The term radiant barrier refers to a reflective coating that is typically applied to the underside of the roof, there are however several different types and different ways that it can be installed. The two major types are spray on and sheet barriers . Sheet radiant barrier comes in large roles that you then roll out in your attic. This can be laid directly on top of your insulation, nailed to the bottom of the roof, or hung from your rafter supports. This is also the type of radiant barrier that is sometimes used underneath shingles. In new construction or re-roofing projects you can roll out sheet radiant barrier on top of the roof but under the shingles.
The second kind of radiant barrier is spray on, which is sprayed onto the underside of the roof like paint. This has the advantage of being more durable and easier to install correctly. It is in fact a paint mixed with metal flecking that provides the reflective property required to reflect the heat. Another advantage of spray on barriers is that they are easier to get total coverage with as compared to the sheets. However, there is one drawback to spray on radiant barriers , they do not provide quite as much heat shielding as properly installed sheets can. However, many experts feel that you will see just a much benefit from spray on radiant barrier due to its greater durability and ease of installation over the entire surface of the roof.
Both of these radiant barriers function on the same basic principle. Reflecting heat away from your house will keep heat away from your insulation, which in turn will lower the amount of thermal radiation that passes through your insulation and makes your house warmer. Radiant barrier is primarily designed to keep heat out and does not show significant benefits in reducing heating costs for people living in cold climates. That being said, it can make a major difference on roofs in southern states and dramatically decrease your cooling bills. The reflective metal in the barrier will prevent the heat on the outside of your roof from the sun turning your attic into an oven. No amount of insulation can totally prevent heat transfer, and the higher the temperature difference the more heat transfer their will be. So by decreasing the temperature in your attic you make it easier for your insulation to do its job and keep the cold air in and the hot air out.
Many people feel that if there house has adequate insulation they either do not need, or won’t see much benefit from, installing a radiant barrier in their house. This is not true primarily because radiant barriers and traditional insulation function very differently in how they keep your house warm. Radiant barrier prevents the transmission of heat radiation into the air around it, it stops heat that is radiating into your attic space from outside. Traditional insulation on the other hand primarily serves to trap air, thus preventing heat transfers due to air movement known as convection. So even if you have adequate traditional insulation preventing convection warming of your house from your attic, you can still benefit from adding a radiant barrier to prevent heat radiating into your house from outside.