Radiant Barrier - Foil or Paint?

The Foil vs Paint Question:

There are two basic types of reflective insulation to choose from when you are considering a radiant barrier to install in your attic.  You can either staple up a traditional Radiant Barrier Foil to the underside of the roof rafters or you can spray a reflective paint to the underside of the roof decking.  In order to evaluate which of these products is right for you, we will look at two different issues: the overall effectiveness of the products and the definition of a true radiant barrier.

How well it works?

The effectiveness of a Radiant Barrier is measured by its emittance value.  Simply put, this is how much heat it gives off to the air below.  You want the emittance value to be as low as possible.  The emittance value of our Radiant Barrier products is 0.03.  In other words, it reflects 97% of any incoming radiant heat.  If you look at the specs on even the best reflective paint products, they have values of ~0.23.  That means they only reflect at best 77% of the heat.  Take a look at the chart below to compare some common brands.

Product	Manufacturer	Emittance
ARMA FOIL™ radiant barrier	Energy Efficient Solutions	.03
Radiance E025	Chemrex	.23
Lo/Mit-1	Solec-Solar Energy Corp	.23
Lo/Mit-2	Solec-Solar Energy Corp	.23
E-Barrier	Sherwin Williams	.36
Therma-Guard Silver	United Community Services of America	.57
Insuladd-RBC	Tech Traders	.59
Formula A Barrier Coat #85	Hy-Tech	.66
Formula B Barrier Coat #85	Hy-Tech	.70
Koolcoat	Pawnee Specialties	.86
Barrier Coat #233	Hy-Tech	.89
Radiosity 3000	Green Building Supply	.89
HeatShield R-20	Kwik Co.	.90

In addition most companies that install the paint only apply it to the roof decking and leave the joists unpainted.  It's true, take a look at their pictures.  All of those exposed roof joists will continue to radiate heat inside the attic.  When the foil is stapled to the bottom side of the roof joists, it covers them as well.

True Radiant Barrier:

To top it off, to even qualify as a true "Radiant Barrier" a material must have an emittance value of 0.1 or less.  None of the paint products in the chart are even close to reaching that qualification.  If someone tells you that they are going to "paint" a Radiant Barrier to the underside of your roof, they are generally misinforming you.

Also Note:

ENERGY STAR does not label so called radiant barrier paints, but they do label foil radiant barriers.