First, let me say: Radiant Barrier is NOT a substitute for “Regular” types of insulation – either fiberglass insulation or cellulose insulation. Radiant Barrier and regular attic insulation work TOGETHER to make your home more comfortable and energy efficient.
People always ask. “Should I Add More Attic Insulation or Install A Radiant Barrier”? I hear this often and the answer is usually BOTH.
Regular attic insulation slows conductive heat. Radiant barrier reflects radiant heat. Your home gets a lot of both. Think of your home as a big refrigerator. How much heat that flows in or out of this big box is determined by basically three things: 1) Inside surface temperature, 2) outside surface temperature and 3) R-Value and thickness of what is between the inner and outer surface.
The walls and top of the refrigerator (or your home) are typically insulated with “regular” insulation. Regular fiberglass or cellulose insulation slows conductive heat. Radiant Barrier acts like shade to reflect radiant heat and make the regular insulation more effective. A perfect home would be built like a well-insulated refrigerator and be wrapped with radiant barrier foil insulation. Common sense says that a refrigerator in the shade will use less energy than one in the direct sun. This is because exterior surface temperatures can easily exceed 150º in direct sun.
If you decrease the outside surface temperature and reduce the difference between the inside/outside temperature, this has a similar effect on reducing heat flow as adding MORE “regular” attic insulation.
Too much of a good thing.
Getting attic insulation up to the recommend level for your area is always a good idea. However, at some point you reach the law of diminishing returns. Why? Regular insulation slows heat, but it also HOLDS heat. At the end of a hot-sunny day, the insulation in your attic can become a big, thick 130º blanket over the top of your home. When the sun goes down, this “hot blanket” effect continues to hold and drive heat into the home. Once the roof cools down, the insulation can easily be the hottest part of your whole house. This can make your air conditioner run later into the night. If you have marginal attic insulation, installing a radiant barrier will make what insulation you already have even more effective.
By combining good attic insulation and radiant barrier, you will get the best of both properties. Reduction of conductive heat flow AND reduction of radiant heat flow by decreasing surface temperatures of the attic insulation. Lower surface temperature is like putting the house in the shade.
So, for most homes I recommend doing both. For maximum benefit, top off your existing insulation AND add a radiant barrier. They will work together for maximum comfort and energy savings.
I've written several other posts on this that you might be interested in. Check these posts below: