Course Learning Objectives:
- Discuss heat transfer, with an emphasis on radiant heat transfer
- Explain the underlying principles of reflective insulation and radiant
barriers
- Clarify the differences between these two reflective technologies and
illustrate applications best suited to each product
- Provide a working knowledge of the effective use of reflective insulation and radiant barriers
- The heat transferred from a hot coffee, though the cup,
- to the hand holding the cup.
- The contents of the kettle boils from heat transferred
- from the burner to the kettle.
- A metal poker becomes hot from contact with
- the hot coals.
- Warm air rising from a furnace register (forced convection)
- Warm air rising from all surfaces of a steam radiator
- (after air in contact with radiator has been heated by conduction)
- Warm air rising from a chimney (free convection)
- Asphalt .90-.98 Paint (white lacquer) .80
- Aluminum foil .03-.05 Paint (white enamel) .91
- Brick .93 Paint (black lacquer) .80
- Concrete .95 Paint (black enamel) .91
- Glass .95 Paper .92
- Fiberglass/Cellulose .8-.9+ Plaster .91
- Iron (polished) .06 Silver .02
- Iron (rusty) .85 Steel (mild) .12
- Limestone .36-.90 Wood .90
- Marble .93
- Reflective insulation has very low emittance values "E-values" (typically 0.03 compared to 0.90 for most insulation) thus significantly reduces heat transfer by radiation;
- A reflective insulation does not have significant mass to absorb and retain heat;
- Reflective insulation has lower moisture transfer and absorption rates, in most cases;
- Reflective insulation traps air with layers of aluminum, paper and/or plastic as opposed to mass insulation which uses fibers of glass, particles of foam, or ground up paper;
- Reflective insulation does not irritate the skin, eyes, or throat and contains no substances which will out-gas;
- Residential Construction, New and Retrofit
- Walls, basements, floors, ceilings, roofs, and crawl spaces.
- Commercial Construction, New and Retrofit
- Walls, floors, basements, ceilings, roofs, and crawl spaces.
- Manufactured Housing Construction, New and Retrofit
Walls, floors, roofs, and crawl spaces. - Water heater covers, cold storage units, poultry, and livestock buildings, equipment sheds, pipe insulation and recreational vehicles.
- Installing Reflective Insulation Systems
- Aluminum Foil Laminates - foil laminated to kraft paper, plastic films, or to OSB/plywood roof sheathing
- Aluminized Plastic Films - a thin layer of aluminum particles deposited on film through a vacuum process
- >Reflective Paints/Coatings- liquids that reduce the emissivity of the surface of the surface to which they are applied.
Course:
The key to maintaining a comfortable temperature in a building is to reduce the heat transfer out of the building in the winter and reduce heat transfer into the building in the summer.
Heat is transmitted across confined air spaces by radiation, convection, and conduction. The goal of all insulation and barriers is to reduce heating and cooling loads. Reflective insulation, radiation control coatings, and radiant barriers are products that perform this function by reducing radiant heat transfer thereby reducing the heating and cooling requirements.
Heat Transfer Basics:
Heat flows from a hot or warm medium to a cold medium in three ways:
By conduction through solid or fluid materials
By convection, which involves the physical movement of air
By radiation from a warm surface to a cooler surface through an air space
Conduction:
Conduction is the direct flow of heat through a material resulting from physical contact. The transfer of heat by conduction is caused by molecular motion in which molecules transfer their energy to adjoining molecules and increase their temperature.
Examples of conduction:
Heat transfer by conduction is governed by the fundamental equation described by Fourier’s law:
(Rate of heat flow) = (k) x (Area) x (Temperature Gradient)
The factor k is called the thermal conductivity and is a characteristic of the material through which heat is flowing, and it varies with temperature and the degree of compaction or density.
Convection:
Convection is the transfer of heat in fluid, such as air, caused by the movement of the heated air or fluid. In a building space, warm air rises and cold air settles to create a convection loop and is termed free convection. Convection can also be caused mechanically, (termed forced convection), by a fan or by wind.
Examples of Convection:
In the flow of heat through a solid body to air, it was observed that the passage of heat into the air was not accomplished solely through conduction. Instead, it occurred partly by radiation and partly by free convection.
A temperature difference existed between the hot solid and the average temperature of the air. In this case, the resistance to heat transfer cannot be computed using the thermal conductivity of air alone. Instead, the resistance has to be determined experimentally by measuring the surface temperature of the solid, the temperature of air, and the heat transferred from the solid to air.
The resistance computed is the combined resistance of conduction, free convection, and radiation. This resistance, denoted by the letter "R", has the units of (hr ft2 °F/Btu) and is commonly used to indicate the thermal characteristics of insulation materials.
