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| Sustainable Building Sourcebook | ||||
| Chapter: Energy | ||||
Energy: Radiant Barriers, Ridge and Soffit Venting | ||||
| CSI Numbers: 07194 Radiant Barrier, 07800 Roof Accessories, 07840 Gravity Ventilators | ||||
| Introduction: | ||||
| Preventing the sun's radiation from entering through the roof can make a significant contribution to comfort and reduction in cooling bills/needs. A radiant barrier is a metallic foil material that, when properly installed next to an airspace, reduces the transmission of radiated heat. To get the best energy savings from a roof radiant barrier system, it should be combined with good attic ventilation -- for example, a ridge-and-soffit attic venting system. This system is comprised of a continuous, weather-shielded opening at the peak of the roof in combination with continuous screened openings along the eaves of the building where the rafters meet the wall. | ||||
| At-A-Glance Notes: | ||||
| Technology: | ||||
| Well-developed. | ||||
| Suppliers: | ||||
| Readily available. | ||||
| Cost: | ||||
| Varies according to the type of radiant barrier material selected. Standard residential radiant barriers cost 12 to 35¢/sf. | ||||
| Public Acceptance: | ||||
| There is moderate interest in radiant barriers and continuous ridge-and-soffit vents among energy conscious buyers. | ||||
| Regulatory: | ||||
| N/A | ||||
| Considerations: | ||||
Buildings gain heat in three ways - conduction, convection, and radiation. Unlike conduction and convection, heat transfer by radiation requires no medium: it can occur in a vacuum. It simply requires two surfaces of differing temperatures that "see" each other -- for instance, the sun and a roof. Without a radiant barrier, the roof then radiates the solar generated heat into the insulation below it, or to the attic floor. In hot weather, almost æ of heat gain in buildings is from radiation, and most of this is from radiation striking the roof. This heat is absorbed by the attic insulation and eventually moves through the ceiling into the occupied space. A roof radiant barrier system keeps the attic up to 30 degrees cooler, and reduces heat transfer into the occupied space up to 40 percent in the hottest part of the day. A radiant barrier is a material that is good at blocking the transfer of radiation across a space. First, it reflects thermal radiation very well. Second, it emits (gives off) very little heat. In other words, it is a good heat reflector and a bad heat radiator. A good radiant barrier has a high reflectivity and a low emissivity . Reflectivity and emissivity are measured on a scale of zero to one. Most building materials, such as wood, concrete, and glass, have emissivity ratings close to 0.9-1.0 and are not good at reducing heat transfer from radiation, even though they may have high R-values and be good at reducing heat transfer by conduction. Aluminum foil, on the other hand, has a very high reflectivity (about 0.95) and a very low emissivity rating (about 0.05). When installed correctly, it blocks up to 95 percent of the radiant heat transfer. Aluminum foil conducts heat very well to any material it touches, so it is a poor insulator. If it is next to an air space, however, it effectively eliminates radiant heat energy exchange across the space. Using a radiant barrier can expand the use of space in a home; for instance, uninsulated and unconditioned spaces such as garages, porches, and workrooms can be more comfortable with radiant barriers. And because radiant barriers keep attics cooler, the space is more useable for storage. One final benefit: a cooler attic transfers less heat into air conditioner ducts and air conditioning equipment located in the attic, so the cooling system operates more efficiently. A radiant barrier can cause an increase in shingle temperature between 2-5 degrees on a hot summer day. Given that shingle temperature at that time is in the range of 160-190 degrees, this increase is negligible and does not accelerate shingle degradation. Although roofing manufacturers were concerned about shingle failure in the years when radiant barriers were first used, it is no longer an issue. Proper attic venting is important to obtain the best performance of the radiant barrier. A continuous ridge-and-soffit vent system is the most effective means to ventilate an attic. As the roof heats air in the attic, the hot air rises and exits out the ridge vent. This air is then replaced by cooler, fresh outside air, which evenly enters the attic space through the soffit vents under the entire eave, regardless of wind currents. This is an example of the thermal chimney effect (see Passive Solar Design section of this Sourcebook). Radiant Barriers, Fig. 1
Ridge-and-soffit venting is a passive system that outperforms mechanical ventilation (fans). If a gas furnace or water heater is located in the attic, passive ventilation is much safer than mechanical. It will not cause backdrafting of the gas flame or of the products of combustion (such as carbon monoxide) that may get sucked into the living space through leaks in attic ducts instead of going out of the vent pipes. An added advantage of ridge vents is that most of them can be shingled over with composition shingles and do not require the penetrations needed for gravity vents. | ||||
