I sat through a free dinner (chicken or shrimp) and heard a guy try to sell me roll-out multi-layer aluminum foil insulation (apparently with mylar in the sandwich). It is perforated so as to allow moisture through, and installed over top of the fiberglass/cellulose/whatever in the attic. It is claimed to reduce heat gain in the summer and heat loss in the winter.
I don't doubt that it would be effective to some degree, but I gather that the optimal placement for heat-gain reflective insulation is on the underside of roof rafters, and I think that the optimal placement for heat loss would be on the warm side of the house in the winter, i.e. just behind the drywall, with an air gap. Laying the stuff over
12 inches of fiberglass is optimal for neither situation, but it still might be ok.
Does anybody know what increase in effective r-value you might get this way? I live in the north and don't use AC, so reducing heat loss is generally more important a consideration. I searched for a while with google but didn't find anything very relevant.
I have truss rafters, incidentally, and installing this would be a total pain in the ass.