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ABOUT RIMA............................................................................................................................................................................
The Reflective Insulation Manufacturers Association (RIMA) is the only trade association representing the reflective insulation, radiant barrier and radiant control coatings industries. RIMA activities are guided by an active board of industry members that participate on national and local levels of building code organizations and governmental agencies.
RIMA’s objective is to further the understanding and acceptance of reflective insulation, radiant control coatings, and radiant barriers. Toward this, RIMA members have contributed many articles that have appeared in magazines and newsletters such as:
Builders Magazine, Journal of Light Construction, Popular Mechanics, Popular Science, Architecture Magazine, RSI, Energy Design Update, Contractor’s Guide, Practical Homeowner, Rural Builder, Frame Builder Professional, Metal Construction News, Metal Architecture.
RIMA has
also contributed technical papers to various conferences and workshops sponsored
by the Department of Energy, ASHRAE, TVA, ASTM, and Oak Ridge National
Laboratory. RIMA members meet twice
a year at the ASTM C-16 Committee meetings to discuss current technical issues
and establish standards that promote the best use of reflective insulation,
radiant control coatings, and radiant barrier products.
RIMA’s members come from a variety of backgrounds including engineers,
scientists, manufacturers, marketers, and academicians.
The RIMA
Handbook aims to provide a simple yet comprehensive guide elaborating on the
fundamentals of heat transfer and the concept of reflective insulation and
radiant barriers.
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,
radiant control coatings, and radiant barriers are products that perform
this function by reducing radiant heat transfer thereby reducing the heating and
cooling requirements.
·
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 handbook does not intend to be a definitive source, but will cover some basic information. There are a large number of excellent authoritative publications about reflective technologies and products. They are listed in section 10, References, and are recommended for additional information and guidance. Our purpose in this section is to inform in an easily understandable way, the virtues of the reflective products represented by RIMA members.
Heat flows
from a hot or warm medium to a cold medium in three ways:
·
By radiation
from a warm surface to a cooler surface through an air space
·
By
conduction through solid or fluid materials
·
By
convection, which involves the physical movement of air
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.
A typical
example of conduction would be the heat transferred from hot coffee, through the
cup, to the hand holding the cup. Another example, as shown above, the contents of the kettle
boils from heat transferred from the burner to the kettle.
Also, the poker becomes hot from contact with
the hot coals.
Heat
transfer by conduction is governed by the fundamental equation described by
Fouier’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 its density.
The thermal
conductivity of typical building and insulation materials is listed below1:
|
Material |
k
(Btu/(h.ft2) (°F/ft) |
Btu
*in/ft2*h*°F |
|
Sawdust |
0.034 |
0.408 |
|
Wood
Shavings |
0.034 |
0.408 |
|
Mineral
Wool |
0.0217 |
0.260 |
|
INSULATION |
||
|
Std.
Fiberglass Batt |
0.313 |
3.2 |
|
High
Performance Fiberglass Batt |
0.263 |
3.8 |
|
Loose-Fill
Fiberglass |
0.400 |
2.5 |
|
Loose-Fill
Rock Wool |
0.357 |
2.8 |
|
Loose-Fill
Cellulose |
0.270 |
3.7 |
|
Expanded
Polystyrene |
0.263 |
3.8 |
|
Extruded
Polystyrene |
0.208 |
4.8 |
|
GASES |
||
|
Air |
0.181 |
5.52 |
|
Carbon
Dioxide |
0.113 |
8.85 |
|
Helium |
1.031 |
0.97 |
|
Methane |
0.234 |
4.27 |
|
LIQUIDS |
||
|
Ethylene
Glycol |
1.80 |
0.56 |
|
Gasoline |
0.94 |
1.06 |
|
Water |
4.19 |
0.24 |
|
METALS |
||
|
Aluminum |
1404 |
0.0007 |
|
Copper |
2636 |
0.0004 |
|
Iron |
468 |
0.0021 |
|
Lead |
241 |
0.0041 |
|
MISCELLANEOUS BUILDING
MATERIALS |
||
|
Acoustical
Tile |
0.40 |
2.5 |
|
Asphalt |
0.43 |
2.3 |
|
Concrete
(D=140 pcf) |
9.7 |
0.1 |
|
Cotton
(D=6 pcf) |
0.42 |
2.4 |
|
Glass |
9.7 |
0.1 |
|
Soil
(D=130 pcf) |
3.6 |
0.3 |
|
Fir Lumber |
0.76 |
1.3 |
|
Oak Lumber |
1.18 |
0.8 |
|
Yellow
Pine Lumber |
1.04 |
1.0 |
|
Plywood |
0.83 |
1.2 |
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.
