By Lee Williamson
Texas Star Roofing, Inc. is a proponent of energy efficient roofing products, and highly recommends the use of Radiant barrier decking when possible. Radiant barrier roof sheathing is the most efficient, cost effective form of radiant barrier available. Radiant barrier plywood or OSB is laminated with a thin sheet of aluminum foil. The result is an e-value (emissivity) of .97. That means 97% of the radiant heat generated by the sun is reflected and therefore does not enter the attic.
This is important because the lower the attic temperature, the less heat surrounding the AC unit and duct work, resulting in more efficient cooling and lower utility bills, possibly increasing the life span of the unit itself. The lower the attic temperature, the less heat is transferred into the interior of the home.
Radiant barrier decking is only economically feasible when installed either when the home is built, or when a wood shingle or fiber-cement roof installed on lathe is replaced and deck installation is required in order to provide a solid nailing surface for the new roof installation.
It is very important to note that the radiant barrier requires a ¾” air space beneath it in order to work. Therefore, Texas Star Roofing removes every other lathe board in order to maximize the benefit from the radiant barrier decking. A lathe system consists of 1×4 planks installed horizontally with a 4” gap between each board. This results in one half of the surface area of the new roof deck being in contact with the 1×4 planks and thus eliminating the air space beneath the radiant barrier. By removing every other plank, we in effect, reduce that amount to one fourth of the surface area and doubling the benefit of the radiant barrier. We do not recommend removal of every lathe board because this can result in structural issues, as the lathe lends support to the rafter system. Also, when the home was built, the framers knew it was going to be a wood shingle roof and therefore, it was not as critical that the rafters be on exactly 2 foot centers. This results in far more waste compared to a home designed for a plywood deck originally and increases the cost of the re-deck.
The plywood comes in 4’x 8’ sheets, and is placed with the 8’ side lying horizontally. Proper installation requires that the lathe boards be moved when necessary to line up with the horizontal seams. This allows for the fastener pattern to go around the entire perimeter and reduce the chance of curling at the seams.
In reroofing applications where the roof deck is already in place, an alternative to the radiant barrier plywood or OSB deck installation would be the installation of Polarium Thermal Barrier underlayment. This is a thermal barrier, rather than a radiant barrier. The test data contained on the manufacturers website, www.heatbarriersystemsinc.com, states that their product has an e-value of .97. It works in a different way. Rather than reflecting the heat, due to the fact there is no ¾” airspace beneath it, it transfers the heat thru the product to the edges of the roof, thus preventing the heat from entering the attic. This product is an aluminum coated underlayment placed face down over the old felt, directly onto the existing roof deck in lieu of new felt.
The only other radiant barrier applications available for retrofit applications are Aluminum Foil Laminates (foil laminated to kraft paper or plastic film) or Reflective Coatings. Both of these methods are installed on the underneath side of the roof deck in the attic.