My attic space currently has fiberglass batt insulation which is falling down. I have vents for a house fan as well and would like to block some fiberglass smell wafting through them on hot days, gain R value, and hold up my falling batts in one swoop. The best product for this seems to be rigid insulation, possibly fanfold for quicker install if two layers, possibly sheets if one. Regarding the two layers of 1/2″ fanfold, I was thinking about one facing up to prevent heat gain in summer, one facing down to prevent heat loss in winter.

My question is, is it worth furring out the rafters first to create an air gap under batt insulation, and having a layer with foil facing the batts? Or is that 97% of radiated heat just going to radiate back down moments later, all day long and eliminate the benefits? Is an air gap only (or exponentially more) effective when it faces an uninsulated space for the heat to escape from?

If this is the case I may just go with a single layer of rigid foil facing down, right against the rafter. Working conditions will be quite unpleasant and I’d rather not spend more time up there than i have to, if it won’t provide benefit.