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 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.
This concept can be understood with the
following example: 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, infra-red
heat rays
There are
two terms commonly encountered while discussing radiant heat transfer:
1.
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
radiated from its surface. Aluminum
foil has a very low emittance which explains its use in reflective insulation.
2.
Reflectance (or reflectivity) refers to the fraction of
incoming radiant energy that is reflected from the surface. Reflectivity and emissivity are 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
emittance of various surfaces is listed in the following table2.
Material
Surface |
Emittance |
|
Asphalt |
0.90-0.98 |
|
*Aluminum
foil |
0.03-0.05 |
|
Brick |
0.93 |
|
Concrete |
0.85-0.95 |
|
Glass |
0.95 |
|
Fiberglass
/ Cellulose |
0.8-1.0 |
|
Iron
(polished) |
0.06 |
|
Iron
(rusty) |
0.85 |
|
Limestone |
0.36-0.90 |
|
Marble |
0.93 |
|
Paint:
white lacquer |
0.80 |
|
Paint:
white enamel |
0.91 |
|
Paint:
black lacquer |
0.80 |
|
Paint:
black enamel |
0.91 |
|
Paper |
0.92 |
|
Plaster |
0.91 |
|
Silver |
0.02 |
|
Steel
(mild) |
0.12 |
|
Wood |
0.90 |
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:
1.
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.
2.
Reduces
energy requirements, which lowers utility bills.
3.
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 with radiation 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.
Concept of Reflective Insulation
Different
types of insulation products reduce the heat transferred by conduction,
convection and radiation to varying degrees.
As a result, each provides different thermal performance and
corresponding “R” values. The
primary function of reflective insulation is to reduce radiant heat transfer
across open spaces, which is a significant contributor to heat gain in summer
and heat loss in winter. The low
emittance metal foil (usually aluminum) surface of the product blocks up to 97%
of the radiation and therefore a significant part of the heat transfer.
There are
many types of materials that reduce heat gain and heat loss. Some materials provide greater resistance than others,
depending on the mode of heat transfer: convection, conduction, or radiation.
Most insulation materials work on the principle of trapped air gas being
a good insulator. Mass insulation like fiberglass, foam, and cellulose use
layers of glass fibers, plastic, and wood fiber respectively to reduce
convection thereby decreasing the transfer of heat.
These materials also reduce heat transfer by conduction due to the
presence of trapped air. (However,
these products, like most building
materials, have very high radiant transfer rates.)
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. This
value is measured in emittance or “E” values ranging from 0 to 1 (lower
“E” value indicates better performance).
Most building materials, including fiberglass, foam and cellulose have
“E” values in excess of 0.70. Reflective
insulation 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 heat flow by radiation.
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.
Understanding a Reflective Insulation System (RIS)
A reflective
insulation system is typically formed by 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. 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.
Like other insulation, reflective insulation is labeled with R-values
which provide a measure of thermal performance.
Reflective
insulation differs from conventional mass insulation in the following:
1.
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;
2.
A reflective
insulation does not have significant mass to absorb and retain heat;
3.
Reflective
insulation has lower moisture transfer and absorption rates, in most cases;
4.
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;
5.
Reflective
insulation does not irritate the skin, eyes, or throat and contain no substances
which will out-gas;
6.
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
Reflective
insulation has been used effectively for decades and is available throughout the
world. The following are the major
types of reflective insulation currently available:
1.
Layer or
layers of aluminum foil separated by a layer or
layers of plastic bubbles or a foam material;
2.
Multiple
layers of aluminum, kraft paper, and/or plastic with internal expanders an
flanges at the edge for easy installation;
3.
Single layer
of aluminum foil laminated to a kraft paper or
plastic material.
Applications for Reflective Insulation Materials
Reflective
insulation materials are designed for installation between or over framing
members and as a result, are applicable to unfinished walls, floors, and
ceilings. Applications for
reflective insulation extend to many commercial, agricultural and industrial
uses, such as panelized wood roofs, pre-engineered buildings,
pole barns and other wood framed structures.
A few representative applications are listed below:
·
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.
·
Other Uses, New and Retrofit
Water heater covers, cold storage units, poultry, and livestock buildings, equipment sheds, pipe insulation and recreational vehicles.
