Energy costs are on the rise. So are the number of calls we get dealing with radiant barriers. The majority of these questions deal with how best to apply a radiant barrier.
Some companies have begun installing radiant barrier on the attic floor. Historically, however, radiant barrier application has mandated that it be installed on the underside of the roof, whether TechShield roof decking, Radiant Barrier Paint, or the sheets nailed directly to the roof rafters, facing down to an air space, eliminating the possibility of being covered by dust and debris that inhibit the effectiveness of the radiant barrier.
Sometimes to get all the facts you just have to go to the source for information, and in this case I have gathered material from the Department of Energy on the proper application of radiant barriers. Periodically we all need to seek out updated information on products. The latest Barrier Attic Fact Sheet from the Department of Energy says the floor installation method is a poor solution.
While I have been unable to locate any studies on the effects of increased humidity levels of attics with this type of application, specifically in hot and humid climates, it does raise concerns about the trapping of humidity in the attic insulation.
Effect of Radiant Barriers on Heating and Cooling Bills
Radiant Barrier Attic Fact Sheet reprint from Department of EnergyAt present, there is no standardized method for testing the effectiveness of radiant barriers in reducing heating and cooling bills. But numerous field tests have been performed that show, depending on the amount of existing conventional insulation and other factors, radiant barriers are effective in reducing cooling bills, and also possibly heating bills.
Most of these field tests have been performed in warm climates where a large amount of air-conditioning is used. The Florida Solar Energy Center (FSEC) at Cape Canaveral has performed tests for a number of years using attic test sections, and has also performed tests with full-size houses.
A test using a duplex house in Ocala, Florida has been performed by the Mineral Insulation Manufacturers Association. The Tennessee Valley Authority has performed a number of winter and summer tests using small test cells in Chattanooga, Tennessee. The Oak Ridge National Laboratory (ORNL) has performed a series of tests using three full-size houses near Knoxville, Tennessee.
The ORNL tests included summer and winter observations. So far, very little testing has been done in climates colder than that of Knoxville. Also, little testing has been done in hot, arid climates such as the southwestern United States.
The tests to date have shown that in attics with R-19 insulation, radiant barriers can reduce summer ceiling heat gains by about 16 to 42 percent compared to an attic with the same insulation level and no radiant barrier. These figures are for the average reduction in heat flow through the insulation path. They do not include effects of heat flow through the framing members.
This does not mean a 16-to-412 percent savings in utility bills can be expected. Since the ceiling heat gains represent about 15 to 25 percent of the total cooling load on the house, a radiant barrier would be expected to reduce the space cooling portion of summer utility bills by less than 15 to 25 percent. Multiplying this percentage (15 to 25 percent) by the percentage reduction in ceiling heat flow (16 to 42 percent) would result in a 2 to 10 percent reduction in the cooling portion of summer utility bills.
However, under some conditions, the percentage reduction of the cooling portion of summer utility bills may be larger, perhaps as large as 17 percent. The percentage reduction in total summer utility bills, which also include costs for operating appliances, water heaters, etc., would be smaller. Tests have shown that the percentage reductions for winter heat losses are lower than those for summer heat gains.
Experiments with various levels of conventional insulation show that the percentage reduction in ceiling heat flow due to the addition of a radiant barrier is larger with lower amounts of insulation. Since the fraction of the whole-house heating and cooling load that comes from the ceiling is larger when the amount of insulation is small, radiant barriers produce the most energy savings when used in combination with lower levels of insulation. Similarly, radiant barriers produce significantly less energy savings when used in combination with high levels of insulation.
Most of the field tests have been done with clean radiant barriers. Laboratory measurements have shown that dust on the surface of aluminum foil increases the emissivity and decreases the reflectivity. This means that dust or other particles on the exposed surface of a radiant barrier will reduce its effectiveness. Radiant barriers installed in locations that collect dust or other surface contaminants will have a decreasing benefit to the homeowner over time.
The attic floor application is most susceptible to accumulation of dust, while downward facing reflective surfaces used with many roof applications are not likely to become dusty. When radiant barriers are newly installed, some testing shows that the attic floor application will work better than the roof applications. As dust accumulates on the attic floor application, its effectiveness will gradually decrease. After a long enough period of time, a dusty attic floor application will lose much of its effectiveness. Predictive modeling results, based on testing, suggest that a dusty attic floor application will lose about half of its effectiveness after about one to ten years.
How does a radiant barrier differ from conventional attic insulation?
Radiant barriers perform a function that is similar to that of conventional insulation, in that they reduce the amount of Radiant Barrier placed on attic floor heat that is transferred from the attic into the house. They differ in the way they reduce the heat flow. A radiant barrier reduces the amount of heat radiated across an air space that is adjacent to the radiant barrier. The primary function of conventional insulation is to trap still air within the insulation, and hence reduce heat transfer by air movement (convection). The insulation fibers or particles also partially block radiation heat transfer through the space occupied by the insulation.
Conventional insulations are usually rated by their R-value. Since the performance of radiant barriers depends on many variables, simple R-value ratings have not been developed for them.
What are the characteristics of a radiant barrier?
All radiant barriers have a 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 sued, 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.
Emissivity is the property that determines how well a radiant barrier will perform. This property is a number between 0 and 1, with lower numbers indicating better potential for performance. The emissivity of typical, clean, unperforated radiant barriers is about 0.03 to 0.05. Hence they will have a reflectivity of 95-97 percent. Some materials may have higher emissivities. It is not always possible to judge the emissivity just by visual appearance. Measured emissivity values should be part of the information provided by the manufacturer.
A radiant barrier used in the attic floor application must allow water vapor to pass through it. This is necessary because during the winter, if there is no effective vapor retarder at the ceiling, water vapor from the living space may condense and even freeze on the underside of a radiant barrier lying on the attic floor. In extremely cold climates or during prolonged periods of cold weather, a layer of condensed water could build up. In more moderate climates, the condensed water could evaporate and pass through the radiant barrier into the attic space.
While most uniform aluminum coatings do not allow water vapor to pass through them, many radiant barrier materials do allow passage of water vapor. Some allow water vapor passage through holes or perforations, while others have substrates that naturally allow water vapor passage without requiring holes. However, excessively large holes will increase the emissivity and cause a reduction in the radiant barrier performance. The ability to allow water vapor to pass through radiant barrier materials is not needed for the roof application.