A "radiant barrier" is a reflective / low-emittance surface, on or near a building component, that intercepts the flow of radiant energy to and from the building component. It is, as the name suggests, a barrier to radiant heat movement, in the same way a vapor barrier blocks water vapor migration and an air barrier stops airflow.
A radiant barrier can be an aluminum foil laminate, an aluminized plastic film or a low emittance coating. The only requirement is that its surface must have low emittance and high reflectivity in the infrared band of the spectrum.
The aluminum foil shields that are commonly inserted behind radiators in older houses are radiant barriers, blocking radiant heat transfer from the radiator to the exterior wall. The invisible glass coating in low-E windows is also a radiant barrier.
It should be clearly understood that although a radiant barrier reduces heat loss and gain through the building envelope, it is not an insulation material per se and has no inherent R-value.
A radiant barrier system (RBS) is a building section that includes a radiant barrier facing an air space. An attic with a radiant barrier on top of the mass insulation on the floor, or under the roof is a RBS. A vent skin wall with a radiant barrier facing the vented air space is also a RBS.
The distinction between a radiant barrier "material" and radiant barrier "system" is not merely academic. In an attic, the effectiveness of a radiant barrier is significantly affected by the amount of attic ventilation. A vented attic with a radiant barrier is a very different system from an unvented attic with the same radiant barrier.
TECHNICAL NOTE: The generally accepted definition of a radiant barrier system specifies that the reflective material faces an open air space. The idea is that a radiant barrier facing an enclosed air space is a "reflective insulation" with a measurable R-value.
Several types of radiant barrier material are available. Although they all have similar surface properties (and consequently similar performance), variations in materials and construction result in significant differences with respect to strength, durability, flammability and water vapor permeability.
Most products available commercially fall into three major categories:
- Aluminum Foil Laminates - foil laminated to Kraft paper, plastic film or OSB/plywood roof sheathing.
- Aluminized Plastic Films - a thin layer of aluminum particles deposited on a film through a vacuum process.
- Reflective Paints/Coatings - liquids that reduce the emissivity of the surface to which they are applied.
The most common location for a radiant barrier system is in attics. Three basic configurations are used.
When an interior barrier is used, all seams should be taped to avoid possible moisture migration. When an exterior barrier is used, it should be perforated, unless a vapor retarder is used on the interior side. Otherwise, it may trap moisture. Application techniques will vary depending on the climate in which radiant barriers are used.
Radiant barriers installed in this manner must be perforated to prevent moisture from being trapped in the mass insulation. In areas where dust accumulation would be an issue, it should be noted that the emittance of the top surface of the radiant barrier would be degraded, reducing the thermal performance of the radiant barrier.
As noted before, a vented attic with a radiant barrier is a very different system from an unvented attic with the same radiant barrier. Common types of attic ventilation are:
- Soffit to ridge
- Soffit to gable
- Soffit to soffit
- Gable to gable
Most codes require at least a 1 to 150 ventilation area. What this means is that for every 150 square feet of floor space, there should be one square foot of free vent area.
Several types of radiant barriers are used in walls, as shown in the figure below. An example is foil faced fiberglass batts stapled to the sides of the wall studs, leaving an air space between the foil facing and interior sheathing. Another less common technique is to use foil faced drywall over furring strips on the interior stud faces. The furring strips create an air space between the foil facing and cavity insulation. The technique used commonly in Florida is to apply a radiant barrier to the exterior of the wall, followed by furring strips and sheathing. In this construction, commonly referred to as "vent skin" construction, the air space created by the furring strips is typically vented top and bottom so that outdoor air can circulate into and through the space.
Radiant barriers can also be used in floor systems above unheated basements and crawl spaces. The radiant barrier is either stapled to the underside of floor joists, creating a single reflective air space, or between the joists, followed by some type of sheathing, creating two separate reflective air spaces as shown below.
Laboratory experiments and computer modeling suggest that floor radiant barrier systems may exhibit R-values as high as R-7.5 to R-8.0 for reducing heat loss to basements and crawl spaces. Radiant barriers make an ideal choice for this application because, in addition to their excellent thermal properties, they are also vapor barriers that prevent ground moisture from migrating into the living space above.