Typical applications (new and retrofit) for reflective insulation
Reflective
insulation in typical basement installation
Installing Reflective Insulation Systems
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, as
shown in the figure and schematic below.
Schematic
of reflective insulation installed between framing members
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.
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.
A radiant
barrier can be aluminum foil laminate, aluminized plastic film or a low
emittance coating. The only
requirement is that its surface must have low emittance and high reflectivity in
the infrared band of the spectrum.
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. The invisible glass
coating in low-E windows is also a radiant barrier.
It should be
clearly understood that although a radiant barrier reduces heat loss and gain
through the building envelope, it is not an insulation material per se and has
no inherent R-value.
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.
TECHNICAL NOTE: 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.
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:
1. Aluminum Foil Laminates - foil laminated to kraft paper, plastic films, or to OSB/plywood roof sheathing
2. Aluminized Plastic Films - a thin layer of aluminum particles deposited on film through a vacuum process
3. Reflective Paints/Coatings - liquids that reduce the
emissivity of the surface to which they are applied
The most
common location for a radiant barrier system is in attics. Three basic configurations are used:
1.
Rafter/truss
installation
2.
Under, or pre-laminated to, roof sheathing
3.
Horizontal installation (directly above ceiling
and/or ceiling 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
gable
·
Soffit to
soffit
·
Gable to
gable
Most codes
require at least a 1 to 150 ventilation rate.
What this means is that for every 150 square feet of floor space, there
should be one square foot of free vent area.
Several
types of radiant barriers are used in walls, as shown in the figure below.
An example is foil faced fiberglass batts stapled to the sides of the
wall studs, leaving an air space between the foil facing and interior sheathing.
Another less common technique is to use foil faced drywall over furring
strips on the interior stud faces. The
furring strips create an air space between the foil facing and cavity
insulation. The technique used
commonly in Florida is to apply a radiant barrier to the exterior of the wall,
followed by furring strips and sheathing. In
this construction, commonly referred to as “vent skin” construction, the air
space created by the furring strips is typically vented top and bottom so that
outdoor air can circulate into and through the space.
TECHNICAL NOTE: When an interior barrier is used, all seams should
be taped to avoid possible moisture migration.
When an exterior barrier is used, it should be perforated unless a vapor
retarder is used on the interior side, otherwise it may trap moisture.
Application techniques will vary depending on the climate in which
radiant barriers are used.
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 as shown below.
Laboratory
experiments and computer modeling suggest that floor radiant barrier systems may
exhibit R-values as high as R-7.5 to R-8.0 for reducing heat loss to basements
and crawl 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
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, it lowers the normal surface emittance of these
materials to 0.24 or lower and may be effectively used as an interior radiant
barrier.
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 and IRCC, it is imperative that after installation the surface
painted with the IRCC face a minimum of a 2” air space.
Typical
installations of an IRCC
Under
Roof
Interior Side Walls
Exterior Side Walls
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 may be a candidate for IRCC technology.
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.
Convection: Convection is the transfer of heat in fluid or air,
caused by the movement of the heated air or fluid itself. 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 by a fan and is termed forced
convection.
Emittance: Emittance refers to the ability of the surface to
emit radiant energy. Emissivity
ranges from 0 to 1 and a lower value indicates a reflective surface with a low
level of radiation.
“R” value:
Property
of an insulation material used to characterize the effectiveness of the
insulation in reducing heat transfer by conduction. The higher the “R” value, the better the insulation’s
ability to reduce this heat transfer.
Radiation: Radiation is the transfer of heat or energy from a
hot surface to a cold surface through air or through a vacuum.
Radiant Barrier: A radiant barrier is a reflective surface, on or
near a building component, that intercepts the flow of radiant energy to and
from the building component.
Radiant Barrier System: A Radiant Barrier System (RBS) is a building
section that includes a radiant barrier facing an air space.
Reflectance: Reflectance refers to the fraction of incoming
radiant energy that is reflected from the surface.
Reflective Insulation
System: Reflective Insulation System is formed by a
combination of low emittance surfaces and air spaces that provide reflective
cavities which have low levels of radiant energy transmission.
The
following list of references is selective rather than exhaustive. Technical papers, reports, sections of books, and important
compliance documents have been included. Many
of the papers and reports contain references that broaden the list and provide
additional insight into the performance of reflective insulation and radiant
barriers.
Reviews
1.
Gross and
R.G. Miller, “Literature Review of Measurement and Predictions of Reflective
Building Insulation System Performance: 1900-1989”, ASHRAE Transactions 95
(2) 651-664 (1989).
