Rigid insulation is typically a hard foam material that is used to help regulate the temperature in buildings. The thermal resistance of this material, or effectiveness at keeping heat from passing through, is known as rigid insulation R value. In general, rigid insulation R value is higher than that of other materials used as insulators. This typically makes it a more efficient choice, although it can also add to the cost.
Insulation is designed to help keep buildings warmer when it's cold by keeping heat inside, and cooler when it's warm out by keeping hot air out. The ability to reduce the flow of heat is typically represented by a positive number called an R value. In general, the higher the insulation R value is, the more the material keeps heat from moving from one side to the other.
One of the main factors affecting R value is the density of the material. Many forms of insulation, such as fiberglass blankets and blown-in cellulose, are made of loosely woven or packed fibers. Rigid insulation, on the other hand, is usually made of substances that can be fused together more compactly, increasing the density of a comparable single layer. This increased density is one of the primary reasons a rigid insulation R value is often higher than that of many other forms of insulation. For example, on average, rigid insulation R values are at least twice that of similarly sized fiberglass blankets and at least one and a half times that of comparable amounts of blown-in cellulose.
Another factor that can affect the R value of insulation is what substances are used to create it. Even the various types of rigid insulation may have different R values, depending on the base material used. For example, a polyiso rigid insulation R value is generally 30% to 40% higher than that of other types of rigid insulation. This is because the polyiso cells that make up the insulation have a special gas that is effective at reducing heat flow added to the inside.
One of the main advantages of rigid insulation having higher R values is the ability to use less. For example, if a person wanted to have R-15 insulation in his or her attic, but only had limited space to add it, some rigid polyiso foam options could allow for using just 2 inches (5 cm) of insulation, as compared to the typical 4 to 5 inches (10 to 12.5 cm) of fiberglass or cellulose insulation that would otherwise generally be required.
The primary downside is that with the higher rigid insulation R value usually comes higher initial purchasing costs. Many experts report, however, that savings in heating and cooling costs over time typically make up for this.