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The Role of Insulation In Saving Energy

Ivan Bazarov, a noted Russian physicist, referred to the second law of thermodynamics as “...without a doubt, one of the most perfect laws in physics.” And, it is the second law of thermodynamics that requires us to take some kind of action to save energy. The second law tells us that heat flows from a warm body to a cooler body. What this means to businesses and consumers is that the spaces they want to stay warm will cool unless heat is added to make up for the losses, and the spaces they want to stay cool will warm unless a cooling system is used to remove the transferred-in heat. There is no preventing it. The best you can do is slow the rate at which heat flows from warm to cold.

So, how do we slow the transfer of heat? We use insulation. Insulation inhibits the flow of heat between source and sink by providing a barrier between them that has a low thermal conductivity. This low heat transfer capability is caused largely by the presence of pockets of stagnant air constructed into the insulation. Air, and gases in general, are very poor conductors of heat. Also, since the air in these pockets is essentially stagnant, convection heat transfer is almost eliminated. The other materials in the insulation are chosen such that they are poor conductors and radiators of heat to further reduce the rate of heat transfer. The net effect is the heat moves more slowly, the heating or cooling system doesn't need to run as frequently, and energy and money are saved.

Insulation effectiveness is given by either its R value or its U value. The R value is a measure of the resistance to heat flow provided by the insulation and is referred to as thermal resistivity. The U value is a measure of the ease with which the material transfers heat and is called the thermal conductivity of the insulation. U is the inverse of R. Mathematically, the R value is the ratio of the temperature difference across the insulation and the rate of heat flow per unit area across the insulation.

In the United States, R has units of 0F∙ft2∙hr per BTU. In the SI system, R has units of m2∙K per W (square meters degrees Kelvin per watt). The higher the R value, or the lower the U value, the better the insulation is at reducing the rate of heat transfer. R values are usually given per unit thickness of material. For example, when you see insulation with an R value of 7, that means that the insulation has an R value of 7 per inch of thickness. Typical R values in U.S. units for standard insulation run from 2 to 10 per inch.

The rate of heat loss could be further slowed if, instead of pockets of stagnant air, there was a void between the heat source and heat sink. Heat transfers only by radiation across a void, and this can be minimized by using materials that minimize radiation heat transfer. This is the principle behind vacuum insulation panels, commonly referred to as VIPs.