Reflective insulation materials are usually fabricated from aluminum foils with a variety of backings such as kraft paper, plastic film, polyethylene bubbles, or cardboard. Reflective insulation is most effectively used to reflect sunlight in the summer which directly affects the heat gain in your house. Therefore, reflective insulation is often used in reducing what we call downward heat flow. Downward heat flow is basically the heat gained during summer. Reflective systems are typically located between roof rafters, floor joists, or wall studs. If a single reflective surface is used alone and faces an open space, such as an attic, it is called a radiant barrier.
Most people are familiar with traditional insulating materials such as fiberglass, cellulose, Styrofoam, and rock wool. These products use their ability to absorb or resist (slow down) convective and conductive heat transfer to insulate (R-value). A third, seldom discussed but dominant form of heat transfer exists: radiant heat transfer.
Radiant heat transfer is due to electromagnetic energy transfer. For instance, when you step outside on a sunny day and feel the sun’s rays on your face, you are feeling radiant heat transfer. All objects above absolute zero (-459.7 degrees F.) emit infrared rays in a straight line in all directions.
Therefore, here comes the radiant barrier to insulate against the radiant heat transfer.
Radiant barrier is basically a type of reflective insulation installed in buildings to reduce summer heat gain and winter heat loss. A radiant barrier insulation system is a layer of foil facing an airspace and is installed in the envelope of a building. A pure aluminum radiant barrier reflective insulation is unaffected by humidity and will continue to perform at a consistent level no matter how humid it may be whereas a 1-1/2% change in the moisture content of fiberglass insulation for example will result in a 36% decrease in performance (referenced from HVAC Manual 10.6; McGraw-Hill).
In new buildings, you can select foil-faced wood products for your roof sheathing (installed with the foil facing down into the attic) or other locations to provide the radiant barrier as an integral part of the structure. For existing buildings, the radiant barrier is typically fastened across the bottom of joists, as shown in this drawing. All radiant barriers must have a low emittance (0.1 or less) and high reflectance (0.9 or more).
Radiant barriers have become increasing important and popular now when this relatively new insulating technology is maturing. Moreover, the cost of radiant barriers installation has also been drastically brought down in recent years which make this insulation method a popular choice in new houses. Studies have shown that as much as 90% of heat gain in the summer, 50% of heat loss through ceiling or roof and 65% of heat loss through wall is through radiant heat transfer.
In conclusion, if you are buying a new house or doing renovation, do consider radiant barriers as part of your insulation work.
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