Radiant barrier
Encyclopedia
Radiant barriers or reflective barriers inhibit heat transfer
by thermal radiation
. Thermal energy may also be transferred via conduction or convection
, however, and radiant barriers do not necessarily protect against heat transfer via conduction or convection.
(also called the "emittance"). Emissivity is expressed as a number between zero (0) and one (1) at a given wavelength. The higher the emissivity, the greater the emitted radiation at that wavelength. A related material property is the reflectivity
(also called the "reflectance"). This is a measure of how much energy is reflected by a material at a given wavelength. The reflectivity is also expressed as a number between 0 and 1 (or a percentage between 0 and 100%). At a given wavelength and angle of incidence the emissivity and reflectivity values sum to 1 by Kirchoff's law
.
Radiant barrier materials must have low emissivity (usually 0.1 or less) at the wavelengths at which they are expected to function. For typical building materials, the wavelengths are in the mid- and long- infrared spectrum
, in the range of 3 - 15 micrometres.
Radiant barriers may or may not exhibit high visual reflectivity. This is because while reflectivity and emissivity must sum to unity at a given wavelength, reflectivity at one set of wavelengths (visible) and emissivity at a different set of wavelengths (thermal) do not necessarily sum to unity. Though, it is possible to create visibly dark colored surfaces with low thermal emissivity.
To perform properly, radiant barriers need to face open space (e.g., air or vacuum) through which there would otherwise be radiation.
Radiant barrier reflective insulation by Eagle Shield is a Space Foundation
Certified Space Technology(TM); radiant barrier insulation was inducted into the Space Technology Hall of Fame
in 1996.
". Sputtering occurs when a metal, most often aluminum, is vaporized and the polyester film is passed through it. This process can be adjusted to control the amount of metal that ultimately coats the surface of the film. In some applications such as the "space blanket", 100% of the surface of the film is covered to cause maxium reflectivity. This is not always the case, however, and films with less than full coverage have been used to fabricate products such as mirrored sunglasses, visors for helmets worn by pilots and astronauts and window shades.
The use of the tinted film to be applied to glass was originally developed by a World War II pilot named Sonny Voges. Personnel in air traffic control towers, especially those on air craft carriers, need to be able to see aircraft as the approach for landing. During World War II this was particularly necessary on aircraft carriers so that air traffic controllers could spot planes with disabled or damaged landing gear to minimize the risk of a landing deck crash. Ships frequently turned into the wind to accommodate landing and this could put the sun aft of the ship. Planes flying out of the sun onto a carrier deck were particularly hard to see.
After the war, Mr. Voges approached a company called Martin Processing located in Martinsville, Virginia to see about developing a film that could be tinted and applied to glass as a means of controlling glare. The original window tint was designed for optical enhancement, but subsequently became known for its thermal control properties as well. The original film made by Martin Processing was made of tinted cellulose acetate and worked well for the purpose of optical enhancement and glare control, but had a substantail shortcoming in that it was incredibly flammable. The silver coating on the Hindenburg
, the zeppellin that famously burst into flames and burned in minutes, was made of cellulous acetate. Largely because there was no available option, the celluslose acetate version of the film was deployed by the military. The film was put to use at Edwards Air Force Base as the first jet aircraft were being tested. It was not unusual for jet wash to hit windows in hangars tinted with the cellulose acetate film only to catch fire.
In 1951, the Du Pont Corporation introduced a polymide material that came to be known as polyester. Almost immediately polyester found its way into film manufacturing. Polyester film was strong, and more importantly, it was not combustible like cellulose acetate. Nearly all window tint film sold since 1960 has been made of polyester or newer polymide materials like Kapton.
It could be metalized and, in fact, is still in use today as a radiant barrier material. Sometime referred to as "MPET" this film makes an excellent radiant barrier for window treatments that becoming part of the passive thermal control strategies used in "green" building.
This is different from the cool roof
strategy which reflects solar energy before it heats the roof. Although a cool roof
has been demonstrated to be more effective, a radiant barrier can be a less expensive retrofit.
Radiant barriers can also reduce indoor heat losses through the ceiling in the winter.
On residential homes radiant barrier is typically installed one of two ways: Radiant Barrier Decking or Radiant Barrier Attic Foil.
New construction: A product called Radiant Barrier Decking is used. This product is made by laminating a highly reflective piece of aluminum foil to one side of OSB board or plywood. The foil side will face the attic which creates the required air space.
It is important to note that if the structure is located in a humid area, the radiant barrier should be perforated to ensure proper passage of moisture. Often pre-laminated radiant barrier sheathing will have vapor barrier properties because the lamination adhesive will fill in the perforations. It is often better to use a perforated, reflective radiant barrier and staple it to the roof decking before installing it on the rafters.
