R-Values and Radiant Barriers

r-value historyMost people are familiar with the term R-Value when discussing insulation properties.  An R-Value is a measure of an insulation product's resistance to thermal heat transfer.  It's a measure of how long it takes a heat "absorbing" product to absorb 100% of its potential and begin to transfer the heat out the other side of the product (referred to as thermal resistance). R-Values are calculated via a controlled test under the following conditions:

  • A constant temperature of 75 degrees Fahrenheit, and
  • 50% relative humidity.

Isn't it odd that products designed to protect us from the heat are rated using a test that is performed under 75 degrees Fahrenheit with 50% humidity?  An interesting fact is that as the temperature rises, the R-Value of an insulation material decreases.

Basically, a product with a higher R-Value is better than a product with a lower R-Value.  A higher R-Value product takes longer to absorb 100% of it's potential and therefore, is more effective at "delaying" the transfer of heat. R-Values themselves don't mean much alone unless they are used to compare the effectiveness of different R-Value rated products.

Radiant barriers do not "absorb" heat.  Instead, they retard heat flow by two means - by reflecting radiant heat away from its surface or by reducing the emission of radiation heat from its opposite side and therefore, have no R-value rating like bulk insulation products.

Installing a radiant barrier between the heat source (sun) and an existing R-Value rated insulation will improve the effectiveness of the R-Value rated insulation.  Why?  Because only 3% of the heat is now hitting the R-Value rated insulation and therefore, it will take longer for it to absorb 100% of it's potential thereby holding the heat longer before passing out the back side into living spaces. 

In summary, you increase the R-Value of your existing insulation by installing a radiant barrier foil insulation product.  Studies have found the following about radiant barriers to be true:

  • A Tennessee Valley Authority study based on radiant barriers used in the summer found that radiant barriers combined with an R-11 insulation has the same affect as an R-19 insulation alone.  Add a radiant barrier to an R-19 insulation and you have the effect of an R-30 insulation alone.
  • The Florida Solar Energy Center specialist Philip Fairey found that a radiant barrier combined with an R-19 insulation produced a LOWER peak demand than an R-30 insulation alone.