for other climates.