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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”, ha