Existing homes: Radiant Barrier Attic Foil is installed inside the attic. This product is a tarp-type material with a layer of aluminum foil laminated on both sides to create a double-sided radiant barrier. The radiant barrier foil material can be either stapled to the bottom of the roof rafters or laid out over the existing insulation. Both methods provide the required air space. If put over existing insulation, care must be taken to ensure moisture is not trapped in the insulation by using a perforated product that is NOT a vapor barrier (in a heating climate, i.e., when the house normally is warmer than the ambient air). In hot humid climates a non-perforated (vapor barrier) type radiant barrier is NEVER recommended in the interior of the structure.The ONLY appropriate placement of a vapor barrier type radiant barrier in hot humid climates in on the outside of the exterior sheathing (hot side of the insulation).
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
by thermal radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
. Thermal energy may also be transferred via conduction or convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
, however, and radiant barriers do not necessarily protect against heat transfer via conduction or convection.
Reflectivity and emissivity
All materials give off, or emit, energy by thermal radiation as a result of their temperature. The amount of energy radiated depends on the surface temperature and a property called the emissivityEmissivity
The emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...
(also called the "emittance"). Emissivity is expressed as a number between zero (0) and one (1) at a given wavelength. The higher the emissivity, the greater the emitted radiation at that wavelength. A related material property is the reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
(also called the "reflectance"). This is a measure of how much energy is reflected by a material at a given wavelength. The reflectivity is also expressed as a number between 0 and 1 (or a percentage between 0 and 100%). At a given wavelength and angle of incidence the emissivity and reflectivity values sum to 1 by Kirchoff's law
Kirchhoff's law of thermal radiation
In thermodynamics, Kirchhoff's law of thermal radiation, or Kirchhoff's law for short, is a general statement equating emission and absorption in heated objects, proposed by Gustav Kirchhoff in 1859, following from general considerations of thermodynamic equilibrium and detailed balance.An object...
.
Radiant barrier materials must have low emissivity (usually 0.1 or less) at the wavelengths at which they are expected to function. For typical building materials, the wavelengths are in the mid- and long- infrared spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
, in the range of 3 - 15 micrometres.
Radiant barriers may or may not exhibit high visual reflectivity. This is because while reflectivity and emissivity must sum to unity at a given wavelength, reflectivity at one set of wavelengths (visible) and emissivity at a different set of wavelengths (thermal) do not necessarily sum to unity. Though, it is possible to create visibly dark colored surfaces with low thermal emissivity.
To perform properly, radiant barriers need to face open space (e.g., air or vacuum) through which there would otherwise be radiation.
Space exploration
In 1954, NASA invented a lightweight, reflective material composed of a plastic substrate with a vapor-deposited coating of aluminum. The material, now commonly known as a "space blanket", is used to protect spacecraft, equipment, and astronauts from thermal radiation or to retain heat in the extreme temperature fluctuations of space. In the vacuum of space, heat transfer is only by radiation, so a radiant barrier is much more effective than it is on earth, where heat transfer can still occur via convection and conduction, even when an effective radiant barrier is deployed.Radiant barrier reflective insulation by Eagle Shield is a Space Foundation
Space Foundation
The Space Foundation is a nonprofit organization that supports the global space industry through information and education programs. It is a resource for the entire space community - industry, national security organizations, civil space agencies, private space companies and the military around the...
Certified Space Technology(TM); radiant barrier insulation was inducted into the Space Technology Hall of Fame
Space technology hall of fame
The Space Technology Hall of Fame annually honors organizations and individuals that "transform technology originally developed for space exploration into products that help improve the quality of life here on Earth"....
in 1996.
Where to install radiant barrier insulation
"Reflective" or "low emissivity" material can be used with equal effectiveness on the outside or the inside of a wall that needs to be insulated.Textiles
- Fire proximity suit
- Space blanket
- Thermo-lite (medical)
Window Treatments
Windows glass can be coated to achieve low emissivity. Some windows use laminate polyester film wherein at least one layer has been metalized using a process called "sputteringSputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
". Sputtering occurs when a metal, most often aluminum, is vaporized and the polyester film is passed through it. This process can be adjusted to control the amount of metal that ultimately coats the surface of the film. In some applications such as the "space blanket", 100% of the surface of the film is covered to cause maxium reflectivity. This is not always the case, however, and films with less than full coverage have been used to fabricate products such as mirrored sunglasses, visors for helmets worn by pilots and astronauts and window shades.
The use of the tinted film to be applied to glass was originally developed by a World War II pilot named Sonny Voges. Personnel in air traffic control towers, especially those on air craft carriers, need to be able to see aircraft as the approach for landing. During World War II this was particularly necessary on aircraft carriers so that air traffic controllers could spot planes with disabled or damaged landing gear to minimize the risk of a landing deck crash. Ships frequently turned into the wind to accommodate landing and this could put the sun aft of the ship. Planes flying out of the sun onto a carrier deck were particularly hard to see.