2.
Ned Nisson:
Radiant Barriers, Principles, Practice, and Product Directory”, Energy Design
Update, Cutter Information Corporation, Arlington, MA (1990).
Technical Papers
1.
Ludwig,
Adams, “Thermal Conductance of Air Spaces”, ASHRAE Journal (March, 1976) pp.
37-38
2.
Cook, D.W.
Yarbrough, and K.E. Wilkes, “Contamination of Reflective Foils in Horizontal
Applications and the Effect on Thermal Performance”, ASHRAE Transactions 95
(1) (1989).
3.
Andre O.
Desjarlais and David W. Yarbrough, ‘“Prediction of the Thermal Performance
of Single and Multi-Airspace Reflective Insulation Materials”, Insulation
Materials: Testing and Applications,
2nd Volume, ASTM STP 1116, R.S. Graves and D.C. Wysocki, Editors, American
Society for Testing and Materials, Philadelphia (1991).
4.
Fairey,
“Effect of Infrared Radiation Barriers on the Effective Thermal Resistance of
Building Envelopes”, Proceedings of the ASHRAE/DOE Conference on Thermal
Performance of the Exterior Envelopes of Buildings II, ASHRAE Special
Publication 38 (1983).
5.
Philip
Fairey, “The Measured, Side-by-Side Performance of Attic Radiant Barrier
Systems in Hot-Humid Climates”, Thermal Conductivity 19, David W.
Yarbrough, Editor, Plenum Press (1988) pp. 481-496.
6.
Robert
Hageman and Mark P. Medera, “Energy Savings and HVAC Capacity Implications of
a Low-Emissivity Interior Surface for Roof Sheathing”.
7.
Joy,
“Improving Attic Space Insulating Values”, ASHRAE Transactions 64 251
(1959).
8.
Levins and
M.A. Karnitz, “Cooling Energy Measurements of Unoccupied Single-Family Houses
with Attics Containing Radiant Barriers”, ORNL/CON-200 (1986), Oak Ridge
National Laboratory, Oak Ridge, TN.
9.
Levins and
M.A. Karnitz, “Cooling Energy Measurements of Unoccupied Single-Family Houses
with Attics Containing Radiant Barriers, ORNL/CON-213 (1987), Oak Ridge National
Laboratory, Oak Ridge, TN.
10.
Levins and M.A. Karnitz, “Cooling Energy Measurements of Unoccupied
Single-Family Houses with Attics Containing Radiant Barriers, ORNL/CON-226
(1987), Oak Ridge National Laboratory, Oak Ridge, TN.
11.
Levins and M.A. Karnitz, “Cooling Energy Measurements of Unoccupied
Single-Family Houses with Attics Containing Radiant Barriers”, ORNL/CON-239
(1988), Oak Ridge National Laboratory, Oak Ridge, TN.
12.
Levins, M.A. Karnitz, and J.A. Hall, “Moisture Measurements in
Single-Family Houses Containing Radiant Barriers”, ORNL/CON-255 (1989), Oak
Ridge National Laboratory, Oak Ridge, TN.
13.
McQuiston, S.L. Der, and S.B. Sandoval, “Thermal Simulation of Attic
and Ceiling Spaces”, ASHRAE Transactions 90 739-163 (1984).
14.
Pratt, “Heat Transmission in Buildings, John Wiley and Sons,
Chapter 3, “The Thermal Resistance of Airspaces in Cavity Building
Structures”, (181) pgs. 66-98.
15.
Robinson and F.J. Powell, “The Thermal Insulating Value of
Airspaces”, Housing Research Paper No. 32, National Bureau of Standards
Project NE-12, National Bureau of Standards, Washington, DC (1954).
16.
Robinson, L.A. Cosgrove and F.J. Powell, “Thermal Resistance of
Airspaces and Fibrous Insulation Bounded by Reflective Surfaces”, Building
Materials and Structures Report 151, National Bureau of Standards, Washington,
DC (1957).
17.
St. Regis, “Reflective Insulation and the Control of Thermal
Environments”, St. Regis-ACI, Diethelm & Co., LTD, Bangkok, Thailand
(1969).
18.
Wilkes, “Thermal Modeling of Residential Attics with Radiant Barriers:
Comparison with Laboratory and Field Data”, Thermal Performance of the
Exterior Envelopes of Buildings IV, ASHRAE (1989) pp. 294-311.
19.
Wilkes, “Thermal Model of Attic Systems with Radiant Barriers”, ORNL/CON-262
91991) Oak Ridge National Laboratory, Oak Ridge, TN/
20.
