Radiant barriers have been shown to provide some savings on cooling bills in hot climates. These barriers are installed in attics as they block some of the heat that is entering a house through the roof. The barriers provide some protection and savings, but should not be seen as a replacement for insulation. A radiant barrier is just one aspect of keeping a house cool in summer and should be seen as a key part, but only a part.
The summer sun beats down on a rooftop, and the surface of the roof can get very hot. That heat eventually works its way down into the house, making it even hotter than the temperature outside in some cases. The barrier reflects much of that heat back up through the roof. A significant drop in temperature in the attic can be realized, and as a result there is less hot air working its way down into the house. An air conditioner will not have to work as hard as a result.
Some big claims have been made for radiant barriers, but studies show realistic savings on an electric bill to be between five and 10 percent. Longer term savings are realized too because the air conditioning unit does not have to work as hard, saving wear and tear. A smaller unit can also be used to cool a house, and that can be a savings as well.
It is not a substitute for regular insulation, however. Regular insulation absorbs heat - just as it would cold in winter - and protects a house from extremes of temperature. Using both together is the most cost-effective solution.
The higher the temperature the more effective a radiant barrier will be as the hotter it gets the more heat it reflects upward. In moderate temperatures, it is not as effective. It can do more in 100-degree heat than 80-degree heat, for example.
It is important that the surface not be touching any other part of the structure because heat is also transferred by conduction as well as radiation. If the barrier is touching something it will still conduct the heat into the rest of the house.
There is still some debate over whether a radiant barrier is effective in winter in colder climates. Some people say you can use the barrier in the reverse way as well, by having a layer face downward. In that scenario, it would keep heat inside the house in winter by reflecting it back inside the house, not allowing it to escape through the roof.
Installing a radiation barrier is a one-time expense. It has no moving parts so should never wear out. The only maintenance is to make sure there is no significant dust buildup as that will reduce its effectiveness. The savings you get from installing a radiant barrier is not huge, but the savings you do get should be permanent. Over the long term that savings could add up to a significant amount of money.