A radiant barrier (sometimes referred to as foil insulation or reflective insulation) is a thin reflective material that is designed to reflect radiant heat. Typically installed in residential attics, radiant barriers can reduce heating and cooling costs 5% to 10% and sometimes up to 15% is hotter climates.
For a product to be classified as a "true" radiant barrier, it must pass very specific and detailed building material tests that fall under the ATSM C1313 "Standard Specification for Sheet Radiant Barriers for Building Construction Applications."
Primarilyy, ASTM C1313 classifies a product as a radiant barrier when:
- at least one side of the material has a reflectivity of infrared (radiant) heat of at least 90%.
When researching comparable radiant barriers, look for the ASTM C1313 testing results which all reputable radiant barrier dealers / manufacturers should provide on their website. If you don't see such results on their website, ask for them. If you still don't get the ASTM C1313 testing results, that should be a red flag that the radiant barrier product has NOT been tested by the American Standard Testing Methods for building materials.
So What Then Is Radiant Heat?
Okay, so now we know for a reflective insulation to be considered a "true" radiant barrier, it must reflect at least 90% of the radiant heat that comes into contact with its reflective surface. But what then is radiant heat?
Radiant heat is just one of three different ways that heat moves from one location to another location:
- via Conduction - the movement of heat through a solid material or between two solid materials that are touching each other. The heat moves from the warmer side toward the cooler side. An example of conductive heat flow would be the heat your feel when you touch the side of a boiling metal teapot on a stove-top. This type of heat transfer can be controlled with
- via Convection - the movement of heat from one location to another through the movement of a fluid or gas (like the air). Warmth rises, due to its higher buoyancy, toward cooler locations. An example of convective heat flow would be the rising of heat through boiling water and steam from the inside of a boiling metal teapot on a stove-top.
- via Radiation - the movement of heat via infrared electromagnetic waves of charged particles. This type of heat transfer doesn't require a solid mass, fluid or gas in order to move from one location to another and it is unaffected by air flow. An example of radiant heat flow would be the heat you feel radiating from the burner underneath a boiling metal teapot on a stove-top.
Radiant heat, therefore, is a method of heat movement that occurs from a heated object ACROSS an air space to another location.
How Does a Radiant Barrier Control Radiant Heat?
As mentioned earlier, a "true" radiant barrier reflects 90% or more of the infrared radiant heat that comes into contact with its reflective surface across an air space. That radiant heat is redirected back in the opposite direction from which it came, thereby protecting the cooler areas on the side behind the radiant barrier from becoming heated.
But high reflectivity isn't the only property available in a radiant barrier that helps control the transfer of radiant heat. A property called low emissivity is also present in a reflective insulation product and it works like this:
- A radiant barrier emits (radiates) very little of its own heat from its reflective surface facing an airspace. In fact, it emits in a percentage quantity directly inverse to the percentage quantity a radiant barrier reflects. For example, if a radiant barrier has a reflectivity of 96%, that same radiant barrier will only emit 4% of heat from itself on the side facing an air space.
Why All This Talk About Air Spaces: Remember, there are three different ways that heat moves from one location to another; conduction, convection, and radiation. Conduction and convection refer to the ways that heat moves THROUGH touching objects and fluids. Radiant heat transfer is the movement of heat ACROSS and air space therefore, in order to control the radiant heat, a radiant barrier must be facing an air space at at least one side.
And, it doesn't matter which side the air space is (on the side of the radiant barrier near the heat source or on the opposite side of the radiant barrier from the heat source). That's because of the two different properties that are in play in a radiant barrier as mentioned above; reflectivety and emissivity.
- If the air space is on the side of the radiant barrier near the heat source, the reflective surface facing the air space with REFLECT a majority of the radiant heat away from itself.
- If the air space is on the opposite side of the radiant barrier from the heat source, the reflective surface facing the air space will EMIT very little radiant heat from itself.
The net affect is EXACTLY THE SAME regardless of which side the air space is when using a radiant barrier: a majority of the radiant heat (97% for the most effective radiant barriers) will be stopped by a radiant barrier thereby protecting the location on the opposite side from heat.
Why A Radiant Barrier is Beneficial in An Attic Space
An ASHRAE study on heat transfer in attics found that approximately 90% of heat transfer in an attic is by radiant heat transfer. The sun beats down on the roof in the from of radiant heat and the roof shingles absorb the heat and transfer heat to the roof decking via conduction. The underside of the roof decking then radiates heat into the attic space via radiant heat transfer. 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.
- "The primary source of heat gain in attics is the solar energy absorbed by the roof." - page 1 (page 122)
- "90 % of the heat transfer in attics occurs via radiation." - page 2 (page 123)
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 resulting in lower utility bills.
A Note on 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, radiant barriers do provide year-round savings. A 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 from the roof in your 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 by keeping your home warmer.
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.
** For more information about radiant barriers and how they work, see our What Is Radiant Barrier page on our website. **