Calculating Conduction
Because conduction is one of
the three significant mechanisms by which homes lose energy, you
may be interested to know how to calculate the amount of energy
a home loses during a year. Such calculations allow you to estimate
the amount of energy saved by reducing heat flow by adding insulation.
To understand where those calculations come from, you first must
understand the factors which influence the conductive flow of
heat through a material.
There are four factors which affect the conduction
of heat from one area to another. They are: The difference in
temperature (^ T) between the warmer area and the colder area;
The length of time (t) over which the transfer occurs; The area
(A) in common between the warmer and the colder area; and the
resistance (R) to heat flow or conductivity (U) of the materials
involved.
Temperature Difference
Much as water moves down hill, heat flows from warm
areas to cold ones. The steeper the gradient between its origin
and its destination, the faster it will flow. In fact, the rate
at which heat is conducted is directly proportional to the difference
in temperature (T) between the warm area and the colder one.
Time
The longer the heat is allowed to flow across the
gradient, the more heat will be conducted. The amount of heat,
measured in BTUs, is directly proportional to the time span (t)
of the transfer. BTU/H is the unit for measuring the amount of
heat transferred in one hour.
Area
The larger the area common to the warmer and colder
surfaces through which the heat flows, the greater the rate at
which heat is conducted. For the same material, for the same length
of time, at the same temperature, the amount of heat transferred
is directly proportional to the area (A) in common.
Resistance to Heat Flow
The rate at which heat flows through a material depends
on its characteristics. Some materials transmit heat more readily
than others. For example, a metal rod transmits heat easily while
a down comforter does not. This characteristic of materials can
be viewed either as their resistance to heat flow or as their
conductivity in allowing the flow of heat. Resistance (R) and
Conductivity (U) are opposite sides of the same coin. Mathematically
they are the inverses of each other. If a material has a high
resistance, its conductivity is low. If its conductivity is high,
its resistance is low.
The unit used to measure conductivity is BTU per
square foot, degree Fahrenheit, per hour.
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