A common question that HVAC technicians who are new to reflective insulation ask is why it must be installed with air gaps at regular intervals between the reflective material and the surface to which it is being applied, such as the outside of a metal duct.
To understand why air gaps are critical to the effectiveness of reflective insulation, we need to look at the science behind the way that heat transfers into unconditioned spaces.
Radiant heat is a form of heat which transfers between matter across a void or air space through electromagnetic waves rather than through convection or conduction. Radiant heat from the sun is what causes attics and other unconditioned spaces in homes or commercial buildings to be so hot during the warmer seasons. When there is not a radiant barrier of reflective insulation present in these spaces, radiation from the sun beats down on the roof and is absorbed by roofing materials, which transfer the heat via conduction to the indoor ceiling of the attic, and finally radiant heat transfers from the ceiling to everything in that unconditioned space, including HVAC ducts.
Certain materials – like plywood, Oriented strand board (OSB), and sheet metal – have properties that make them strong radiators, meaning that they emit a lot of radiant heat, causing high temperatures in attics and other similar spaces. Reflective insulation serves as a barrier against this radiation because it is not a strong radiator, essentially stopping heat in its tracks before it reaches the indoor space. However, this only works if there is an air gap between the reflective insulation and the wall, duct, or other surface to which you are applying the insulation. Without a gap, heat will conduct from the other materials straight through the reflective insulation and into the room. Typically, materials that do not emit much radiant heat tend to be strong conductors, meaning that heat transfers quickly and easily to them when touching other hot materials. The air gap eliminates the chance of heat transferring inside via conduction and allows the radiant barrier to do its job.
From a scientific standpoint, radiant heat cannot exit unless there is an air gap; otherwise the heat is conductive and installing reflective insulation on an HVAC duct is pointless, as it would not achieve its purpose in making the system’s performance more efficient.