What amount of heat savings can I expect using reflective shield in my attic over twelve inches of fiberglass insulation? The house was built with 6″ craft faced fiberglass between the joists, to which the previous owner added blown in cellulose(?). There is no flooring, there is no AC ductwork or exposed wiring. Our heat cost is very high in central NV (AC season is short). Along comes a man promoting his radiant barrier product consisting of two layers of aluminum foil with about 1/8″ bubble material between the two. The product features holes about 1/8″ in diameter every 1 1/2″ apart. He proposes to simply lay the product over the existing blown in insulation and guarantees a 55% fuel savings. I have my doubts about the savings, but my real concern is moisture and then mold being trapped under the radiant barrier. Will the cellulose plug the holes? Would I be better off adding unfaced fiberglass roll over the top of the cellulose?
First and foremost — DO NOT EXPECT 55% SAVINGS or anything close to that. On the face of it I would expect little to no savings — opinion offered without the benefit of inspecting the attic. As to moisture, I would expect if you were going to have a problem with that, you already would have one unless the kraft backing to the fiberglass was perforated. Putting the added cellulose on top could have created a moisture trap.
In your application, I would only endorse applying a radiant barrier to the underside of the roof — attached to the bottom of the rafters.
You should take a look at what’s going on in the attic during hot months. Does it get very hot? Does your AC continue to run well into the night, long after the sun has gone down? Are your walls contributing to the heat buildup? Does your attic have adequate ventilation? If the attic air is getting very much hotter than the outside ambient air temperature, then the mass on the attic floor is contributing to it and the heat that is built up in the mass will go to the coolest place, which will be the living space.