Kenneth E. Wilkes, “Analysis of Annual Thermal and Moisture Performance
of Radiant Barrier Systems”, ORNL/CON-319 (1991), Oak Ridge National
Laboratory, Oak Ridge, TN.
21.
Wu, “The Effect of Various Attic Venting Devices on the Performance of
Radiant Barrier Systems in Hot Arid Climates”, Thermal Performance of the
Exterior Envelopes of Buildings IV”, ASHRAE (1989) pp. 261-270.
22.
Yarbrough, “Assessments of Reflective Insulation for Residential and
Commercial Applications”, Oak Ridge National Laboratory Report ORNL/TM 8819,
Oak Ridge, TN (1983).
23.
Yarbrough, “Estimation of the Thermal Resistance of a Series of
Reflective Air Spaces Bounded by Parallel Low Emittance Surfaces”, Proceedings
of the Conference on Fire Safety and Thermal Insulation, S.A. Siddiqui, Editor,
(1990) pp. 214-231.
24.
Yarbrough, “Thermal Resistance of Air Ducts with Bubblepack Reflective
Insulation”, Journal of Thermal Insulation 15 137-152 (1991).
25.
Queer, “Importance of Radiation and Heat Transfer Through Air
Spaces”, American Society of Heating and Air Conditioning Engineers.
Documents
1993
ASHRAE Handbook Fundamentals - IP Edition,
American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.,
1791 Tullie Circle, NE, Atlanta, GA.
·
“Surface
Conductances and Resistances for Air”
Table 1 - page 22.1
·
“Thermal
Resistances of Plane Air Spaces”
Table 2 - page 22.2
·
“Emittance
Values of Various Surfaces and Effective
Table 3 - page 22.3
·
Emittances
of Air Spaces”
·
“Effective
Thermal Resistance of Ventilated Attics”
Table 5 - page 22.11
Federal
Trade Commission,
Part 460, “Labeling and Advertising of Home Insulation”
Para. 460.5
R-value Tests 2(b) Aluminum Foil systems
2(c)
Single sheet systems
2(d)
Foil facings
International
Conference of Building Officials (ICBO),
“Acceptance Criteria for Reflective Insulation,” (1987, revised 1997).
U.S.
Department of Energy,
“Attic Radiant Barrier Fact Sheet,” (1991).
ASTM Standards
C 236-89
“Standard Test Method for Steady-State Thermal Performance of Building
Assemblies by Means of a Guarded Hot Box.”
1995 Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 52-62.
C 727-90
“Standard Practice for Use and Installation of Reflective Insulation in
Building Constructions.” 1995
Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 339-341.
C 976-90
“Standard Test Method for Thermal Performance of Building Assemblies by
Means of a Calibrated Hot Box.” 1995
Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 463-481.
C 1158-90
“Standard Practice for Use and Installation of Radiant Barrier Systems
(RBS) in Building Construction.” 1995
Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 655-657.
C 1224-93
“Standard Specification for Reflective Insulation for Building
Applications.” 1995 Annual
Book of ASTM Standards Vol. 04.06 (1995) pp. 670-673.
C 1313-95
“Standard Specification for Sheet Radiant Barriers for Building
Construction Applications.” 1996 Annual Book of ASTM Standards Vol. 04.06 (1996)
pp.
C 1340-96
“Standard Practice for Estimation of Heat Gain or Loss Through Ceilings
Under Attics Containing Radiant Barriers by Use of a Computer Program.”
1997 Annual Book of ASTM Standards Vol. 04.06 (1997) to be
published.
C 1371-96
“Standard Test Method for Determination of Emittance of Materials Near
Room Temperature Using Portable Emissometers.”
1997 Annual Book of ASTM Standards Vol. 04.06 (1997) to be
published.
E 84-95b
“Standard Test Method for Surface Burning Characteristics of Building
Materials.” 1995 Annual Book of ASTM Standards Vol. 04.07 (1995).
E 96-95
“Standard Test Method for Water Vapor Transmission of Materials.”
1995 Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 697-704.
APPENDIX
A
INTRODUCTORY
COMMENTS ON THERMAL
RESISTANCES
FOR REFLECTIVE INSULATION SYSTEMS
________________________________________________________________
Reflective
insulation materials (RIMs) are available in a variety of forms that includes
one or more low emittance (emissivity) surfaces. The low emittance surfaces are generally provided by aluminum
foils or deposited aluminum surfaces which exhibit very low emittances and high
reflectances for long wavelength radiation.
