Properties of Good Thermal Insulators

Written by tony oldhand
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Properties of Good Thermal Insulators
Insulators, such as these boot inserts, prevent thermal loss. (Jupiterimages/ Images)

The role of an insulating material is to prevent hot or cold temperature transference from one side of the insulator to the other. This may seem obvious on the surface, but there is an entire science behind the field of insulation. Materials engineers, chemists and physicists all investigate the hows and whys of different materials, and even develop new materials with very efficient insulating qualities. All insulation is rated in terms of "R factor," and understanding what role the R factor plays will help you understand what defines a good thermal insulator.

R Factor

If you shop at a home improvement store for insulation, for instance, you will see the R factor rating is posted for particular products, such as "R factor 11." Understanding what this number means will give you a reference point for insulators. Through an equation that uses time, temperature and distance researchers determine how much heat is conducted through a material over a certain time period. The final answer to this equation is a number, which is the R-factor. The higher the R factor, the less heat transference the material allows; that is to say, the higher the R factor, the greater the insulating properties. .

Interpreting R Factor

Just a number for the R factor is not enough information; you have to interpret what the number actually means and how it relates to the material, especially as regards thickness. According to James Dulley, writing for the Deseret News, commercial R factors are defined for material 1 inch thick. To put this is into perspective, plywood has an R factor of 1.25, whereas fibreglass insulation has an R factor ranging from 3.14-4.3. This tells you that 1 inch of fibreglass batting has three to four times more thermal blocking than 1 inch of plywood.

Cumulative R Factor Rating

When you walk through the insulation aisle at a home improvement store, a common R factor printed on the insulation may be "R-12." According to Mr. Dulley, R factors add up with increased thickness; the number that is advertised is the total R factor of the material given the thickness of the product. Fibreglass batting has a true R factor of about R-4; therefore, for a 3-inch thick piece, the total R factor is 12.

Cumulative R Factor Interpretation.

You also have to look at the cumulative R factor to find insulators that have the best thermal properties. A 3-inch thick piece of fibreglass batting has an R-12 rating. To achieve the same total rating for plywood, which has a R factor of 1, the plywood would have to be 12 inches thick. A concrete block has an R factor rating of 1.11 for an 8-inch thick piece; this calculates to a true R factor of .139; to achieve R-12, you would need a concrete block 86 inches thick.

Making Sense of It All

You have to relate back to the "true" R factor number; the properties of a good thermal insulator directly relate to the R factor. The greater the R factor number -- for a 1-inch thick piece -- the greater the insulating property and the less heat transference a material provides.

Specific Applications

Look at the specific application to make a final determination on the best insulator to use. A myriad of factors play into this; you have to look at the moisture characteristics of the environment, the temperature ranges encountered and the limitations of space. An insulation engineer or other qualified professional can assist you with the final determination for the best insulator to use. Vibrational stress may also be a factor; for example, the tiles used on the space shuttle are very good insulators, protecting the shuttle during the high heat generated during re-entry. However, NASA states the tiles are also very fragile, so they're not suitable for all applications.

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