| Guidelines: | ||||
| Radiant Barriers
There must be an airspace 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. 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 percent aluminum and the emissivity value should be 2-5 percent. Radiant barriers should be installed in such a manner as to minimize the collection of dust, which will seriously reduce their effectiveness. Radiant barriers are available in several configurations. Applied to OSB or plywood roof decking: the reflective surface is factory-applied to one side of the sheathing, which is installed facing downward toward the attic air space. This is the preferred form for new construction because there is no added installation process. Reinforced sheet radiant barrier material: with one or both reflective surfaces and with and without perforations. It may be run over the top of the roof rafters and draped down about 2 inches between every two rafters. In a retrofit situation it can be tacked to the underside of the rafters. In this location, an airspace will be present on both sides of the material. Other considerations:Radiant barrier material should not be applied over the top of the attic insulation. It would touch the insulation and conduct heat to it. Secondly, dust would settle on the upward facing radiant barrier surface causing it to lose effectiveness. There is no point in using more than one layer of radiant barrier. Although there is an added R-value because of dead air spaces between two layers, one layer will block about 95 percent of heat gain. Continuous Ridge-and-Soffit VentingThis is a passive system that is considered the most effective means of ventilating an attic. It can operate effectively regardless of wind direction and velocity. Airflow in an attic created by this type of venting will be a continuous wash of air directly under the roof moving upward from the eaves (soffits) to the roof’s ridge. Soffit vents are made in long perforated plastic or metal strips, typically 8 feet long and 3 inches wide, shaped to fit between sections of soffit material. They are installed in a continuous run on the underside of the eaves where the rafters meet the wall. They should be located as far out on the eave as possible, closest to the fascia. Care should be taken to ensure good airflow over the top of attic insulation located at the eave, as well as insuring that attic insulation does not block the soffit vents. Insulation baffles are available for this very purpose. There are several types of ridge vents: Panel vents are made of plastic, some in a corrugated form, and come in 4-foot lengths, which can be connected end to end. Pieces to cap off the ends are available. Roll-out vents are made in a long continuous roll of fibrous plastic. No waste is generated in installation. Both of these types of vents are very low- profile and can be shingled over with composition shingles. Metal ridge vents for metal roofs come in 8-foot lengths and also connect end to end. Baffles (vertical flanges at the vent openings) prevent rain from entering the attic. Some ridge vents are better adapted to a very steep pitch than others, so it is important to select the right type for the roof. Ridge vent installation:A gap or space must be maintained between the sections of roof decking on either side of the ridge so attic air can reach the roof vent. This space is typically about 2 inches wide on each side, but manufacturer’s instructions should be followed. They should also be followed in regard to final section installation at the ends of the ridges, and in regard to fasteners (nails, staples, screws, etc.). “Architectural” shingles may need to be stripped back since they are thicker than standard shingles and could push the vent up. Shingles installed at the peak over the vent may need to be trimmed. The joint between the vent and the roofing should be caulked. If a vent does not have alignment tabs, an inch-wide space needs to be allotted between sections to allow for expansion. Additional attic ventilation (such as gable, turbine or gravity vents) should not be used because they upset the natural (and ideal) air path of the ridge and soffit system.
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| Resources: | ||||
| Professional Assistance: | ||||
Florida Solar Energy Center |
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| Components/Materials/Systems: | ||||
See "Building Materials - Retail" in Yellow Pages Radiant Barriers:Radiant barriers are now readily available in most lumberyards & home improvement stores. Energy Conservation Technologies Innovative Insulation Inc. International Paper Louisiana-Pacific Corp. Solar Energy Corporation Ridge Vents:Ridge vents are now readily available in most lumberyards and home improvement stores. Air Vent Inc. (a CertainTeed Corp.) American Wick and Drain Corp. Benjamin Obdyke Inc. Builder's Edge Cor-A-Vent GAF Greenstreak Plastic Products Marco Industries, Inc. (Southwest office) Mid-America Building Products Northwest Building Products Inc. Ridglass Shingle Manufacturing Co. The Solar Group |
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