The foils are attached to other materials for mechanical strength or
support. In some cases, supporting
materials add to the thermal resistance of the reflective insulation system that
is created upon installation of a reflective insulation in a building or vehicle
cavity. The following discussion of
thermal resistances will be limited to one-dimensional heat flow across
reflective air spaces.
A
reflective insulation system (RIS) is formed by a RIM positioned to form one or
more enclosed air spaces. A good
RIS design will have at least one low-emittance major surface bounding each air
space. The purpose of the low-emittance
high-reflectance surfaces is to significantly reduce the radiative heat transfer
across the enclosed air space. The
enclosed air spaces that make up a RIS are not ventilated.
There should be no air movement in or out of the enclosed space.
The reflective air spaces (enclosed spaces) are positioned so that the
major surfaces are perpendicular to the anticipated heat flow direction. When this is done, the thermal resistances of the air spaces
in series are additive. If the
reflective insulation material has thermal resistance, then this resistance is
added to that provided by the reflective air spaces.
The thermal resistance for one-dimensional heat-flow through a series of n reflective air spaces is:
RTOTAL = RAIRSPACE ONE + RAIRSPACE
TWO + ... RAIRSPACE “n” + RREFLECTIVE MATERIAL
Heat is transferred across air spaces by conduction and convection as well as radiation. Convective heat transfer within the air space is related to the movement of air caused by temperature differences. The density of air at constant pressure decreases as the temperature increases. A temperature difference between two regions will result in air density differences which will result in buoyant forces and air movement or natural convection. The magnitude of the buoyant forces increases as the temperature increases and the induced movement of air depends on the buoyant force magnitude and its direction relative to gravity. Since heat flow is in the direction of decreasing temperature, the direction of the buoyant force will depend on the orientation and temperatures of the bounding surfaces. As a result, the convective contribution to the overall heat transfer depends on heat flow direction. Convective heat flow upward is the greatest, and convective heat flow down is the least and can be zero in an idealized system with stagnant air.
Estimates of the thermal resistance of a single reflective air space that has parallel bounding surfaces perpendicular to the direction of heat flow can be made using the following equations.
(1)
(2)
(3)
(4)
Îi
IR emittance for surface “i”, i = 1 or 2
E
Effective emittance for an air space
hc
Convective heat transfer coefficient, Btu/ft2·hr·°F
hr
Radiative heat transfer coefficient, Btu/ft2·hr·°F
l
Thickness of air space, inches
Q
Heat flux, Btu/hr·ft2
R
Thermal resistance, ft2·hr·°F/Btu
Tm
Average of hot and cold surface temperatures, °F
DT
Difference between hot and cold surface temperatures, °F
Equation
(1) expresses mathematically the fact that R-value depends on heat transfer by
radiation, E·hr,
and heat transfer by conduction-convection, hc.
The multiplying factor, E, is often called an effective emittance and takes
on values between 0 and 1. Its
value depends on the emittances of the two major bounding surfaces, Îi and Î2, as
shown by Equation (2). The “E” value for an air space with one low-emittance
aluminum boundary is very low, usually in the range 0.03 to 0.05.
Equation
(3) is the heat transfer coefficient for radiation, h r, between two
parallel surfaces. The hr
is multiplied by “E”
to introduce the effect of surface emittances.
Equation (2) has been derived for infinite parallel planes and discussed
in most texts dealing with radiative heat transfer.
The
equation for hc is the complication in the R-value calculation.
Equation (4) indicates that hc depends (is a function of) four
variables for one-dimensional heat flow between parallel surfaces.
Values for hc are developed from experimental data for total
heat flow such as that obtained with
a hot-box facility such as that
described in ASTM C 236. The terms
R, E, and h r are obtained from emittance
and hot-box measurements. Values
for hc are derived from sets of hot box measurements done for a
specific heat-flow direction. Robinson
and Powell (see references) have provided hc in graphical form and
Yarbrough (see references) has provided hc in analytical form.
One-dimensional
heat flow and R-values between large parallel surfaces held at different
temperatures and separated by distance “l”
are established by the above equations and discussion.
The procedure has been used to generate the following three tables for
single air space R-values for Tm = 50°F and DT = 30°F.
These temperatures match the requirements of the FTC labeling rule for
“single-sheet” products.
Tables
1, 2, or 3 can be used to estimate the R-value for a RIS provided that the
overall temperature difference across each element in the RIS is known.
