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Heat and Thermodynamics

Thermal Insulation

In some situations, it is desirable to keep heat from escaping a container or from entering an area. Normally, heat is transferred by from one material to another by conduction, convection and/or radiation. Insulators are used to minimize that transfer of heat energy. In home insulation, the R-value is an indication of how well a material insulates.

Questions you may have are:

  • Where is thermal insulation used?

  • How does insulation work?

  • What is the R-value?

Where thermal insulation is used

If you have an object or area that is at a certain temperature, you may want to prevent that material from becoming the same temperature as neighboring materials. This is usually done by employing thermal insulation barrier.

For example:

  • If the air outside is cold, you may want to protect your skin by wearing clothes that keep the cold out and the body warmth in

  • If your house has cool air inside during the summer, you may want to prevent the temperature from becoming the same as the hot air outside by having the house well insulated

  • If you have a hot drink, you may want to prevent it from becoming room temperature by putting it in a thermos bottle

Any place where there are materials of two drastically different temperatures, you may want to provide an insulating barrier to prevent one from becoming the same temperature as the other. In such situations, the effort is to minimize the transfer of heat from one area to another.

How insulation works

Insulation is a barrier that minimizes the transfer of heat energy from one material to another by reducing the conduction, convection and/or radiation effects.

Insulating materials

Most insulation is preventing conduction. In some cases radiation is a factor. A good insulator is obviously a poor conductor.

Less dense materials are better insulators. The denser the material, the closer its atoms are together. That means the transfer of energy of one atom to the next is more effective. Thus, gases insulate better than liquids, which in turn insulate better than solids.

An interesting fact is that poor conductors of electricity are also poor heat conductors. Wood is a much better insulator than copper. The reason is that metals that conduct electricity allow free electrons to roam through the material. This enhances the transfer of energy from one area to another in the metal. Without this ability, the material--like wood--does not conduct heat well.

Insulation from conduction

Conduction occurs when materials--especially solids--are in direct contact with each other. High kinetic energy atoms and molecules bump into their neighbors, increasing the neighbor's energy. This increase in energy can flow through materials and from one material to another.

Solid to solid

To slow down the transfer of heat by conduction from one solid to another, materials that are poor conductors are placed in between the solids. Examples include:

  • Fiberglass is not a good conductor nor is air. That is why bundles of loosely packed fiberglass strands are often used as insulation between the outer and inner walls of a house.

  • Heat cannot travel though a vacuum. That is why a thermos bottle has an evacuated lining. Heat cannot be transferred from one layer to the other through the thermos bottle vacuum.

Gas to solid

To slow down the heat transfer between air and a solid, a poor conductor of heat is placed in between.

A good example of this is placing a layer of clothing between you and the cold outside air in the winter. If the cold air was in contact with your skin, it would lower the skin's temperature. The clothing slows down that heat loss. Also, the clothing prevents body heat from leaving and being lost to the cold air.

Liquid to solid

Likewise, when you swim in water, cold water can lower your body temperature through conduction. That is why some swimmers wear rubber wet suits to insulate them from the cold water.

Insulation from convection

convection is transfer of heat when a fluid is in motion. Since air and water do not readily conduct heat, they often transfer heat (or cold) through their motion. A fan-driven furnace is an example of this.

Insulation from heat transfer by convection is usually done by either preventing the motion of the fluid or protecting from the convection. Wearing protective clothing on a cold, windy day will inhibit the loss of heat due to convection.

Insulation from radiation

Hot and even warm objects radiate infra-red electromagnetic waves, which can heat up objects at a distance, as well as lose energy themselves. Insulation against heat transfer by radiation is usually done by using reflective materials.

A thermos bottle not only has an evacuated lining to prevent heat transfer by conduction, but it also is made of shiny material to prevent radiation heat transfer. Radiation from warm food inside the thermos bottle is reflected back to itself. Radiation from warm outside material is reflected to prevent heating cold liquids inside the bottle.

R-value

The R-value of a material is its resistance to heat flow and is an indication of its ability to insulate. It is used as a standard way of telling how good a material will insulate.The higher the R-value, the better the insulation.

Definition

The R-value is the reciprocal of the amount of heat energy per area of material per degree difference between the outside and inside. Its units of measurement for R-value are:

(square feet x hour x degree F)/BTU in the English system and

(square meters x degrees C)/watts in the metric system

Table

Insulation for the home has R-values usually in the range of R-10 up to R-30.

The following is a listing of different materials with the English measurement of R-value:

Material R-value
Hardwood siding (1 in. thick) 0.91
Wood shingles (lapped) 0.87
Brick (4 in. thick) 4.00
Concrete block (filled cores) 1.93
Fiberglass batting (3.5 in. thick) 10.90
Fiberglass batting (6 in. thick) 18.80
Fiberglass board (1 in. thick) 4.35
Cellulose fiber (1 in. thick) 3.70
Flat glass (0.125 in thick) 0.89
Insulating glass (0.25 in space) 1.54
Air space (3.5 in. thick) 1.01
Free stagnant air layer 0.17
Drywall (0.5 in. thick) 0.45
Sheathing (0.5 in. thick) 1.32

The R-value is proportional to the thickness of the material. For example, if you doubled the thickness, the R-value doubles.

In conclusion

Thermal insulation is used minimize the heat transfer in many everyday situations. It is done by reducing conduction, convection and/or radiation effects. The R-value is a standard of measurement of this insulation.