R-Values and Radiant Barriers
Most 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:
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
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
A Tennessee Valley Authority study based on radiant barriers used in the summer
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.