A common question is “What Is The R-Value of Radiant Barrier Foil?”
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:
Interesting but I don’t wrap my house with foil to reflect the sun. Adding foil to the underside of my roof does NOT stop any of the conductive heat. Putting a sheet of foil on top of the ice inside the cooler does not make the cooler absorb any less heat.
Brad,
Thanks for the comment. Everything you say is correct. I really don’t get your point?
I have a client that has a vaulted ceiling with 2×6 planks for the ceiling. The top side of the planks is the roof, underside is the ceiling. They currently have a foam roof, but are concerned that if it is removed and a composition roof is installed, the room will get hot. Currently with the foam roof, they have about R-14. Without the foam, they would have about R-2. My question is if they installed a composition roof with the Radiant barrier installed on the deck before the roof is installed, would that give them the equivalent of the foam roof for R rating?
It’s like the example above – It is an apples to oranges comparison, so I can’t fairly compare the foil plus roof to the foam plus roof. They address different types of heat. I can tell you that the foam (or any R-value material) doesn’t STOP heat gain, it only slows it. The foil however actually BLOCKS 97% of radiant heat when there is an air space on at least one side of the foil. Is there an option to install the foil and some R-value? That would be the best set up for a cathedral ceiling, to combine the foil AND the R-value, this is how it is done: Installing Radiant Barrier & Insulation in a Cathedral Ceiling.