Radiant heat by definition is heat transfer across an air space by non-contact.  If you don't have an air space, you can't have radiant heat.  If you don't have radiant heat, you can't have a radiant barrier.

The difference between radiant heat transfer and conductive heat transfer is really quite simple. Radiant heat is heat transferred across an air gap/space (or a vacuum), while conductive heat transfer is heat transferring through materials/objects that are in contact (touching) one another.

This is important because many times people would like to use a radiant barrier to stop heat transfer but they do not have an air space in the assembly they are working with. Without an air space, a radiant barrier will NOT work!

Why? Because radiant heat transfer requires an air space for the heat to transfer. When the air gap does not exist, the materials/objects are touching and so the heat is transferring via conductive heat transfer (conduction).

• Radiant Heat: heat transfer across an air space
• Conductive Heat: heat transfer between touching objects
• In an attic you have BOTH radiant heat and conductive heat.

Surface Temperatures in the Attic

When the goal is having a cooler home, we need to examine the surface temperatures in the attic. Remember, the attic has both surface temperatures and air temperatures.

Bottom line: getting a cooler, more efficient home is all about the surface temperatures.

Surface temps can only be measured with an infrared thermometer, while air temps can be measured with probe thermometers. The surface temperatures in your attic are basically what tell your home whether or not it is hot outside; this directly effects whether or not your air conditioner turns on and runs. When the attic gains radiant heat from outside, it re-radiates that heat through the attic air and the insulation on your attic floor absorbs it. When your insulation is 130 degrees, your house thinks it's 130 degrees outside because it essentially has a 130 degree blanket on top of it! By stopping the radiant heat gain in the insulation, with a layer of radiant barrier foil, we can reduce the top surface temperature of the insulation and when the insulation is cooler, the home is cooler and the air conditioner is more efficient.