Powerful Ideas for Powerful SolutionsMeasuring Insulation Effect
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.
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.
