Insulation - Radiant Barrier Introduction What is a radiant barrier? Radiant barriers are materials that are installed in buildings to reduce summer heat gain and winter heat loss, and hence to reduce building heating and cooling energy usage. The potential benefit of attic radiant barriers is primarily in reducing air-conditioning cooling loads in warm or hot climates. Radiant 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. These substrates include kraft paper, plastic films, cardboard, plywood sheathing, and air infiltration barrier material. Some products are fiber reinforced to increase the durability and ease of handling.
How are radiant barriers installed in a residential attic? 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. This is often called the attic floor application. 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 the 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.
How do radiant barriers work? Radiant barriers work by reducing heat transfer by thermal radiation across the air space between the roof deck and 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 the "emissivity" (also called the "emittance"). The emissivity is a number between zero (0) and one (1). The higher the emissivity, the greater the emitted radiation.
A closely related material property is the "reflectivity" (also called the "reflectance"). This is a measure of how much radiant heat is reflected by a material. The reflectivity is also a number between 0 and 1 (sometimes, it is given as a percentage, and then it is between 0 and 100%). For a material that is opaque (that is, it does not allow radiation to pass directly through it), when the emissivity and reflectivity are added together, the sum is one (1). Hence, a material with a high reflectivity has a low emissivity, and vice versa. Radiant barrier materials must have high reflectivity (usually 0.9, or 90%, or more) and low emissivity (usually 0.1 or less), and must face an open air space to perform properly.
On a sunny summer 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. 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 makes 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.
Under the same conditions, a roof mounted radiant barrier works by reducing the amount of radiation incident on the insulation. Since the amount of radiation striking the top of the insulation is less than it would have been without a radiant barrier, the insulation surface temperature is lower and the heat flow through the insulation is reduced.
Radiant barriers can also reduce indoor heat losses through the ceiling in the winter. Radiant barriers reduce the amount of energy radiated from the top surface of the insulation, but can also reduce beneficial heat gains due to solar heating of the roof. The net benefits of radiant barriers for reducing winter heat losses are still being studied.
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 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 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.
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 to 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 applications.
What should a radiant barrier installation cost? Costs for an attic radiant barrier will depend on several factors, including the following:
* Whether the radiant barrier is installed by the homeowner or by a contractor.
* Whether the radiant barrier will be installed in a new home (low cost) or in an existing home (possibly higher cost if done by a contractor).
* What extra "features" are desired; e.g., a radiant barrier with perforations and reinforcements may be more expensive than a "basic" radiant barrier.
* Any necessary retrofit measures such as adding venting (soffit, ridge, etc.)
* Whether the radiant barrier is installed on the attic floor or on the rafters.
Radiant barrier costs vary widely. As with most purchases, some comparison shopping can save you money. A survey of nine radiant barrier manufacturers and contractors representing 14 products, taken by the Reflective Insulation Manufacturers Association (RIMA) in 1989, shows installed costs of radiant barriers to range as shown in Figure 66. In some cases, radiant barriers are included in a package of energy saving features sold to homeowners. When considering a "package deal", you may want to ask for an itemized list that includes material and installation costs for all measures included. Then shop around to see what each item would cost if purchased individually before you make a decision.
What should conventional insulation cost? Heating and cooling bills can also be reduced by adding conventional attic insulation. So that you can have some basis for comparison shopping, typical installed costs for adding various levels of insulation are given in Figure 67. These costs are typical for insulation installed by contractors. Actual insulation costs will vary from region to region of the country, will vary with the type of insulation selected (blown, or loose-fill, insulation is usually lower in price than "batt" insulation), and may vary from one local contractor to another. You can expect to deduct 20% to 50% for a do-it-yourself application. Radiant Barriers Radiant barriers-usually fabricated from aluminum foils-can be used to reduce cooling loads. These reflective insulation systems are usually installed directly under the roof rafters to reduce heat gain from the sun. They can also be very effective when used for walls that absorb direct sunlight, especially if an effective roof overhang is not practical (for example walls facing west).
Radiant barriers are more effective in hot climates than in cool climates. In heating dominated climates, they aren't very economical nor recommended in most cases.
Unlike other insulation, there currently isn't a standard method for equating how well a radiant barrier works. Many manufacturers use the term "equivalent R-value." This really has no scientific meaning, and it often reflects optimum conditions and not necessarily climate conditions.
