How does your attic feel on sunny summer days? Many people find their attic spaces unlivable on summer days because of soaring temperatures. On a sunny day, solar energy is absorbed by the roof, heating the roof sheathing and causing the underside of the sheathing and the roof framing to radiate heat downward toward the attic floor. A hot attic is not only uncomfortable, it can also conduct heat into the interior space of your home, making the air conditioner work harder and increasing your energy bills.
Hot attics are caused by heat from the sun, also known as radiant heat. Radiant heat is heat that is transmitted from a heat source through space. It is the heat we feel from distant objects like the sun or a fire. Radiant heat is unique because it does not require a medium to travel through (like a pan that feels warm on the outside because of the warm water inside it). Radiant heat is also called infra-red heat or infra-red energy.
Radiant barriers are materials that are installed in buildings to reduce summer heat gain and winter heat loss. They reduce building heating and cooling energy usage. A radiant barrier reflects radiant heat back towards its source, reflecting as much as 97%. Radiant barriers are designed to block the effects of radiant heat gain in homes by reflecting radiant heat rather than absorbing it. They provide substantial energy savings in warm climates. When a radiant barrier is placed on the attic floor, much of the heat radiated from the hot roof is reflected back toward the roof. This keeps the top surface of the insulation cooler than it would have been without a radiant barrier and thus reduces the amount of heat that moves through the insulation into the rooms below the ceiling. Studies have shown that radiant barriers can lower a cooling bill by between 5 and 10 percent when used in warm, sunny climates.
The effects of radiant heat gains can be reduced with the aid of highly reflective surfaces. Traditional forms of insulation absorb radiant heat energy. Radiant barriers reflect it. Reflective barriers usually consist of a thin sheet or coating of a highly reflective material, usually aluminum, applied to one or both sides of a number of substrate materials. Radiant barriers can also reduce indoor heat losses through the ceiling in the winter. However, radiant barriers reduce the amount of energy radiated from the top surface of the insulation, but can also reduce beneficial heat gains in winter due to solar heating of the roof. The net benefits of radiant barriers for reducing winter heat losses are still being studied.
Radiant barriers are made from materials that are excellent at reflecting heat and poor at absorbing it. Radiant barriers work by reducing thermal radiation heat transfer from the roof sheathing to the attic floor, where conventional insulation is usually placed. All materials give off or emit energy by thermal radiation as a result of their temperature. The amount of energy emitted depends on the surface temperature and a property called "emissivity" (also called the "emittance"). Emissivity is the property that determines how well a radiant barrier will perform. A closely related material property is the "reflectivity" (also called the reflectance"). This is the measure of how much energy is reflected and not absorbed by the barrier. Radiant barrier materials must have high reflectivity (90%) and low emissivity (10%) and must face an open air space to perform properly.
Radiant barriers come in various forms, including: reflective foil, reflective paint coatings, and reflective chips. Radiant barriers, which do not provide a significant amount of thermal insulation, can be combined with thermal insulation for increased energy efficiency. They reduce heat gains without the need for increasing wall cavity thickness in order to accommodate bulky insulation.
A roof exposed to the sun for a prolonged period will absorb a great deal of heat, sometimes reaching temperatures in excess of 170° Fahrenheit. Radiant barriers can help prevent overheated attics from warming the interior of a home. Conventional thermal insulation can slow down radiant heat transfer, but will not stop it.
All radiant barriers have at least one reflective (or low emissivity) surface, usually a sheet or coating of aluminum. Some radiant barriers have a reflective surface on both sides. Both types work about equally well, but if a one-sided radiant barrier is used, the reflective surface must face the open air space. For example, if a one-sided radiant barrier is laid on top of the insulation with the reflective side facing down and touching the insulation, the radiant barrier will lose most of its effectiveness in reducing heating and cooling loads.
Dust can seriously impair the performance of a radiant barrier by dulling the reflective surface. This problem is most likely to occur if the barrier is installed on the attic floor with the reflective surface facing up. Instead of the radiant heat being reflected, it would be absorbed.
Radiant barriers also act as a vapor barrier, preventing the passage of moisture. If the barrier is installed on the cold side of the cavity, warm, moist air passing through the cavity will condense on the cold surface.Foil with perforations to allow moisture to pass through is available.
Most types of radiant barriers add another step and cost to the process, which may be viewed negatively by some builders.
|
|
|
|
Radiant barriers reduce attic temperatures. In hot climates, especially if ductwork is located in attics, this can translate into cooling energy savings.
|
|
|
|
|
|
|
|
|
|
Today there are many manufacturers making various forms of radiant barriers with a network of suppliers across the country. These include wholesale builders suppliers selling to homebuilders and retail outlets selling to homeowners.
Not Applicable
Not Applicable
Codes require radiant barriers to undergo stringent testing for tensile strength, flame spread rate, smoke development, water vapor permeability and thermal properties. Testing methods are generally in accordance with the American Society for Testing and Materials (ASTM). ASTM C1224 states the standard specifications for reflective insulation for building applications.
Asdal Builders: Henderson, Nevada
Shea Homes: San Diego, California
Typical locations for radiant heat barriers include: between the roof sheathing, above attic floor insulation, in cavity walls, around door openings, water heaters, and pipes. Placing a radiant barrier on top of conventional attic insulation effectively blocks the path for radiant heat into a home.
Radiant barriers may be installed in attics in several configurations. The simplest is to lay the radiant barrier directly on top of existing attic insulation, with the reflective side up. Another way to install a radiant barrier is to attach it near the roof. The roof application has several variations. One variation is to attach the radiant barrier to the bottom surfaces of attic truss chords or rafter framing. Another is to drape the radiant barrier over the tops of the rafters before the roof deck is applied. Still another variation is to attach the radiant barrier directly to the underside of the roof deck.
Foil barriers can be applied to rigid insulation, structural sheathing, or as a one- or two-sided reflective sheet material. They are installed by stapling and taping to the appropriate surface. Reflective paints are sprayed on to a surface. Thermal insulation is available with a reflective surface pre-applied at the factory. Reflective chips are suitable for use in attics where they are spread between joists. Chips are spread about 1/2-inch thick on top of attic insulation. They can be blown-in using special equipment, and can be installed in a 1500 square foot house in about one hour.
Radiant barriers can be incorporated into new or existing homes, although installation is easier in new homes.
Warranties vary by manufacturer.
Radiant barriers are fairly simple to use; installation in new or existing homes is relatively easy. The radiant barrier material is clean, lightweight, easy to handle, non-toxic, resistant to harsh environments, and durable. Once installed, it is virtually maintenance-free.
The installation of radiant barriers can help reduce utility costs for a home. Some studies have shown that radiant barriers can reduce cooling costs by between 5 and 10 percent. They can increase comfort and improve thermal performance in hot climates without substantially increasing building envelope thickness.
The material cost of radiant barriers is between 15 and 75 cents per square foot. The cost is higher for products that combine insulation with reflective material.
|