After the war, Mr. Voges approached a company called Martin Processing located in Martinsville, Virginia to see about developing a film that could be tinted and applied to glass as a means of controlling glare. The original window tint was designed for optical enhancement, but subsequently became known for its thermal control properties as well. The original film made by Martin Processing was made of tinted cellulose acetate and worked well for the purpose of optical enhancement and glare control, but had a substantail shortcoming in that it was incredibly flammable. The silver coating on the Hindenburg
LZ 129 Hindenburg
LZ 129 Hindenburg was a large German commercial passenger-carrying rigid airship, the lead ship of the Hindenburg class, the longest class of flying machine and the largest airship by envelope volume...
, the zeppellin that famously burst into flames and burned in minutes, was made of cellulous acetate. Largely because there was no available option, the celluslose acetate version of the film was deployed by the military. The film was put to use at Edwards Air Force Base as the first jet aircraft were being tested. It was not unusual for jet wash to hit windows in hangars tinted with the cellulose acetate film only to catch fire.
In 1951, the Du Pont Corporation introduced a polymide material that came to be known as polyester. Almost immediately polyester found its way into film manufacturing. Polyester film was strong, and more importantly, it was not combustible like cellulose acetate. Nearly all window tint film sold since 1960 has been made of polyester or newer polymide materials like Kapton.
It could be metalized and, in fact, is still in use today as a radiant barrier material. Sometime referred to as "MPET" this film makes an excellent radiant barrier for window treatments that becoming part of the passive thermal control strategies used in "green" building.
Construction
Solar energy is absorbed by a 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 directly underneath the roofing material incorporating an air gap, much of the heat radiated from the hot roof is reflected back toward the roof and the low emissivity of the underside of the radiant barrier means very little radiant heat is emitted downwards. 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.This is different from the cool roof
Cool roof
Cool roofs are the roofs that can deliver high solar reflectance and high thermal emittance...
strategy which reflects solar energy before it heats the roof. Although a cool roof
Cool roof
Cool roofs are the roofs that can deliver high solar reflectance and high thermal emittance...
has been demonstrated to be more effective, a radiant barrier can be a less expensive retrofit.
Radiant barriers can also reduce indoor heat losses through the ceiling in the winter.
On residential homes radiant barrier is typically installed one of two ways: Radiant Barrier Decking or Radiant Barrier Attic Foil.
New construction: A product called Radiant Barrier Decking is used. This product is made by laminating a highly reflective piece of aluminum foil to one side of OSB board or plywood. The foil side will face the attic which creates the required air space.
It is important to note that if the structure is located in a humid area, the radiant barrier should be perforated to ensure proper passage of moisture. Often pre-laminated radiant barrier sheathing will have vapor barrier properties because the lamination adhesive will fill in the perforations. It is often better to use a perforated, reflective radiant barrier and staple it to the roof decking before installing it on the rafters.
Existing homes: Radiant Barrier Attic Foil is installed inside the attic. This product is a tarp-type material with a layer of aluminum foil laminated on both sides to create a double-sided radiant barrier. The radiant barrier foil material can be either stapled to the bottom of the roof rafters or laid out over the existing insulation. Both methods provide the required air space. If put over existing insulation, care must be taken to ensure moisture is not trapped in the insulation by using a perforated product that is NOT a vapor barrier (in a heating climate, i.e., when the house normally is warmer than the ambient air). In hot humid climates a non-perforated (vapor barrier) type radiant barrier is NEVER recommended in the interior of the structure.The ONLY appropriate placement of a vapor barrier type radiant barrier in hot humid climates in on the outside of the exterior sheathing (hot side of the insulation).
See also
- Aluminized clothAluminized clothAluminized cloth is a material designed to reflect thermal radiation. Applications include fire proximity suits, emergency space blankets, and insulation for building and containers.-See also:* Fire proximity suit* Reflectivity* Space blanket...
- Bivouac sackBivouac sackA bivouac sack is an extremely small, lightweight, waterproof shelter, and an alternative to traditional tent systems. It is used by climbers, mountaineers, hikers, ultralight backpackers, soldiers and minimalist campers...
- EmissivityEmissivityThe emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...
- Fire proximity suitFire proximity suitA fire proximity suit is a suit designed to protect a firefighter from high temperatures, especially near fires of extreme temperature such as aircraft fires....
- InsulationThermal insulationThermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
- ReflectivityReflectivityIn optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
- R-valueR-value (insulation)The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux through it or R = \Delta T/\dot Q_A.The R-value being discussed is the unit thermal resistance...
- Space blanketSpace blanketA space blanket is a blanket used in emergencies to reduce heat loss in a person's body caused by thermal radiation, water evaporation and convection.-Manufacturing:First developed by NASA in 1964 for the US space program, the material consists of a...
- Aluminize
External links
- List of Emissivity Values for Common Materials
- How a radiant barrier works
- Radiant Barriers: A Question and Answer Primer
- This entry incorporates public domain text originally from the Oak Ridge National Laboratory and the U.S. Department of EnergyUnited States Department of EnergyThe United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...
. http://www.ornl.gov/sci/roofs+walls/radiant/rb_01.html - Effect of Radiant Barriers on Heating and Cooling Bills Department of Energy
- The Physics of Foil: Heat Gain / Loss in Buildings