|
|
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 (Δ 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 downhill, heat flows from warm to cold areas. 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 between the warm and the colder area.
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 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 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.