All radiant barriers must have a low emittance (0.1 or less) and high reflectance (0.9 or more). Most of them on the market today have about the same emissivity values. Therefore, you should consider other characteristics (strength, flammability, availability, and cost) before you buy.
Radiant barriers can be anything that is very reflective (has a very low emissivity.) A good example of a radiant barrier is aluminum cooking foil since it reflects heat into or away from food while cooking. Since the 1930s (which is about when aluminum foil was invented) radiant barrier materials have proven themselves to be a potentially effective method of keeping unwanted heat out of buildings too. They are predominantly beneficial for buildings in cooling dominated climates.
Unlike the more common types of insulation (i.e., fiberglass, cellulose, etc. that trap pockets of a gas which in turn reduces heat conduction) radiant barriers reduce only radiant heat transfer. A single layer of reflective material, properly installed between the hotter roof deck and the attic floor, may reduce radiant heat transfer to the attic by about 95%.
Radiant barriers used in buildings to reduce cooling needs are usually installed directly under the roof to reduce heat gain from the sun. You may also see recommendations to apply them directly over any other attic floor insulation, however this strategy is NOT recommended since dust will quickly cover the reflective surface and reduce the insulating effect significantly.
Radiant barriers are also very effective for walls that get direct sunlight hitting them, and where building an effective roof overhang is not practical (for example walls facing west.) However, unless you plan on rebuilding the wall for another reason (or the cost of labor is nominal) this application may be too expensive to have a reasonable economic payback in retrofit situations.
It is also worth noting that research has proven that radiant barriers installed in heating dominated climates are not economic at all unless the material and labor costs are extremely low (almost free.) And, in some cases, radiant barriers have been seen to actually increase heating costs since the attic was kept cooler during sunny winter weather. This increased heat loss from the living space below during those sunny daytime hours.
How Radiant Barriers Work: Heat travels from a warm area to a cool area by a combination of conduction, convection, and radiation. Heat flows by conduction from a hotter material to a colder material when the two materials touch. Heat transfer by convection occurs when a liquid or gas is heated, becomes less dense, and rises. Radiant heat travels in a straight line away from the hot surface and heats anything solid as the wave of energy hits it.
When the sun heats a roof, it is primarily the sun's radiant energy that makes the roof hot. A large portion of this heat travels by conduction through the roofing materials to the attic side of the roof. The hot roof material then radiates it's gained heat energy into the cooler attic (some of the roof's heat will radiate in other directions too.) A radiant barrier reduces the radiant heat transfer from the roof to the attic space.
Some radiant barrier installations work better than others. The key to understanding them is 1) the emissivity of the material's surface; 2) the temperature of the reflective surface and the surface temperature of the source of the heat; 3) and the angle the heat hits the receiving surface.
Of these items, the angle the heat wave strikes the surface has the most influence on how well any shiny surface acts as a thermal insulator. All radiant barriers work best when heat hits the surface at a right angle (perpendicular.) From one brand of radiant barrier to another, how shiny the material is, and it's emissivity, is so similar that it makes little difference as far as thermal performance. (Most products have emissivities between 0.03 to 0.05, which is the same as a reflectivity of 97% to 95%.) Also the greater the desired temperature difference between the sides of the radiant barrier material, the greater the benefits a radiant barrier can offer.
It may also be worth noting that a very glossy white paint is within 10% of as good reflector of heat as most of the common radiant barriers currently available in residential construction.
Since insulation of any type is intended to keep heat from moving in an undesired direction, the amount of thermal insulation already in the attic affects the benefits a radiant barrier may have in energy savings. For example, installing a radiant barrier in an attic that already has at least the DOE minimum recommendation of attic insulation for that climate will be less cost effective than an attic insulated to an inferior level. It is also necessary that an air space be on at least one side of the reflecting surface. If the material intended as a radiant barrier is "sandwiched" between two other materials without the air space the insulating effect from the foil surface is reduced to zero.
Selection: First determine if a radiant barrier is worthwhile for your climate. As mentioned before, they are intended for very sunny, hot climates to reduce cooling loads. Radiant barriers tend to offer little if any benefits in Northern heating dominated climates. They are also of minor value if your home's roof is already heavily shaded by trees or has at least the minimum insulation recommended for your climate.
Most radiant barrier materials on the market today have about the same emissivity values. Therefore, you should consider other characteristics (strength, flammability, availability, and cost) before you buy.