Radiation:
Radiation is the transfer of heat or energy from a hot surface to a cold surface through air or through a vacuum. All surfaces including a radiator, stove, a ceiling or roof
and ordinary insulation radiate to different degrees. The 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.
Example of Radiation:
On a bright sunny day, radiant heat from the sun travels through a car’s window, strikes the steering wheel and is absorbed, causing it to rise in temperature. In summer, radiation from the sun strikes the outer surfaces of walls and ceilings and is absorbed causing the surface to heat up. This heat flows from the outer wall to the inner wall through conduction which is then radiated again, through the air spaces in the building, to other surfaces within the building.Radiation between surfaces is through invisible, infrared heatrays
There are two terms commonly encountered while discussing radiant heat transfer:
Emittance (or emissivity), refers to the ability of a material’s surface
to emit radiant energy. All materials have emissivities ranging from zero
to one. The lower the emittance of a material, the lower the heat (infra-red
radiant energy) radiated from its surface. Aluminum foil has a very low
emittance, which explains its use in reflective insulation and radiant barriers.
The emittance of several surfaces is listed below:
Reflectance (or reflectivity) refers to the fraction of incoming radiant
energy that is reflected from the surface. Reflectivity and emissivity are
inversly related and a low emittance is indicative of a highly reflective
surface. For example, aluminum with an emissivity of 0.03 has a reflectance
of 0.97.
The Need for Insulation:
When installed correctly, insulation reduces the heat transfer through the envelope of a building. Whenever there is a temperature difference, heat flows naturally from a warmer space to a cooler space. To maintain comfort in winter, the heat lost must be replaced by the heating system; and in summer, the heat gained must be removed by the cooling system.
Statistics show that 50% to 70% of the energy used in the average home in the United States and Canada is for heating and cooling. It makes sense to use thermal insulation to reduce this energy consumption, while increasing comfort and saving money. Naturally, less consumption of fossil fuels and the energy produced from them relieves the burden our ecosystem must bear.
To summarize, insulating the envelope of a building’s conditioned space yields these key benefits:
Provides a much more comfortable, productive and livable structure. In addition,
the effects of moisture condensation and air movement are minimized in well-insulated
buildings. This results in lower maintenance costs and increased longevity
of the building structure.
Reduces energy requirements, which lowers utility bills.
Supports economic, environmental and energy conservation goals. This is
evidenced by the numerous studies sponsored by the Department of Energy.
Heat moves through wall cavities or between roofs and attic floors by radiation, conduction, and convection. In some buildings, radiation is the dominant method of heat transfer. A reflective insulation is an effective barrier against radiant heat transfer because it reflects almost all of the infrared radiation striking its surface and emits very little of the heat conducted through it. By virtue of its impermeable surface, reflective insulation also reduces convective heat transfer. Mass insulation like fiberglass or foam board primarily slows conductive heat transfer, and to a smaller extent, convective heat transfer. However, mass insulation is not as effective against infrared radiation, actually absorbing it rather than reflecting or blocking it.
Reflective Insulation:
Standard types of insulation, such as fiberglass, foam, and cellulose primarily reduce heat transfer by trapping air or some type of a gas. Thus these products or technologies reduce convection as a primary method of reducing heat transfer. They are not as effective in reducing radiant heat transfer, which is often a primary mode of heat transfer in a building envelope, in fact, these products, like most building materials, have very high radiant transfer rates. In other words the surfaces of standard types of insulation are good radiators of heat.
Reflective insulation uses layers of aluminum, paper, and / or plastic to trap air and thus reduce convective heat transfer. The aluminum component however is very effective in reducing radiant heat transfer. In fact, the metalized and foil materials commonly used in reflective insulation will reduce radiant heat transfer by as much as 97%.
Heat flow by radiation has been brought to the public’s attention with high efficiency windows, which commonly use the term "Low E" to advertise the higher performance ratings. The "E" stands for emittance and the values range from 0 to 1 with 0 being no radiation and 1 is the highest measure of emittance or radiation. Most building materials, including fiberglass, foam and cellulose have surface emittances or "E" values in excess of 0.70. Reflective insulations typically have "E" values of 0.03 (again, the lower the better).
Therefore, reflective insulation is superior to other types of insulating materials in reducing radiant heat. The term reflective, in reflective insulation, is in some ways a misnomer because aluminum either works by reflecting heat (reflectance of 0.97) or by not radiating heat (emittance of 0.03). Whether stated as reflectivity or emissivity, the performance (heat transfer) is the same.
