The R-Value Myth

R-value is the term given to the property of any material to “resist” the conduction of heat.  R-Value as a property is legitimate, however, the way at which the laboratory determines this property, in relation to the types of insulation materials available, is flawed at a fundamental level.  Yielding what can be called the R-Value myth.  This flaw forces the engineering community to calculate heat loads using the “myth” that this R-Value property of various insulation materials, derived by flawed test methods, can accurately predict the performance of the insulation material in real life.  Not true.

How is the test method flawed?  By the two “test criteria” that are used in the laboratory to determine R-Value. The first criteria relates to "standard temperature". This stipulates that the test be conducted at a constant temperature of 75-degrees Fahrenheit.  The question is, “Who is heating or cooling there home when it’s 75 degrees?”  It isn't surprising that fiberglass performs well at 75-degrees. However, the effectiveness of fiberglass decreases in efficiency at hotter and cooler temperatures while foam performs very well at hotter and cooler temperatures. Now do you see why we're suspicious of R-Values as a rule of thumb?

Secondly, the "test criteria" requires that R-value testing not start until the materials reach "steady state". Steady state occurs when a material is exposed to a heat source on one side and allowed to become thermally saturated so that for every single unit of heat entering on one side of the material, a single unit of heat exits on the opposite side. This seems very scientific. It appears logical, but it misses a single important issue relevant to predicting real world performance: The amount of time it takes to reach steady state.

Fiberglass reaches steady-state within a few hours, cellulose takes about a day and EPS foam takes several days to get to steady state.  In the real world, foam will not actually reach a steady-state condition and, in the final analysis, is a far better insulator than fiberglass.  In addition, foam acts as an air-barrier that impedes air infiltration.  Fiberglass can’t make that same claim.

Providing an effective air-barrier is just as important as a high R-Value.  Think of the cooler example.  If you were to fill a cooler with ice on a summer day and keep the lid closed, you would still have ice in the cooler the next day.  Take that same cooler filled with ice and crack the lid open just a half-inch on a summer day and by the next day you will have a cooler full of water.  The R-Value of the cooler walls and lid didn’t change, by allowing air to enter and escape the cooler, the ice melted faster than by blocking the air-flow.  This is exactly what happens in your house everyday. 

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