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WHAT IS A RADIANT BARRIER?
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Heat flows in buildings occur in three ways: conduction, convection and radiation. Radiant barriers address heat flows by radiation. Radiation is the transfer of heat by electromagnetic waves known as thermal radiation. Thermal radiation is a form of invisible light that travels through the air from a warm to a cool surface. Warm surfaces, such as roof decks warmed by the sun, will radiate or emit their heat to cooler interior building surfaces. A radiant barrier is a low emissivity surface placed in the path of thermal radiation that blocks heat transfer.
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WHAT IS EMISSIVITY?
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Emissivity is the ability of a surface to radiate or emit energy in the form of long wave electromagnetic radiation. It is represented by a value from 0 to 1. The closer the value is to 1, the less effective the surface is at impeding radiant heat transfer. Wood has an emissivity of 0.72. Thus, when wood is heated above the temperature of surrounding surfaces, it will tend to transfer 72% of its heat energy by radiation.
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WHAT IS LO/MIT?
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LO/MIT is a silver colored, non-thickness dependent, low emissivity coating. When applied to non-porous building materials such as plywood, OSB, or plasterboard, LO/MIT lowers their surface emissivity to 0.24--allowing only 24% of the heat energy to be transferred by radiation. It is classified by ASTM as an Interior Radiation Control Coating (IRCC) and may be effectively used as an interior radiant barrier.
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HOW DOES LO/MIT WORK?
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LO/MIT works by changing the emissivity of the surface where it is applied. Most building products such as wood, brick, and plasterboard have high emissivities (0.7-0.95). When heated they radiate most of their heat to cooler surfaces. LO/MIT lowers their surface emissivity to 0.24, lessening their ability to radiate heat.
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WHERE IS THE AIRSPACE WHEN USING LO/MIT?
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To function effectively, the low emissivity surface of a radiant barrier must face an airspace of 2” or greater. A radiant barrier will not function if sandwiched between two solid surfaces. LO/MIT, when installed on the underside of roof decking, normally faces downward into the attic, an airspace substantially larger than 2”.
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DOES LO/MIT ADVERSELY EFFECT ROOF SHINGLES?
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Absolutely not! LO/MIT has never caused shingle damage in any installation. Extensive testing at the Florida Solar Energy Center has shown that the use of a radiant barrier results in, at most, a 2 degree rise in shingle temperature.
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DOES LO/MIT DEGRADE OVER TIME?
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Unlike some insulation products, LO/MIT does not loose it effectiveness over time. Test samples installed experimentally over 15 years ago still have the same emissivity levels today as they did when originally installed.
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IS LO/MIT PERMEABLE TO WATER?
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Yes! LO/MIT allows water vapor to permeate, and does not trap water vapor within roof decks or sidewalls.
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CAN LO/MIT BE USED ON SIDEWALLS?
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Yes, but LO/MIT is normally more effective when used under roofs. Low emissivity surfaces become more effective in controlling radiant heat transfer as the temperature of the radiant heat source increases. Roof surfaces are generally much hotter than sidewalls. However, in hotter climates where no insulation is used in sidewalls, LO/MIT can substantially help increase interior comfort levels.
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SHOULD LO/MIT BE SPRAYED ON THE RAFTERS?
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No, there is no benefit to spraying the rafters only the underside of the decking should be coated.
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IS ATTIC VENTILATION IMPORTANT?
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Very important! Radiant barriers only control radiant heat loads. Good attic ventilation helps control convection loads. Together they raise the comfort levels and lower cooling costs.
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DO RADIANT BARRIERS LOWER HEATING COSTS?
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Yes, but only modestly. They are most effective at lowering cooling costs.
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DO RADIANT BARRIERS HAVE AN “R” VALUE?
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No. R-values apply only to solid insulation or to insulating systems that control heat flows other than radiation. However, when LO/MIT is installed with R-19 insulation, the combination yields a total R-value of R-35 for heat influx thus giving LO/MIT an “apparent” R-value of R-16.
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DO YOU HAVE ADDITIONAL QUESTIONS?
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Call A. & B. Construction toll-free at: 281-660-0604
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