The Foil vs Paint Question:

When you are considering the installation of a heat reflective radiant barrier in your home or office, you are typically presented with two basic options. You can either staple up a traditional Radiant Barrier Foil or spray Radiant Barrier Paint on the underside of the roof decking. In order to help answer the question of which is better – foil or paint, we will look at two issues: the overall effectiveness and the definition of radiant barrier.

How does a radiant barrier work?

There are three modes of heat transfer to consider: Conduction, Convection, and Radiation (infrared). Of the three modes of heat transfer, radiation is the primary mode of heat transfer. By way of example let’s consider what happens on a hot and sunny summer day. The suns solar energy heats the roof shingles which in turn is transfers this heat into the roof decking and framing. Our heat absorbing roof decking then radiates heat down into the attics insulation and continues to the rooms below. A professionally installed attic radiant barrier reduces the amount of radiated heat that strikes the insulation and the living space below by reflecting the heat back toward the roof decking.

A radiant barrier's performance is determined by three factors:

Emissivity (or emittance) – the ratio of the radiant energy (heat) leaving (being emitted by) a surface to that of a black body at the same temperature and with the same area. It's expressed as a number a between 0 and 1. The higher the number, the greater the emitted radiation.

Reflectivity (or reflectance) – a measure of how much radiant heat is reflected by a material. It's also expressed as a number between 0 and 1 (sometimes, it is given as a percentage between 0 and 100%). The higher the number, the greater the reflectivity.

The angle the heat wave strikes the surface—a right angle (perpendicular) usually works best.

 

How Well Does It Work?

The effectiveness of a Radiant Barrier is measured by it emittance value.  Simply put, this is how much heat it gives up 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 the radiant heat.  Now if you look at the specs on even the best radiant paint products, they have values of about 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.

According to the US Department of energy "All radiant barriers must have a low emittance (0.1 or less) and high reflectance (0.9 or more).

A True Radiant Barrier

Note:  ENERGY STAR does not label so called radiant barrier paints, but they do label Energy “Q”TM Radiant Barriers. 

More on the Physics of Foil