BRIELLE, NEW JERSEY - January 10, 2011 -- Insulation advertisements - including those for spray foam insulation - often cite a statistic called R-value to illustrate the effectiveness of a given solution.

But what exactly is R-value?

Simply put, R-value is thermal resistance, or how hard it is to heat or cool a given substance. It's calculated by dividing the temperature difference between in the inside and the outside of an insulated space by the heat flux, or flow of heat, through a given area of insulated space.

The first number might be the difference between a 40-degree Fahrenheit day outside and an interior temperature of 65 degrees Fahrenheit, while the second is a more technical measurement of British thermal units per square foot per hour.

Spray foam insulation solutions typically have an R-value of between 5 and 6 per inch of thickness, though some types have R-values as high as 7 or 8 per inch. Fiberglass insulation, by contrast, has an R-value of just 3.5 per inch.

Comparing R-values for different insulation solutions - or different types of spray foam - is an effective and scientific way of assessing the efficiency of different options.

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