The steady-state temperature difference (DT) across each element is related to the R-values of
the RIS elements, Ri, by Equation (5).
(5)
(6)
(7)
Unfortunately,
Ri values are related to DTi.
The only known quantity in Equation (5) is the overall temperature
difference DT.
An approach to solving for R is to first estimate the DTi values. This
should be done in such a way that Equation (6) is satisfied. Given a trial set of DTi, the
average temperature T in each element can be calculated and R i can
then be estimated from Tables 1, 2, and 3.
This, of course, limits the accuracy since the tables are for 50°F.
The total R is calculated by adding the Ri as indicated by
Equation (7). The calculated Ri
are used to recalculate DTi by
means of Equation (5). This
iterative procedure is continued until constant values for DTi and
Ri are obtained.
The
calculational procedure can be improved by using the iterative procedure and
Equation (1) to calculate Ri values. Table 4 has been prepared to expedite the calculation for a
mean air space temperature of 75°F.
Table 1. Calculated
R-Values for an Enclosed Air Space at 50°F
and DT = 30°F
-- Heat Flow Down
-----------------------------------------------------------------------------------------------------------------------------------
l
E/ 0.030
0.050
0.100 0.150
0.250 0.500
0.750 0.820
-----------------------------------------------------------------------------------------------------------------------------------
0.50
2.63
2.51
2.25
2.04
1.72
1.24
0.97
0.91
0.75
3.72
3.48
3.01
2.64
2.13
1.44
1.08
1.01
1.00
4.69
4.32
3.61
3.10
2.42
1.56
1.15
1.07
1.25
5.57
5.06
4.11
3.46
2.63
1.65
1.20
1.11
1.50
6.36
5.70
4.53
3.75
2.80
1.71
1.23
1.14
1.75
7.03
6.23
4.86
3.98
2.92
1.76
1.25
1.16
2.00
7.60
6.68
5.12
4.15
3.01
1.79
1.27
1.18
2.25
8.08
7.04
5.34
4.29
3.09
1.81
1.28
1.19
2.50
8.49
7.36
5.51
4.41
3.15
1.83
1.29
1.20
3.00
9.15
7.84
5.78
4.58
3.23
1.86
1.31
1.21
-----------------------------------------------------------------------------------------------------------------------------------
Table 2. Calculated
R-Values for an Enclosed Air Space at 50°F
and DT = 30°F
-- Heat Flow Horizontal
-----------------------------------------------------------------------------------------------------------------------------------
l
E/ 0.030
0.050
0.100 0.150
0.250 0.500
0.750 0.820
-----------------------------------------------------------------------------------------------------------------------------------
0.50
2.41
2.31
2.09
1.91
1.63
1.19
0.93
0.88
0.75
2.88
2.74
2.43
2.19
1.83
1.29
1.00
0.94
1.00
2.76
2.63
2.35
2.12
1.78
1.27
0.98
0.93
1.25
2.67
2.55
2.28
2.07
1.74
1.25
0.97
0.92
1.50
2.62
2.50
2.25
2.04
1.72
1.24
0.97
0.91
1.75
2.60
2.48
2.23
2.02
1.71
1.23
0.96
0.91
2.00
2.59
2.47
2.22
2.02
1.70
1.23
0.96
0.90
2.25
2.58
2.47
2.22
2.02
1.70
1.23
0.96
0.90
2.50
2.59
2.47
2.22
2.02
1.71
1.23
0.96
0.91
3.00
2.61
2.49
2.24
2.03
1.72
1.23
0.96
0.91
-----------------------------------------------------------------------------------------------------------------------------------
Table 3. Calculated
R-Values for an Enclosed Air Space at 50°F
and DT = 30°F
-- Heat Flow Up
-----------------------------------------------------------------------------------------------------------------------------------
l
E/ 0.030
0.050
0.100 0.150
0.250 0.500
0.750 0.820
-----------------------------------------------------------------------------------------------------------------------------------
0.50
1.61
1.56
1.46
1.37
1.22
0.95
0.78
0.75
0.75
1.69
1.64
1.53
1.43
1.27
0.98
0.80
0.76
1.00
1.76
1.70
1.58
1.47
1.30
1.00
0.82
0.78
1.25
1.81
1.75
1.62
1.51
1.33
1.02
0.83
0.79
1.50
1.85
1.79
1.66
1.54
1.35
1.03
0.84
0.79
1.75
1.89
1.83
1.69
1.57
1.37
1.05
0.84
0.80
2.00
1.92
1.86
1.71
1.59
1.39
1.06
0.85
0.81
2.25
1.95
1.88
1.74
1.61
1.40
1.06
0.86
0.81
2.50
1.98
1.91
1.76
1.63
1.42
1.07
0.86
0.82
3.00
2.02
1.95
1.79
1.66
1.44
1.09
0.87
0.82
-----------------------------------------------------------------------------------------------------------------------------------
Table 4. Conduction-Convection
Coefficients, hc, for use in Equation (1)
-----------------------------------------------------------------------------------------------------------------------------------
Heat
Flow Down
Width of Air Space (l,
in.)
