The article makes the point that the intensity of solar radiation is far higher than that usually generated by most household heat sources Sol air temps can reach up to 140 F in summer Radiant barriers a white roof can reflect much of this energy away even before it reaches a conductive insulator However if you place foam directly directly in contact under a dark roof much of that solar radiation is not reflected but will conduct inward and result in high surface temperature at the foam Even at good R values the underside surface temperature of the foam can be still quite significant and the energy re radiated downward on to the ceiling However the article does not consider the effect of a white roof to reflect most the energy away and then spray foam below the much cooler roof to deal with the rest of the heat energy