When a radiant barrier is installed in an enclosed air space, such as in a wall or ceiling joist space, it is called "reflective insulation." When a radiant barrier is installed in this way a calculation can be made to determine the "effective" R-value of the product. Keep in mind, the calculated effective R-value of the product will vary depending on the direction of heat flow (up, down, sideways) and the size of the enclosed air space. Be aware that anyone claiming an effective R-value for a reflective insulation product is making a very specific assumption about how the products is installed and exactly how much enclosed air space is next to the reflective surface. Such claims can be very misleading. The performance and long-term cost-effectiveness of the product depends on number of factors, including where the product is installed, how the product is installed, and the amount of existing insulation currently in the home.