When reflective insulation is installed in building cavities, it traps air (like other insulation materials) and therefore reduces heat flow by convection thus addressing all three modes of heat transfer. In all cases, the reflective material must be adjacent to an air space. Aluminum, when sandwiched between two pieces of plywood for example, will conduct heat at a high rate.
All insulation products including reflective insulation are measured by R-values, whereby the "R" stands for reduction of heat transfer. The larger the R-value, the greater the thermal performance of the insulating material.
Reflective insulation is a non-toxic building material that is user and building owner safe. In addition, the products are typically recyclable and thus can be termed a Green Building Material.
Another benefit is that the reflective insulation also can serve as a high performance and thus effective vapor barrier.
Understanding a Reflective Insulation System (RIS):
Layers of aluminum or a low emittance material and enclosed air spaces, which in turn provide highly reflective or low emittance cavities adjacent to a heated region, typically form a reflective insulation system.
Some reflective insulation systems also use other layers of materials such as paper or plastic to form additional enclosed air spaces. The performance of the system is determined by the emittance of the material(s), the lower the better, and the size of the enclosed air spaces. The smaller the air space, the less heat will transfer by convection.
Therefore, to lessen heat flow by convection, a reflective insulation, with its multiple layers of aluminum and enclosed air space, is positioned in a building cavity (stud wall, furred-out masonry wall, floor joist, ceiling joist, etc.) to divide the larger cavity (3/4" furring, 2" x 4", 2" x 6", etc.) into smaller air spaces. These smaller trapped air spaces reduce convective heat flow.
Reflective insulation differs from conventional mass insulation in the following:
The change in thermal performance due to compaction or moisture absorption, a common concern with mass insulation, is not an issue with reflective insulation.
Types of Reflective Insulation Materials:
Other Uses, New and Retrofit
Reflective insulation products incorporate trapped air spaces as part of the system. These air spaces, which may be layered or closed-cell, can be included in the system either when the product is manufactured or while it is being installed. In either case, the advertised performance of the insulation requires that these air spaces be present after the product is installed. The labeled R-values will not be achieved if the product is not installed according to the instructions of the manufacturer.
The thermal performance of the reflective system varies with the size and number of enclosed reflective spaces within the building cavity. Most reflective systems range from one to five enclosed air spaces.
Air spaces in typical Reflective Insulation System:
There are other beneficial considerations for using reflective insulation. Generally, these products have a very low water vapor and air permeance. When installed properly, with joints taped securely, reflective insulation materials are efficient vapor retarders and an effective barrier to air and radon gas.
Since reflective insulation materials are effective vapor retarders, care should be taken to ensure that they are installed correctly within the structure. Correct installation depends on the climatic conditions and moisture sources involved. An appropriate installation ensures that all joints and seams are butted against each other and taped, or overlapped and taped. This will reduce the possibility of moisture condensation within the cavity and improve performance.
Radiant Barriers:
Definition: The generally accepted definition of a radiant barrier system specifies that the reflective material face an open air space. The idea is that a radiant barrier facing an enclosed air space is a "reflective insulation" with a measurable R- value.
Physics of Radiant Barriers:
A "radiant barrier" is a reflective/low-emittance surface, on or near a building component that intercepts the flow of radiant energy to and from the building component. It is, as the name suggests, a barrier to radiant heat movement, the same as a vapor barrier blocks water vapor migration and an air barrier stops air flow.
The aluminum foil shields that are commonly inserted behind radiators in older houses are radiant barriers, blocking radiant heat transfer from the radiator to the exterior wall.
It should be clearly understood that although a radiant barrier reduces heat loss and gain through the building envelope because they are installed in vented cavities (like attics), it is not an insulation material per se and has no inherent R-value.
Radiant Barrier Systems (RBS):
A "radiant barrier system" (RBS) is a building section that includes a radiant barrier facing an air space. An attic with a radiant barrier on top of the mass insulation on the floor, or under the roof. is an RBS. A vent skin wall with a radiant barrier facing the vented air space is also an RBS.
The distinction between a radiant barrier "material" and radiant barrier "system" is not merely academic. In an attic, the effectiveness of a radiant barrier is significantly affected by the amount of attic ventilation. A vented attic with a radiant barrier is a very different system from an unvented attic with the same radiant barrier.
Types of Radiant Barrier Material:
Several types of radiant barrier materials are available. Although they all have similar surface properties (and consequently similar performance), variations in materials and construction result in significant differences with respect to strength, durability, flammability and water vapor permeability.
