Posted in Builders
Don't Forget Your Sunblock
Energy efficiency is nothing new to most architects and specifiers, but for those seeking an easy way to specify efficiency into a home, radiant barrier roof sheathing is a product that provides energy efficiency and does not cost more to install.
Understanding Radiant Heat
To understand how a radiant barrier works, one must first consider the process of heat transfer, which can occur in one of three ways: by conduction (through solids or liquids), by convection (through liquids or gases in motion), and by radiation (through vacuums or air spaces via the electromagnetic spectrum). Radiant heat is invisible and has no temperature, just energy. When this energy strikes another surface, it is absorbed and increases the temperature of that surface. According to the Reflective Insulation Manufacturers Association International (RIMA-I), 93 percent of solar radiated heat enters a building through the attic, where only 7 percent enters through the walls. The result of this heat infiltration is higher energy consumption by the home’s air conditioning unit to compensate for the attic, ceiling and living space heat gains.
In the summer months it is common to see roof temperatures reach 160° F, with attic temperatures exceeding 120° F. Standard roof sheathing is a poor insulator, allowing 90 percent of that heat to radiate into the attic.
How a Radiant Barrier Can Help
Radiant barrier roof sheathing can block up to 97 percent of that heat from entering the attic. Radiant barriers have two properties, high reflectivity and low emissivity (low-e), that allow them to reduce radiant heat transfer. Radiant barriers should have an emissivity of 0.1 or less (ASTM 1313); however, many radiant barriers on the market have an emissivity of 0.05 or less, as required by some code jurisdictions.
Aluminum foil makes an excellent radiant barrier, with pure grades having an emissivity of ~0.03 –~0.05. In contrast, the emissivity of common building materials found in attics, such as wood, fiberglass and cellulose, range between 0.8 and 0.9. Because these materials are highly emissive, installing a radiant barrier in an attic, with its low-e surface facing the air space, is a very effective strategy for reducing summer heat gain into the attic.
Effects of Radiant Barriers on Energy Costs
According to the U.S. Energy Information Agency, the largest component of residential energy use is heating and cooling. It follows then that the most dramatic effect on home energy savings is tied to these uses of energy.
Installing a radiant barrier reduces heat movement from the roofing materials into the attic space, which then reduces the attic space air temperature and heat radiated from the roof to the attic floor. This further reduces heat from the attic floor to the living space, additionally making ducting and air handling equipment in the attic cooler. Studies have shown the benefits of building with radiant barrier sheathing, revealing monthly cooling energy savings of up to 17 percent.
There are many factors to consider when calculating energy savings—climate, roof slope, ventilation rate, insulation levels, location of HVAC equipment and ductwork, orientation of the home, and window area and efficiency.
Selecting a Radiant Barrier Sheathing
Radiant barrier sheathing is made of a thin, durable sheet of aluminum overlay laminated to OSB or plywood. These products may absorb moisture during storage as well as during early phases of construction.
Perforations can help reduce trapped moisture, provided they are large enough to allow the passage of water vapor but spaced properly so that they do not negatively affect the barrier’s emissivity.
Of these, post-incision can best help reduce trapped moisture in the OSB substrate and reduce drying time. Tests have shown that this technology allows the panel to dry almost as efficiently as OSB sheathing without a radiant barrier, as it dries within two percent of standard OSB sheathing. Other products that use pre-perforated foil showed a significant amount of moisture still in the panel after up to 80 days of drying. LP® TechShield® Radiant Barrier Sheathing is the original and premier brand of radiant barrier sheathing, and also the only post-incised product on the market.n
There are a variety of types of radiant barrier sheathing, including:
- Non-perforated foil: A solid foil overlay laminated to the structural panel
- Pre-perforated foil: A pre-perforated foil laminated to the structural panel
- Post-perforated foil: A solid foil overlay laminated to the structural panel then run through a pin roller or incising station
- Post-incised: An LP patented process by which a solid foil overlay is laminated to a structural panel then put through an incising process, creating serrations through the foil and into the wood fibers
To learn more about the benefits of radiant barrier sheathing, take the Radiant Barrier Structural Roof Sheathing Course (HSW credit) at www.LPCorp.com/University.
This information and the websites identified above are provided solely as a convenience to the reader. They are not intended to state or imply that the editors of Engineered Wood or LP Building Products sponsor, recommend, endorse or are affiliated or associated with the companies or products listed.