William Robinson, Evans, Audrey A., Reichel, Claudette H.
A radiant barrier under the roof decking (foil side down) can block 95% of the roof’s radiant heat. This is most beneficial when attic insulation levels are R-19 or less or when the air conditioner or ducts are located in the attic. A radiant barrier may produce little energy savings when the roofing already has a heat-reflective coating or is a light-colored metal or tile or when the HVAC is within the conditioned space and the recommended level of attic insulation is installed. Certain low-e paints and coatings can perform as radiant barriers, although they are typically less effective than foil materials at blocking heat.
Should I insulate under the roof decking or on the attic floor?In a vented attic, do not spray foam under the roof decking. That’s a total waste of money. However, a radiant barrier under the roof decking (shiny side down) is helpful to reduce heat gain in summer, especially if air conditioner and ducts are in the attic.
An unvented attic is essentially part of the conditioned space within the air barrier and insulation envelope. To prevent possible moisture problems in the decking, you have to either use spray foam to insulate and air seal under the roof (and roof/wall joint) or have rigid foam insulation on top of the roof decking (between decking and shingles) plus any sort of insulation under the roof deck (netted cellulose, fiberglass, etc.). Don’t use a radiant barrier in an unvented attic.
Foil-faced sheathing or a radiant-barrier housewrap behind brick veneer walls can provide a worthwhile radiant barrier benefit, particularly on west and east sides. It can help block the heat radiated by the sun-heated bricks with the added benefit of a vapor barrier that can resist water vapor diffusion into the wall from the high vapor pressure drive of a rain-wetted, sun-heated brick veneer.
A radiant barrier in either location should have a shiny side facing a vented air space of at least one inch and an emissivity rating of 0.05 or less. In climate zone 2 (South Louisiana), foil-faced plywood or OSB sheathing can be used, yet a perforated foil is suggested to allow some drying to the outside in cold weather. In cooler climate zones (North Louisiana), the foil should be on an insulating material to prevent moisture condensation problems; a foil-faced rigid foam insulating sheathing is a good choice.