Resistance to tearing is important especially for the "do-it-yourself" installation. A logical method for you to test its strength is to obtain a sample and try to tear it by hand. A barrier that tears easily may rip at fastening points and make installation difficult, if not impossible. The types that are least susceptible to tearing usually have a woven mesh laminated between two sheets of foil. Other types use a "bubble-pack" between the foil faces. This is similar to the plastic sheets typically used to cushion packages for shipping.
You should also check the flammability rating. Choose one that has a Uniform Building Code (UBC) Class I, or National Fire Protection Association (NFPA) Class A, flammability rating. If you are uncertain ask your local fire department or building inspectors office about suitability of the product you are considering.
Radiant barriers also direct heat back through the roofing materials and may raise shingle temperatures 1° to 10° F (17.2° to 12.2° C) but this has been proven to have no detrimental effects on roofing materials.
Since there is currently no standard method for equating how well a radiant barrier works when comparing it to other insulation types, many manufacturers use the term "equivalent R-value." Be wary of such claims since this has no scientific or legal meaning, and manufacturers' claims often reflect optimum conditions and not necessarily climate conditions at your home site.
Installation: There are a variety of installation options for radiant barriers: There are rolled foil types, radiant barrier paints, metal shingles, and roof sheathing that has a radiant barrier laminated to it. There is even aluminum foil "chips" mixed with blown-in cellulose insulation. All of these work in similar ways, but some types work a little better than others under specific circumstances.
When installing a rolled foil barrier, the easiest way to install it is during new construction. The installer typically drapes the radiant barrier, foil-face down between the roof rafters to minimize dust accumulation on the reflective faces (double-faced radiant barriers are available.) This is generally done just before the roof sheathing goes on if it's not too windy, but it can also be done afterwards from inside the attic by stapling it to the bottom of the rafters.
When installing a foil-type barrier it is important to allow the material to "droop" between the attachment points to make at least a 1.0 inch (2.5 cm) air space between it and the bottom of the roof. This air space has mainly two functions: it creates an air channel for the soffit and ridge ventilation system to work more effectively; and acts as a second reflector since there are two shiny sides (one facing up/ one facing down.)
Some builders also try to attach the radiant barrier directly onto the roof sheathing prior to their installation on the roof rafters, but a more effective method is to simply buy foil-faced plywood sheathing instead. There are also metal roof shingles that have a reflective underside. If you needed roof shingles anyway, these are a practical option although the cost of the material is considerably higher than other types of radiant barriers.
As mentioned earlier, a radiant barrier on top of attic floor insulation is more susceptible to dust accumulation. This undesirable method may also trap moisture in the fiber insulation since, during cold weather, a radiant barrier on the cold side of the insulation acts as a vapor barrier in the wrong location. When warm moisture carrying house air leaks into the attic in the winter, it may condense on the underside of the barrier. Even a perforated radiant barrier can trap moisture in cold climates since the water can freeze in the small holes and seal them. Because of these hazards it is strongly recommend that you NOT apply radiant barriers directly on top of the attic floor insulation. Of course, installing it at all in a cold climate is not generally cost effective anyway.
How Effective are Radiant Barriers? During the summer, an attic radiant barrier, combined with existing R-19 attic insulation, will usually result in a total cooling load savings of 2%-15%. Buildings with little to no attic insulation and more than the usual amount of attic ventilation typically provides the most dramatic energy savings from a radiant barrier. The hotter and sunnier the climate is, the more beneficial the radiant barrier installation becomes. The reduced heat gain may also allow you to install a smaller air-conditioning system, which results in even more saved energy.
For buildings in heating dominated climates (and with poor insulation on the attic floor) it is generally far more cost effective to install more than the minimum recommendation of ordinary insulation rather than a radiant barrier. This is because attics are often vented to the outdoors and heat entering the attic in the winter (through the inferior floor insulation) simply leaves the building through the attic vents whether or not a radiant barrier is present.
Also, in cold climates air-conditioning is usually a much lower priority than heating. Cold-climate homes also tend to have higher attic insulation levels when compared to more Southern climates. As mentioned before, large amounts of common attic insulation negates much of the usefulness of a radiant barrier even under favorable circumstances.
Two field tests, one in Minnesota and one in Canada, both found that a radiant barrier placed over R-19 attic floor insulation (which is less than half the DOE minimum recommendation for those climates,) found that the radiant barrier contributed to less than a 1% reduction in energy consumption for heating and cooling. |
