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Definitions
Basic Heat Flow
A fundamental principle of heat flow is that heat will always flow
from a warm space or object toward one that is colder. When this
principle is applied to a dwelling, it is apparent that in the
winter, heat necessary to maintain a comfortable environment tends
to flow upward through the roof/ceiling system, outward through
the exterior walls and downward through the floor system.
Conversely, in the summer, the flow of heat is reversed since the
dwelling's interior is kept cooler than the outside air. The rate
at which heat is transferred, in either case, is a function of the
temperature difference between the inside and outside air and the
types of materials it must pass through. Accordingly, the rate of
heat loss or gain through this natural transfer can be reduced by
the proper selection of a dwelling's construction and insulating
materials. There are three methods by which the natural transfer
of heat takes place: (1) conduction, (2) convection and (3)
radiation.
Conduction - is the flow of
heat directly through a solid material. The measure of a
conductor's efficiency is based on how rapidly heat moves through
it. The better conductor a material is, the worse insulator it is.
Convection - is the transfer
of heat by a fluid, usually water or air since they are readily
available.
Radiation - is the transfer
of heat by electromagnetic waves such as heat generated by the
sun.
Thermo-Siphoning - or
convection looping is caused by warm air movement in conventional
(fiberglass) insulated walls. The rising warm air creates a
current in the cavity, drawing away heated or cooled inside air.
Damaging condensation is formed when the cool and warm air meet.
Temperature - is a measure
of the intensity of heat and not the quantity or amount of heat
that is present. Temperature is measured by degrees of heat on a
definite scale such as a thermometer.
Relative Humidity (RH) - is
a measure of the amount of water vapor in air and is stated as a
percentage. The ratio of the amount of water vapor in the air as
compared to the maximum amount the air can contain at a specific
temperature and atmospheric pressure. For instance, the air at 50
percent RH contains 1/2 the amount of vapor it could hold at a
given temperature and atmospheric pressure; at 100 percent RH it
contains the maximum, and such air is said to be saturated. The
temperature at which saturation or 100 percent relative humidity
occurs is called the dew point.
Thermal Inertia - is the
term used when a material has the ability to store heat and
retards the transfer of heat loss or gain.
British Thermal Unit (BTU) -
is a measure of the amount of heat required to change the
temperature of an object and is a common way of comparing fuels
and insulating materials. A BTU is the amount of heat required to
raise the temperature of one (1) pound of water one (1) degree
Fahrenheit.
Thermal Resistance (R) - is
a property which indicates a material's ability to resist the flow
of heat and is given in units of square foot-hour-degree
Fahrenheit per BTU. When "R" values are given, they reflect ideal
conditions and do not account for installation practices or
techniques that are common in the field that result in a reduction
of "R" values.
Thermal Transmittance (U) -
is a measure of the time rate of heat flow (air to air) expressed
in units of BTU per hour per square foot per degree Fahrenheit.
This term is most often used in energy conservation codes as an
expression of the overall coefficient of heat transmission of an
assembly of materials. The "U" values of materials cannot be added
as "R" values can. However, the sum of the "R" values of the
individual components of an assembly of materials divided into one
(1) will give the "U" value of the assembly. It should be
emphasized that the component "R" values must be added together
first then divided into one (1) to determine the "U" value. In the
preceding example U = 1/R = 1/13.62 =0.07
Thermal Conductance (C) - is
a measure of the amount of heat in BTU's that will pass through
one square foot of a material of any thickness each hour for each
degree Fahrenheit difference in temperature on opposite sides of
the material.
Thermal Conductivity (K) -
is the measure of the number of BTU's that will pass through one
(1) square foot of a material one (1) inch thick each hour for
each degree Fahrenheit difference in temperature on opposite sides
of the material.
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