Most products available commercially fall into three major categories:
Installing Radiant Barriers:
Attics: The most common location for a radiant barrier system is in attics
Rafter/truss installations
Prelaminated to roof sheathing
Top of attic floor insulation
As noted before, a vented attic with a radiant barrier is a very different system from an unvented attic with the same radiant barrier. Common types of attic ventilation are:
*Soffit to ridge *Soffit to soffit
*Soffit to gable *Gable to gable
Most codes require at least a 1 to 300 ventilation rate. What this means is that for every 300 square feet of floor space, there should be one square foot of free vent area.
Walls:
A very effective technique for walls is a vented skin wall using a radiant barrier. Furring strips are used to separate the outer skin from the internal structural wall. The wall is wrapped with a radiant barrier facing the vented air space. Vents are used top and bottom
to allow the heated air to rise naturally to the attic where it is vented out through the roof vents.
TECHNICAL NOTE: Radiant barriers, which are non-perforated, are vapor barriers. Care should be exercised with placement!
Floors:
Radiant barriers can also be used in floor systems above unheated basements and crawl spaces. The radiant barrier is either stapled to the underside of floor joists, creating a single reflective air space, or between the joists, followed by some type of sheathing, creating two separate reflective air spaces.
Radiant barriers make an ideal choice for this application because, in addition to their excellent thermal properties, they are also vapor barriers that prevent ground moisture from migrating into the living space above.
Interior Radiation Control Coatings (IRCC):
Definition of an IRCC - As characterized by ASTM, an Interior Radiation Control Coating is a non-thickness dependent, low emittance coating. When applied to non-porous building materials such as plywood, OSB, metal siding or plasterboard, according to the manufacturer’s installation instruction, it lowers the normal surface emittance of these materials to 0.24 or lower.
Physics of an IRCC:
An IRCC works by changing the emittance of the surface where it is applied. Building products, such as wood, brick, painted surfaces and plasterboard exhibit high emissivities (0.7 - 0.95). When heated above the temperature of adjacent surfaces, they radiate most of their heat energy to cooler surfaces. An IRCC works by lowering their surface emittance to 0.24 or lower, lessening their ability to radiate heat.
Definition of an Interior Radiation Control System (IRCCS):
A building construction consisting of a low emittance (normally 0.25 or less) surface bounded by an open air space. An IRCCS is used for the sole purpose of limiting heat transfer by radiation and is not specifically intended to reduce heat transfer by convection or conduction. (ASTM C 1321, section 3.2.3)
Thus, an IRCCS is similar to a Radiant Barrier System (RBS) but is somewhat less efficient due to its higher emissivity and is comprised of a coating on a building surface, not a foil or film product.
Advantages of an IRCC:
An IRCC is normally applied using airless spray equipment, resulting in very low labor costs and greatly reduced installation times. Also, a water based IRCC can be safely installed in existing structures where the costs of installing foil or film products may be prohibitive or impractical. An IRCC may also be used in many manufactured products (such as infrared heat reflectors of automotive parts) where it is impractical to adhere foil or film radiant barriers.
Installation methods for an IRCC:
Since an IRCC is a paint product, spray painting, either air atomization or airless is the most effective method of installation. Where spray painting is not practical. An IRCC may be applied using a low nap roller. Brush painting is usually impractical since these coatings are very low viscosity and not formulated for brush application.
The IRCC may be applied to a building surface already in place (such as the underside of an installed roof deck or the inside of a wall) or it may be applied to a building component before it is installed (such as roof decking painted while laying on the ground before it is lifted into place. Regardless when a building component is painted with an IRCC, it is imperative that after installation the surface painted with the IRCC face a minimum of a 2" air space.
Other Possible Uses - Construction:
An IRCC is a paint product therefore it can be used on almost any solid surface where paint can be applied and where radiant heat transfer is a problem. An example would be painting the inside of a boiler room to retain heat that might make adjacent areas uncomfortable. Even painting the boiler, itself, might make it operate more efficiently. Freestanding heat shield in welding bays or at foundries can be painted with an IRCC. Exterior roof surfaces may also be painted with an IRCC to repel summer heat and lower radiation losses in the winter.
Other Possible Uses of an IRCC:
IRCC technology has many applications in manufacturing and industry. It is used in the automotive industry to keep temperature sensitive parts and automotive interiors cool. It is used in the lighting industry to make plastic reflectors for heat lamps and radiant heating devices. It is used as a heat-reflecting surface in industrial ovens. It is used on high temperature process piping and storage tanks in chemical plants to lessen heat loss. Any process or device that is temperature sensitive to infrared heat problems or uses reflected heat in its operation might be a candidate for IRCC technology.
End of Course: