Radiant Barriers: Don’t Become a Baked Potato!
Radiant barriers are everywhere now. You can’t walk through a home and garden show without witnessing multiple companies selling different kinds of barriers. You can even find them in your local big box lumberyard. If you do a quick search on YouTube you can find a plethora of marketing videos. But do these barriers actually work, or are they “snake oil” marketing?
The Basics of Building Science: Conduction, Convection, & Radiation
To properly understand whether or not these barriers work, a basic understanding of building science is necessary. We’ll focus on the three basics of how heat is transferred: conduction, convection, and radiation.
Today we will focus on radiation, leaving convection and conduction for another blog. Some basic things are needed to allow heat transfer to happen with radiation. First, it requires a direct line of sight from some radiant source (like the sun). The sun’s rays hit the surface and transmit heat to the surface that it is touching. An example of radiation is when you are outside on a sunny day. You can feel the heat from the sun’s rays on the surface of your skin. When you walk under a shady tree the surface of your skin feels cooler.
Common Radiant Barriers
Why exactly are the basics of building science important to radiant barriers? Well, many of the products that are advertised to you are simply laid down in your attic. Being installed in this manner does not work properly. To recall what we learned about the basics of radiation, these barriers require a direct line of sight from the radiant source. If the barrier is being laid down in your attic (under your roof), how is the sunlight getting in? It’s not.
The other problem with this method of installation is that it is not truly radiation. Laying these barriers on the attic floor in contact with the insulation is in fact conduction, not radiation. These radiant barriers even lose their emissivity when they become covered with dust. This is also problematic to install on the attic floor because most of the products used will qualify to be a vapor barrier. Vapor barriers should be on the “warm” side of the wall or attic. When they are in the vented attic, they are on the cold side of the wall and could potentially be trapping moisture in the insulation. Needless to say, this is not a good thing.
So what about attaching the radiant barriers to the underside of the roof? It still doesn’t work.
How to Lower Attic Temperatures
Many factors can help to lower your attic temperature. The color of your roof can make a big difference in the temperature. Choosing a lighter color of roofing material can help. An experiment to help you understand: go to a parking lot on a sunny day and check the surface temperature of a white and black car with an infrared temperature gun. An informal study testing this theory was done here. It was concluded that simply choosing a lighter shade can reduce the inside temperature greatly.
Back to the line of sight issue we discussed earlier. If you park both vehicles in a parking garage and repeat the test, what are the results? Both cars in the parking garage have about the same temperature. It’s starting to look a lot like snake oil to me.
So what if we covered both of the vehicles in the radiant barrier and parked them outside? That would work as long as there is an air gap between the barrier and the surface of the vehicle. Once the barrier touches the surface of the vehicle it becomes conduction.
But what about those reflective heat shields people place in the windows of their cars? They work well installed in the window, but how do they work when laying on the back seat? Not well at all.
Reduce Your Energy Use
We aren’t the only people shooting holes into the radiant barrier industry. This article here articulates the downsides of radiant barriers as well.
So there’s your proof that radiant barriers don’t work in all applications. If you wrap a potato in foil and put in your oven, what happens? It cooks.
The good news is there are many good ways to reduce your energy use simply by air sealing and insulating with spray foam (along with keeping HVAC equipment and duct work inside the thermal envelope).
Consider creating a condition attic space, click here learn more why condition attics are a good idea..
Contact KC Spray Foam today for more information about radiant barriers and attic insulation to help lower your energy cost and have a more comfortable home.