Myth: Adding a radiant barrier below the roof line inside the attic will make the roof hotter and damage the roofing materials, including shingles.
There is a common thought that because radiant barrier reflects 97% of heat back toward the source, it will increase your roof temperature substantially and cause damage to the roof decking and shingles. However radiant barrier doesn't change the amount of heat going through your roof, it simply changes the direction of the heat flow.
Understanding Radiant Heat
Radiant heat by definition is heat that travels as radiation in waves across either a void (air gap) or vacuum until it is either absorbed or reflected. Radiant heat is the INVISIBLE part of the light spectrum; it travels at the speed of light until it is either absorbed or reflected.
A simple way to understand this is to think of a light bulb in a dark room. When you turn the light on, it shines in all directions across the air space in the room – lighting up the space.
The same is happening in your attic; radiant heat from the sun travels across the atmosphere where it then is absorbed by your roof. The heat travels via conduction though the roofing materials and then re-radiates across your attic space, in all directions until it is absorbed or reflected by the items in your attic.
What Happens When Radiant Barrier Reflects Heat
Heat is coming in to your roof - period. Right now, without a radiant barrier, that heat is hitting the roof and being absorbed into the attic. Then it radiates across the attic until the items in your attic, such as your insulation, an A/C unit, duct work, etc., absorb it. When you add the radiant barrier the same amount of heat is still coming in, the difference is that it is not heating up the contents of the attic since the heat is blocked at the roof line (for a staple-up method). So the amount of heat in/on your roof is essentially the same, but now it is being redirected to stay between the foil and the roof, instead of being directed in various directions all over the attic.
Let's revisit the lamp analogy from above. If you take the reflector (lampshade) off the lamp, light goes in all directions (this is how your roof is without a radiant barrier). Once you add the reflector, you redirect the light to mainly go downward, from the bulb. The amount of light the lamp puts off did not change by adding a reflector, you redirected the light to a concentrated area. Do the things in the current path of the light get brighter? Maybe ever so slightly, but it's not as though they have doubled in brightness. The same is true in your attic. The heat that was radiating in all direction is now only radiating upward, away from the attic, instead of upward and downward. This means there is a slight increase in temperature, but the amount is so trivial that it doesn't really have an impact on the overall temperature of the roof or the shingle life.
Measured Temperature Differences
The government, privately owned companies, RIMA and even AtticFoil.com have all done studies to test the measurable temperature differences in a roof with radiant barrier versus a roof without radiant barrier. Some of these studies go back 20 years or more, but they all have the same conclusions. All testing concludes that the temperature of a roof does not increase enough to warrant concern about shingle life or roofing damage.
The average roof temperature increase in the studies ranged from 2°F - 10°F. On roofs with darker shingles, the temperatures overall were measured higher than on roofs with lighter shingles, as was to be expected. One example of a roof we measured showed that without radiant barrier the temperature was 166°F -167°F (this was for a medium/dark shingled roof). On the area that did have radiant barrier installed the readings were 173°F - 174°F. The tests conclude an increase of about 6°F to 7°F degrees in roofs with a radiant barrier installed below the shingle line. This is exactly within the parameters found by major studies done on the effect of radiant barrier on roof temperatures, proving that radiant barrier only increases the shingle/roof temperature by a nominal amount. This small increase is considered negligible and not having any sort of negative impact on either shingle or roofing material life.
Shingle Warranty and Radiant Barrier
Many people often wonder if installing a radiant barrier will void their shingle warranty. While it's best to check with the manufacturer of your specific shingles to see what guidelines are in place for your particular warranty, oftentimes shingles are made to handle a much hotter load than they do – even with radiant barrier installed. Also, some cities and counties require a radiant barrier on all new homes constructed. If you are unsure about your particular roof, check your paperwork before installing a radiant barrier. Remember, on a 100°F day, with a roof getting up to 180°F – a temperature increase within 10°F is nominal in the overall assembly. Make sure you install the radiant barrier with the proper air gaps, have adequate and uninterrupted air flow in the attic and start reaping the benefits of a radiant barrier today.
For more information on RIMA's testing of roof shingle temperatures, click here.