What does the R value of foam mean? The r value is a term used to measure the insulation of the material. Insulation is used to increase thermal resistance. There are two different factors that determine the effectiveness of the insulation. The first is the thickness of the foam. The second is the density. They determine how much air is conducted through a structural barrier. Your outside climate is not the only determining influence. You also have to take into account which section of your home needs the most protection from the elements. Also keep in mind the type of heating and cooling system you have installed. This is essential in making your home comfortable and energy efficient.