My material has only one foil side; should the foil face the roof?
No. In attics, single-sided radiant barrier material should be installed with the foil side facing down. Even if you use a double-sided radiant barrier material, it is best to install it at the rafter level so that the bottom side faces the attic airspace and will not collect dust. This may run counter to our intuitive feel for "how things work," but it does work, and work well.
To understand how it works, remember the two properties of aluminum foil from our Thanksgiving turkey and light bulb analogies; foil reflects radiant energy very well but does not radiate heat well. It does not emit heat to the cooler surfaces around it.
If you install a single-sided radiant barrier with the foil side facing up, the aluminum will (for a time) reflect the thermal energy radiated by the hot roof.
If you install a single-sided radiant barrier with the foil side facing down, the aluminum simply will not radiate the heat it gains from the roof to the cooler insulation it faces.
At first, a single-sided radiant barrier will work equally well with the foil facing up or down. However, over time, dust may accumulate on the surface of foil facing up. The dust will reduce the radiant barrier effect by allowing the foil to absorb rather than reflect thermal radiation. However, a radiant barrier with the foil side facing down will not collect dust on the foil and will continue to stop radiant heat transfer from the hot roof to the insulation over the life of the installation.
Does the radiant barrier have to be airtight in order to work?
Can't I just roll the material out on top of the insulation?
Placing the material directly on top of insulation is not recommended. In this type of installation, dust will accumulate on the foil surface facing the roof. In time, the dust will negate the radiant barrier effect. An exception to this is the “chip” and multi layer products on the market. Their multiple layers give them resistance to the effects of duct build up, making them suitable for floor installations. In all installations beware; problems could develop with moisture condensation.
Will foil-faced batt insulation work as a radiant barrier?
While some conventional batt-type insulations have an aluminum foil backing, it's probably not a good idea to simply flip your insulation over to use it as a radiant barrier. Not only will you encounter the eventual dust problem, you may also encounter a fire hazard in the glue that bonds the foil to the batt.
At least one batt insulation manufacturer has introduced a product with a foil face that is bonded to the insulation with a fire-retardant glue. This product meets fire codes, but it still has the potential for dust problems.
Will heat build up in the roof and damage my shingles?
It's extremely unlikely. The Florida Solar Energy Center has measured the temperatures of roof shingles above attic radiant barriers on hot, sunny summer days. Depending on the color of the shingles, their peak temperatures are only 2-5° F higher than the temperature of shingles under the same conditions without a radiant barrier.
Roofing materials are manufactured to withstand the high temperatures to which they are frequently exposed. A 2-5° F increase in peak temperatures that normally reach 160-190° F should have no adverse affect.
What about my shingle warranty?
Shingle warranties should not be subject to cancellation by the manufacturer on the basis of radiant barrier installation. However, it may be wise to review the warranty to be sure that work of this nature will not void it. You may want to inquire directly of the manufacturer. Any changes in warranty should be substantiated in writing.
Can I just roll the material out on the roof before reshingling?
This will not provide a radiant barrier. Remember, to be a radiant barrier, the aluminum foil must be installed facing an airspace. If there is no airspace, the foil acts as a conductor and quickly passes heat by conduction from a hot surface to a cooler one.
If your re-roofing job requires decking replacement, however, it is a good time to consider a radiant barrier, especially the foil-faced roof sheathing materials on the market.