I have heard of aluminum foil radiant barriers to reduce heat gain and loss in a roof. They sell roof plywood with foil bonded on one side. I know some people, say it does not do much. living in a desert I think it can't hurt.
So today I found found some legitimate paint companies make what they call radiant barrier paint.
Would be easy to apply to the bottom of the roof deck with my airless sprayer and a long extension pipe. Not sure if it is watertight so the roof deck would not dry to the inside or not.
Of course the companies selling the aluminum foil say the paint is a gimmick and you should buy their products instead. You do have to install air chutes near the eaves either way to keep blown insulation from blocking soffit vents.
Bil lD.
http://104.130.6.56/roofing/reflective-coatings/liquidfoil-attic-barrier
on edit: the Henry product says it allows water vapor to escape
It is 84% reflective compared to 97% for foil.