Your roof acts the same way.  It can easily reach 150º or hotter on a sunny day.  This heat will then be re-radiated through the attic and will be absorbed by your attic insulation causing high energy bills and comfort issues.

This video illustrates how a radiant barrier works – plain and simple.  Rather than absorbing the radiant heat from the roof, let’s bounce it back and keep the attic insulation cooler. If you keep the insulation cooler, then less heat enters the home and you save money and stay more comfortable.

Think of it this way:  Radiant barrier is your 1st line of defense against radiant heat and your attic insulation is your 2nd line of defense against conductive heat.  They will actually work TOGETHER for maximum efficiency and comfort.

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Basically it works this way.  Radiant heat is heat that is transferred across either an air space or a vacuum.  This is how the heat from the sun reaches the earth.  Radiant heat acts similar to a sound wave.  By definition, you MUST have either an air space or a void for radiant heat to even exist.

If you don’t have an air space then you basically have a solid.  Heat can essentially only move through a solid by conduction. This is by direct contact.  This is how an egg cooks on a hot skillet.

So, without an air space, you cannot have radiant heat. Without radiant heat there is no way to have a “Radiant Barrier”.  In fact, because of the conductive nature of pure aluminum, if you install radiant barrier foil WITHOUT the required air space it will actually work AGAINST you and INCREASE Heat flow.

The bottom line is you MUST have an air space on one side of a radiant barrier for it to work.

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The short answer is NO damage will occur. Tests have shown that installing a radiant barrier in attics generally only cause the roof and shingles to increase about 2° – 5° Fahrenheit.

This may seem unbelievable since radiant barriers are sold on the fact that they will reflect 97% of radiant energy away from their surface. The question is asked “Where Does The Heat Go?” It is assumed that since the heat is reflected back towards the roof then the roof must experience a significant increase in temperature.

Let me explain exactly what is happening between the sun, the roof (shingles & decking material), and the attic.

First, radiant energy from the sun heats up the shingles. The actual temperature of the shingles is primarily determined by two things: 1) Shingle color – darker shingles absorb more heat and get hotter, and 2) outside air temperature. Obviously, your roof will get hotter on a hot-sunny day compared to a cold-sunny day.

Once the heat is absorbed by the shingles, it is transferred via conductive heat flow to the roof deck. The roof then becomes a “sponge” to hold the heat. On the other hand, air has a cooling effect. How much is determined by the actual outside air temperature and the amount of airflow on the roof either by wind above the roof or attic ventilation below the roof.

These factors: Amount or Angle of the Sun, Shingle Color, Outside Air Temperature, and Airflow will determine the “Maximum Roof Temperature”. At this point, the roof will not get any hotter. Your geographic location will also affect this. A roof in Las Vegas will get hotter than a roof in Kansas. In general, maximum roof temperatures will range from 130° up to 180°.

I mentioned that the roof becomes a heat “sponge”. As the temperature goes up it will want to release the heat by converting it to radiant energy. This radiant energy is emitted in all directions, both upward to the sky and downward into the attic.

By installing a radiant barrier, the energy heading towards the home will be reflected back up towards the roof and cause MORE heat to be sent towards the sky away from the home. This is similar to a light bulb with a reflector behind it. The amount of heat and light given off by the bulb is constant, but you would feel less heat behind the reflector and the light is directed to one side.

Shingle manufacturers also provide a full warranty on products installed over radiant barriers. Your geographic location and color of shingles are the two biggest factors in determining roof temperatures.
The bottom line is that shingles usually see a SMALL increase in temperature (usually 2°-5°) over radiant barriers. These results are from tests performed by the Department Of Energy. Plus, many tests performed by customers and installers support this fact. This slight increase in roof temperature is considered nominal and will have virtually no impact on the performance or life expectancy of your roof. Rest assured, Your shingles will NOT BAKE.

The answer: ZERO

Heat moves in 3 different methods:  1) Conductive 2) Radiant 3) Convective.  R-Value is a term used to measure how resistant a product is to CONDUCTIVE HEAT ONLY.  The best way to illustrate this is to picture an ice chest with a big block of ice inside.  Common sense tells us that WHAT the ice chest is made of and how THICK the walls of the ice chest are will determine how long it will take for the ice to completely melt.  An ice chest made of foam will obviously keep the ice from melting longer than an ice chest made of metal.  This is R-Value.

R-Value examples:  Wood has an R-Value of about 1 per inch of wood. So, three inches of wood has an approximate R-Value of 3.  Styrofoam has an R-Value of about 3-4 per inch resulting in an R-Value of about 11 for 3 inches.  This is why we don’t make coffee cups out of wood.  The coffee would not stay hot very long.

Radiant Barrier has NO R-value.  Why? Because R-Value is a measurement to determine the effectiveness of a material to slow CONDUCTIVE heat flow. It is an apples to oranges comparison. This is like asking how effective is an umbrella to protect you against the wind?  Obviously, an umbrella is designed to stop rain and not wind.  Just like a radiant barrier is designed to stop radiant heat as compared to conductive heat.

So, if radiant barrier has no R-Value, how is it effective?

Think of radiant barrier as SHADE.  Does shade have R-Value? No, but it sure is nice to have on a sunny day to help keep cool and comfortable.

Here is an example:  Let’s take two identical ice chests and fill them with ice.  Then, put one ice chest in the sun and the other in the shade. Which one is most effective in keeping the ice from melting?  The one in the shade, right?  Although the AIR TEMPERATURE will be exactly the same in the shade and the sun, the OUTSIDE SURFACE TEMPERATURES of the ice chest will be higher in the sun.  This will cause the ice chest in the sun to be less effective than the one in the shade.

If you were to wrap an ice chest with radiant barrier foil and put it in the sun, the foil would act like shade by reflecting 97% of the radiant heat from hitting the ice chest.  Decreasing the outside surface temperature is as effective as adding more insulation or more R-Value in most circumstances. Therefore, although radiant barrier by itself has NO R-value, it WILL increase the effectiveness of the existing insulation or R-Value.

Your home is very similar.  Regular insulation is like the ice chest and a radiant barrier will act like shade to keep the box cooler.  Install a radiant barrier to make the insulation in your home even more effective. Combining the two will maximize comfort and efficiency by slowing BOTH conductive and radiant heat flow.

I've written several other posts on this that you might be interested in. Check these posts below: