Infrared Heat Transfer
Infrared solar energy heat transfer through building materials and assemblies is a complicated subject. However, if we can grasp one key idea, understanding how things work becomes a lot more simple. The key idea is that heat always flows in the direction of warmer to colder. Whether the mechanism of heat transfer is by conduction, convection or radiation, this is always true.
Heat transfer by conduction requires two materials (including air molecules) in direct contact with each other. Convection is heat transfer by air movement. But while conduction and convection can only occur through a medium, infrared heat energy—which is a form of radiation—can be transferred across a perfect vacuum. Radiation travels in a straight line through space.
Attic Shield™ includes layers of a highly reflective material that will stop that transfer of radiation across an airspace. It’s important to understand that Attic Shield™ cannot perform this reflective function if it is in contact with another material: An airspace must be present between the reflective surface and the radiation heat source. (Incidentally, this explains why foil products attached to the exterior surface of roof sheathing—underneath the shingles—have no Attic Shield™ capability. No air space, no effective blocking of infrared energy.
A highly reflective material has a second very desirable characteristic, which is that it emits very little energy that strikes its “hot” surface from its other side. Any opaque material’s emissitivity is always one minus its reflectivity. Pure aluminium foil, for example, typically reflects about 97 percent of the infrared heat energy that strikes it, so its emissivity is 1.00 - 0.97 = 0.03. Materials that absorb a large percentage of radiation that strikes them have high emissivities. Most common building materials have emissivities of 0.90 or more.
Aluminum foil is a good thermal conductor, so it has an extremely low R-value. However, if it is placed between two surfaces, like a roof deck and an attic floor, that are attempting to transfer thermal energy by radiation (rather than conduction) and if it is separated from these materials by an air layer, the foil almost completely eliminates the infrared heat energy exchange across the airspace.
Installation of Attic Shield™ in the attic of a typical Florida home offers excellent energy saving potential. This is because in Florida’s climate, the roof is the surface of the structure most exposed to solar radiation, and because most of the solar gain absorbed by the roof is radiated downward to the attic floor (which is in contact with the ceiling below). Two very simple common sense reasons explain why virtually all of an attic’s heat gain comes from infrared heat energy transfer. First, the underside of the roof and the attic floor are separated by a large airspace, so there can be no significant conductive heat transfer between the two. Second, hot air rises, so any convective air flow inside the attic will not move from the underside of the roof downward toward the attic floor.
Attic Shield™ installed in the airspace between the hot roof surface and the cooler attic floor (which is typically insulated with fiberglass or cellulose), will eliminate almost all infrared heat transfer and approximately 40 percent of total heat transfer.
Sunfire™ Attic Shield™ is ideally installed against the roof trusses on the underside of the roof sheathing, leaving an airspace between the sheathing and the Attic Shield™ system. While you could in some cases install Attic Shield™ on the attic floor, this is a less than ideal solution for two reasons. First, significant air conditioning performance improvement and operating cost savings can be achieved by keeping the ducts cooler. The ducts for virtually all air conditioning systems in Florida run through the attic. Blocking infrared heat energy transfer at the roof keeps the attic—and the air conditioning ducts—cooler. Second, virtually all of the winter heat loss to a home’s attic is by convection (this airflow is encouraged by pressure differences between the indoor and outdoor air). Attic Shield™ installed on an attic floor must be perforated to prevent moisture buildup in the fiberglass or cellulose insulation below, so it can’t stop this convective heat transfer.