-----------------------------------------------------------------------------------------------------------------------------------
DT
0.5
1.0
1.5
2.0
2.5
3.0
-----------------------------------------------------------------------------------------------------------------------------------
5
0.359
0.184
0.126
0.097
0.080
0.068
10
0.361
0.187
0.129
0.100
0.082
0.072
15
0.363
0.189
0.131
0.101
0.085
0.075
20
0.364
0.190
0.132
0.103
0.087
0.078
25
0.365
0.191
0.133
0.105
0.090
0.081
30
0.366
0.192
0.134
0.106
0.092
0.082
-----------------------------------------------------------------------------------------------------------------------------------
Heat Flow Horizontal Width of Air Space (l, in.)
-----------------------------------------------------------------------------------------------------------------------------------
DT
0.5
1.0
1.5
2.0
2.5
3.0
-----------------------------------------------------------------------------------------------------------------------------------
5
0.360
0.204
0.169
0.179
0.185
0.189
10
0.366
0.267
0.223
0.233
0.238
0.241
15
0.373
0.247
0.261
0.271
0.275
0.276
20
0.380
0.270
0.292
0.301
0.303
0.303
25
0.387
0.296
0.317
0.325
0.327
0.326
30
0.394
0.319
0.339
0.347
0.347
0.345
-----------------------------------------------------------------------------------------------------------------------------------
Heat Flow Up Width of Air Space (l, in.)
-----------------------------------------------------------------------------------------------------------------------------------
DT
0.5
1.0
1.5
2.0
2.5
3.0
-----------------------------------------------------------------------------------------------------------------------------------
5
0.381
0.312
0.295
0.284
0.275
0.268
10
0.429
0.381
0.360
0.346
0.336
0.328
15
0.472
0.428
0.405
0.389
0.377
0.368
20
0.511
0.465
0.440
0.423
0.410
0.400
25
0.545
0.496
0.469
0.451
0.437
0.426
30
0.574
0.523
0.494
0.475
0.460
0.449
-----------------------------------------------------------------------------------------------------------------------------------
Example 1.
Calculation of Thermal Resistance for a Single Air Space.
Specifications
Surface One: T = 70°F, e1 = 0.03
Surface Two: T
= 80°F, e2 = 0.80
Space between surfaces, l,
2.0 inches
Heat flow down
Equation
2 for E
E
= (1/0.03 + 1/0.8 - 1)-1 = 0.0298
Tm = (70 + 80)/2 = 75
DT = 80 - 70 = 10
hc
from Table 4
hc = 0.100
hr
from Equation 3
hr = 1.049
R
from Equation 1
R = (0.0298 x 1.049 + 0.100)-1 = 7.6 (ft2·h·°F/Btu)
Example 2.
Estimation of Thermal Resistance for Two One-inch Reflective Air Spaces
in Series.
Specifications:
Air space 1: 1.0 inch wide
Side one
e1 = 0.80
Side two
e2 = 0.03
Air space 2:
1.0 inch wide
Side one
e1 = 0.03
Side two
e2 = 0.80
Cold side temperature
70°F
Warm side temperature
80°F
First
Approximation for DT
DT across air space 1:
DT1 = 5°F
DT across air space 2:
DT2 = 5°F
Use hc at mean temperature 75°F as an
approximation.
Tm for air space 1:
72.5°F
Tm for air space 2:
77.5°F
E1 = E2 =
0.0298
From Table 4
hc1 = 0.184
hc2 = 0.184
From Equation 3
hr1 = 1.034
hr2 = 1.064
From Equation 1
R1 = 4.66
R2 = 4.64
R = R1 + R2 = 9.3
Check approximation for DT
DT1 = 10 x 4.66/9.3 = 5.01
DT2 = 10 x 4.64/9.3 = 4.99
These DT values agree with the assumed values. If the agreement is not satisfactory then the calculation should be repeated using the calculated DT values.