In today’s economy with rising utility costs, people are constantly looking for cost effective ways to reduce their monthly utility bills and save money. Radiant barriers have gained a lot of attention as a result and yet very little is understood about what makes them so effective.
Definition of a Radiant Barrier
Radiant barriers reflect radiant heat back to it’s source. Typically installed in attics or as a house wrap, a radiant barrier reduces summer heat gain and winter heat loss which reduces cooling and heating costs. While highly reflective, a radiant barrier works by greatly reducing the transfer of radiant heat using two unique properties; high reflectivity and low emissivity.
For a product to be classified as a true radiant barrier, it must be proven to reflect 90% or more of the radiant heat the strikes it’s surface. How do you know a radiant barrier has been proven to reflect 90% of the radiant heat? Make sure the radiant barrier has an ASTM C1313 classification. The manufacturer should communicate this classification as part of their product specifications.
What is radiant heat?
A common, and very easy to understand, example of radiant heat is the heat you feel on your skin when standing out under direct sunlight. More specifically, radiant heat is the transfer of heat across an air space from a heated surface. Stand in your attic space on any sunny day and you’ll instantly feel the heat bearing down on you from the underside of the roof decking above.
More about Reflectivity & Emissivity
Radiant barriers are unique in that they work very differently than traditional insulators (fiberglass, rock wool, cellulose, etc) which merely absorb and slow down the transfer of heat. A radiant barrier reflects 90% or more of the radiant heat that strikes it’s surface from across an air space. Conversely, a radiant barrier emits 10% or less heat from itself across an air space.
The high reflectivity property of a radiant barrier is the easiest concept to understand. When facing a heat source across an air space, the highly reflective surface of the radiant barrier will reflect the radiant heat away from its surface. For example, if a radiant barrier were installed on the attic floor, it would greatly reduce the amount of radiant heat that reaches the ceiling of the living spaces below by reflecting the radiant heat that strikes it’s surface from above.
The more difficult, yet equally as important, concept to understand is that a radiant barrier has a very low emittance factor. A radiant barrier emits (radiates) very little heat from itself. For example, if a radiant barrier was attached directly to the underside of the roof decking in an attic space, the low emissivity factor is what makes the radiant barrier effective in this location as it greatly reduces the transfer of radiant heat by NOT emitting the heat from itself.
Note: Both the high reflectivity and low emissivity properties work only when a radiant barrier is facing an air space. Without an air space on at least one side of a double-sided radiant barrier, there will be no reduction in the transfer of radiant heat. If you’re using a single-sided radiant barrier (paper backed), then the aluminum side MUST face the air space to be effective.
Radiant Barrier Benefits
Studies have shown that a majority of heat gain in a living space during summer months is due to radiant heat transfer. Radiant heat from the sun, penetrates a home’s roof and radiates into the attic space. As the attic space heats up, this heat begins to transfer through the attic floor/living space ceiling and into the living spaces below. Even with traditional insulation on the attic floor, up to 93% of the ceiling heat gain in summer months can be attributed to this radiant heat from the attic space above.
By adding a radiant barrier to your attic space, a majority of the radiant heat can be BLOCKED from transferring into your living space. This can equate to reducing heat transfer into your living spaces by up to 50%. This means using your air conditioner less often resulting in lower utility bills.
Winter Savings with Radiant Barriers
While most of the formal studies on the use of radiant barriers are conducted in warmer climates during summer months, double-sided radiant barriers do provide year-round savings. A double-sided radiant barrier in the attic space will not only block summer heat from penetrating your livings spaces from your attic, but will also help keep heat generated within your home from escaping out the attic. And since studies have shown that up 75% of heat loss in the winter time is radiant heat, the addition of a radiant barrier is a very effective way to reduce winter utility bills.
However, a radiant barrier also blocks beneficial winter heat gain from the solar heating of your roof so the consumer should evaluate how much this winter heat gain contributes to the warming of their home before installing a radiant barrier. The net benefit of radiant barriers on winter heat savings are still being studied.
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