Rigid Insulation

There are four basic categories of insulation. They are loose fill, blankets & batts, spray on and rigid insulation. Rigid insulation is the category that we will discuss in detail in this article. We will examine the several types of rigid insulation and outline some best practices that you may want to use in your home. Rigid insulations are mostly used for insulating roofs, basements and exterior walls. We will cover what type to use and where to use it below.

While there are four basic categories of insulation, there is also many subcategories and types, so we will help you to understand the two main categories of rigid insulation and the several types if rigid insulation that fall into these two main categories. We concentrate on only rigid insulation here because to have a discussion of every type of insulation would be like writing an entire book.

Types of Rigid Insulation

First, there are two main categories of rigid insulation; fibrous materials and plastic foam materials.

Fibrous materials such as wood, cane and vegetable fibers are mixed with binders and fillers to form rigid boards of insulation. Wood is the most commonly used fibrous material. Wood fibers can only be used to make flat boards whereas other types of rigid insulations can be formed into pipe coatings and other useful shapes.

These types of rigid insulation boards are often coated with asphalt or impregnated with asphalt to make them more moisture resistant. Wood fiber is one of the most used materials for cover board insulation today. It is most widely used as cover board in roofing systems.

Fibrous materials are inexpensive, but are not nearly as effective as our next category of rigid insulation, plastic foams. For example, it would take 7.5 inches of wood fiber board insulation to equal the R value of 3.3 inches of Polyisocyanurate plastic foam insulation, the most effective of the plastic foam type rigid insulations.

Plastic Foams

Plastic foams make up the second and largest category of rigid insulation. There are at least four main types of plastic foam rigid insulation and we will explain each here:

    Extruded polystyrene, abbreviated XEPS, solid polystyrene crystals, additives and a blowing agent are combined in an extruder where they are heated to a liquid and poured into a die. They come out of the die in the form of foam boards. It is a closed cell board that has both high and low densities. High density extruded polystyrene boards have an R value of 5.0 per inch whereas low density boards have an R value of 4.7 per inch. Both densities are designed for areas where space is limited and they can be subjected to high pressure, making them good for under the slab applications. They degrade when exposed to light and need a fire retardant surface when used on interiors.

    Expanded polystyrene, abbreviated EPS, is made with expanded polystyrene beads bonded together to form rigid boards. Expanded polystyrene comes in both high density and low density. High density expanded polystyrene boards can be used on exterior foundations with good soil and drainage conditions. High density board has an R value of 4.0 per inch and is more moisture resistant than low density board. Low density boards are for using above the grade and have an R value of 3.7 per inch.

    Polyurethane is a closed cell insulation material that contains a low-conductivity gas that increases the insulation’s R-value. Polyurethane rigid boards are often coated with foil or plastic facings and can have R-values of 7.1 to 8.7 per inch with the facings. The R value of the plain non-faced boards can deteriorate over several years as the gas leaks out slowly, but with a foil or plastic face, the deterioration is slowed significantly. Polyurethane is commonly used as an insulation board that also adds structural strength to walls and ceiling structures in the home.

    Polyisocyanurate as a type of insulation has the best r value per inch of material. This type of rigid board also contains a low-conductivity gas, like polyurethane, that gives it a higher R value. The R value is typically 5.6 per inch. It is commonly used as a roofing insulation. It is both fire resistant and moisture resistant.

That is not all of the types of rigid board insulations that exist, but it is a good overview of the most popular types used in home construction. There are about 10 other less popular types that are available on the market.


Now you should be armed with enough information on rigid insulation choices to help you decide what type to use and where to use it. Different types of rigid insulations are used throughout the home for many different reasons. If you have more questions about what type to use where, consult with your local home supply store and they would be happy to assist you. Just remember to consider space, R values and densities when picking insulation for an area in your home.