Recommendation
Install radiant barriers on the underside of roofs in residential buildings. Radiant barriers can also be installed in walls and in vaulted (cathedral) ceilings. This recommendation applies primarily to residential buildings, as the need for a radiant barrier is reduced by Title 24's cool roof requirement for nonresidential buildings.
Description
A radiant barrier is a reflective surface, usually aluminum foil, installed on the underside of a roof to minimize heat gain in the summer, reducing the amount of energy needed for cooling. Radiant barriers also minimize heat loss in winter, reducing energy demands for heating.
In hot weather, almost all heat gain in buildings is from radiation, mostly from radiation striking the roof. This heat is absorbed by attic insulation, then moves through the ceiling into rooms below. A roof radiant barrier system keeps the attic up to 30° cooler.
Radiant heat from the sun penetrates the roof and walls on warm days, but a radiant barrier blocks that heat from entering the building.
In cold weather and in the evenings, a radiant barrier keeps indoor heat from escaping through the roof and walls and reflects it back inside.
Radiant Barriers Highlights
- Lower heating and cooling costs
- Least expensive way to reduce air conditioning use
- Potential to downsize or eliminate heating & cooling systems
- Can be installed in both new and existing construction
- No additional installation costs when part of new roof sheathing
Types
There are several types of radiant barriers.
Applied to roof sheathing: The reflective surface is factory-applied to one side of the sheathing, which is installed facing downward toward the attic air space. Use in place of, and install in the same manner as, conventional roof sheathing. This is the preferred form for new construction because there is no added insllation process and very little additional cost.
Reinforced sheet radiant barrier material: This material comes with one or both surfaces reflective, and with or without perforations. It may be run over the top of the roof rafters and draped down about two inches between each pair. In a retrofit situation it can be tacked to the underside of the rafters, creating an air space on both sides of the material.
Radiant barriers work in several ways:
- By reflecting heat from your home, which lowers the attic temperature, allows the insulation to work more effectively and causes your A/C unit to work less.
- By keeping heat from being absorbed into holes in ducts passing through the attic. This reduces the cooling load and helps the A/C operate more efficiently. Ductwork also lasts longer because it isn't exposed to high temperatures that induce drying and cracking, creating leaks.
Radiant barriers can also expand a building's usable space; for instance, uninsulated and unconditioned spaces such as garages, porches and workrooms can be more comfortable with radiant barriers.
Design Details
- There must be at least 0.75 inch of air space adjacent to the radiant barrier for it to work. This is true for all radiant barrier applications. The shiny side of the radiant barrier faces the air space. Anything contacting the foil side of the barrier will negate the effect of the barrier.
- Unlike insulation or an air barrier system, air tightness does not affect radiant barrier performance. It works wherever it is installed. Seams do not have to be sealed, and gaps, which are likely in retrofits, do not seriously compromise the system. The material should be 99% aluminum and the "emissivity value" should be 2% to 5%. Radiant barriers should be installed in such a manner as to minimize the collection of dust, which will seriously reduce their effectiveness.
- To get the best energy savings from a roof radiant barrier system, it should be combined with good attic ventilation.
- The reduction of attic temperatures is less important in highly insulated attics (R-30 and higher), since less heat will be conducted through the ceiling into the house.
- Radiant barrier material should not be applied over the top of the attic insulation. It would touch the insulation and conduct heat to it, and its top surface would collect dust, causing the barrier to lose effectiveness. It is better to staple the foil to the rafters. Even if you cannot cover the entire underside of the roof, it will help reduce heat gain to the attic.
- There is no point in using more than one layer of radiant barrier. Although there is an added R-value because of dead air space between two layers, one layer will block about 95% of heat gain.
Cost and Cost Effectiveness
If the radiant barrier is installed in new construction as part of the roof sheathing, the additional cost is typically less than 50 cents per square foot. The radiant barrier will typically pay for itself in reduced air conditioning costs within a few months.
Other forms of radiant barriers, especially those installed after construction, will cost more, with most of the expense being for installation.
Code Considerations
If Title 24's prescriptive approach is used, radiant barriers are required in California's hotter climates, including inland areas of Ventura County (climate zone 9). Radiant barriers are not required under the performance compliance approach, but they are recommended in cooling-dominated climate zones as a cost-effective approach to energy-efficient construction.
Radiant barrier sheathing is placed in the attic with the foil face toward the interior.
Other Ways to Reduce Build Up of Heat in the Attic
Install a Cool Roof: Light-colored roofs reflect a large amount of the heat generated by the sun. Cool roofs, which combine reflective surfaces with materials that resist heat storage, dissipate as much as 50-60% more heat than dark roofs. Attic temperatures in a building with a cool roof can be as much as 30° F cooler during the day.
Insulate: If your attic has less than R-19 insulation-six inches of fiberglass or rock wool or five inches of cellulose-you should add more. Click here for more info on insulation.
