Radiant Barriers...

A radiant barrier system, used with traditional energy conservation strategies, could help you reduce your home's cooling bill. A radiant barrier basically is a layer of aluminum foil that reflects thermal radiation without transferring heat to other materials in your home.

Heat Gain

A building gains heat in three ways: conduction, convection, and radiation. Conduction is the transfer of heat directly through a solid material and on to another material with which it is in contact. Convection is the transfer of heat by air movement in which lighter warm air rises and heavier cold air sinks. Radiation is the transfer of heat from one object to another through space.

Radiant heat gain is not affected by most traditional conservation strategies. For example, insulation in walls and ceilings primarily restricts conductive heat gain, and double-glazed windows restrict both conducive and convective heat gain. On the other hand, a radiant barrier system can restrict the thermal radiation entering a building and thereby decrease the building's cooling load.

Radiant Barrier Systems

A radiant barrier system is composed of an airspace with one or more of its boundaries acting as a radiant barrier. A radiant barrier is a material that restricts the transfer of thermal radiation by reflecting the radiation that strikes it and by preventing the radiation of heat to other surfaces.

Aluminum foil is a good radiant barrier. Although aluminum foil is a good conductor it is highly reflective and absorbs very little of the thermal radiation that strikes it. So if it is placed between materials that are attempting to transfer heat by radiation (rather than conduction) and if it is separated from these materials by an airspace, the foil effectively eliminates the normal radiant heat exchange across the airspace.

Roof Systems

The attic of a Texas house offers excellent potential for the use of a radiant barrier system. First because the roof is the surface most exposed to solar radiation, and second, because most of the solar gain absorbed by the roof is transmitted down to the attic floor by radiation. Because the attic airspace separates the hot roof surface from the ceiling, no heat will move down by conduction. Because heated air rises, no heat will move down by convection from the hot roof to the ceiling.

If you place a radiant barrier (layer of foil) in the airspace between the hot roof deck and the cooler attic floor (insulation), you can eliminate almost all radiant heat transfer. Studies at the Florida Solar Energy Center (FSEC) indicate that, under peak daytime conditions, total heat transfer down through attics can be reduced by more than 40 percent through the use of a radiant barrier system. The temperature of attic insulation also is reduced significantly. During the period of peak daytime temperatures (noon to 2:00 p.m.) the temperature at the top of the attic insulation was about 10 degrees cooler beneath the roof with the radiant barrier system than beneath the roof without the system.

Heat transferred upward through attics (winter heat loss) will not be affected as much because a greater part of total upward heat transfer occurs by convection. That is why radiant barriers in roof systems are a more effective cooling rather than healing strategy, and why they may be a great benefit to Texas homeowners. FSEC studies show that in a typical Florida home, a radiant barrier roof could cut annual cooling loads by 4 to 8 percent and peak cooling loads by 9 percent, depending on the level of attic ventilation.

Guidelines

Based on the results of full-scale tests conducted at the FSEC Passive Cooling Laboratory, the FSEC has developed climate guidelines for the use of radiant barriers. Attic or roof radiant barrier systems are likely to be effective where there are 3000 or fewer annual heating degree days and 2000 or more annual cooling degree days (both measured at a base temperature of 65 degrees F). This area includes all of Texas except the Panhandle (figure 1). Radiant barriers are most effective on homes and smaller buildings that receive most of their cooling load from the outside conditions, rather than on larger buildings that receive most of their cooling load from inside the building.

Placement

Most types of roofs already contain some kind of attic or airspace that can accommodate an effective radiant barrier system. In new construction, it should be easy to install radiant barrier systems regardless of roof pitch. Figure 2 shows three possible generic locations for radiant barriers in attics. When first installed there will be no significant difference in the effectiveness of these locations. But in time, location 3 will suffer because of dust accumulation, which decreases performance. Dust is not as likely to collect on the underside of the radiant barriers at locations 1 or 2.

 

Figure 1. Climate region recommendations for use of radiant barriers.

Figure 2. Typical attic section with three possible locations for a radiant barrier.

Location 2 is best for two reasons. First it can easily have two radiant barrier surfaces (top and bottom). Second-and more important-it offers the potential for separately ventilating the space between the radiant barrier and hot roof deck and the attic space itself. This results in an attic air temperature somewhat closer to the conditioned space temperature in both winter and summer. As with location 3 dust may collect on the top of location 2, but a radiant barrier surface facing downward will perform as well as one facing upward. Therefore, for reasons of dust accumulation use location 1 or 2 and depend on the downside for radiation control.

In new construction, there is an alternative, which offers the advantages of location 2 while providing the construction ease of location 1. This technique places the radiant barrier on top of the roof rafters (or trusses) before the roof decking is applied. It is installed so that it droops 1 1/2 to 2 inches below the upper surface of the roof structure. When the roof decking is applied, an airspace separates it from the radiant barrier in a way similar to that of location 2. This airspace also can be vented separately from the attic. As with location 2 the most reflective radiant barrier surface should face downward toward the attic airspace.

Economics show that more than one radiant barrier in an attic is not cost effective. The first barrier surface eliminates about 95 percent of the radiant heat transfer across the attic. Adding more layers can affect only 95 percent of the remaining 5 percent.

Tightness

It is not necessary to form airtight seals with radiant barriers; radiant energy travels in a straight line through the air but is not transported by the air. In fact, if you choose location 3 (figure 2), you should use a perforated foil product that will allow the free passage of vapor out of the Insulation during winter This may also apply to location 1 in some cases because the barrier is in contact with the roof decking. Location 2 should not have moisture condensation problems because it has an airspace on both sides of the radiant barrier.

Radiant barriers can reduce energy consumption and / or improve comfort in many buildings. But the radiant barrier strategy and construction technique will have to meet individual building needs.

Conclusions

Summer heat gain can be driven by forces different from those that cause winter heat loss. In southern climates, radiant barriers offer significant potential for impeding solar-driven heat gains in buildings. Their effectiveness depends on their location in the building structure, direction of heat flow, building type, and occupant needs. Widespread use of radiant barriers in overheated climates will prove their potential for energy conservation in small buildings.

Information for this article was taken from "Radiant energy transfer and radiant barrier systems in buildings " (FSEC-ON-6-84) and "Designing and installing radiant barrier systems" (FSEC-ON-7-84), by Philip Fairey. Both items were published as part of the Florida Solar Energy Center Design Note series.