Rigid Foam Board or Fiberglass Insulation: Which to Choose?

Fiberglass and rigid foam board are two of the most popular home insulation options currently available. 

They are both readily available at most home improvement stores across the US. Many homeowners apply both fiberglass and rigid foam board insulation in different parts of their residence. 

Made from polymers like polyurethane and polystyrene, rigid foam boards are denser, more expensive, and offer better resistance to heat flow than fiberglass. Conversely, fiberglass insulation is cheaper and more flexible, as it is made from raw materials like limestone, silica sand, and recycled glass.  

There can also be quite a bit of overlap between these two insulation materials since they can both be used to insulate interior walls, roofs, floors, and other parts of the house. 

To help you decide which of these insulating materials is suitable for your home, we will discuss and compare the features of both materials at length. By the end of this article, you will know all there is to know about fiberglass and rigid foam board insulation and the significant differences between the two.  

What is Rigid Foam Board Insulation?

Spray foam and rigid foam board insulation are the two most popular types of foam insulation. This article will focus mainly on the latter. (To learn more about the former, click here.)

Rigid foam board is made from polyurethane, polystyrene, polyisocyanurate, and other polymers. It is available in sheets or panels that can be installed in an existing wall or one still under construction. 

However, if you are re-insulating your home, you will have to remove the drywall before installing the foam board. This type of insulation is best for homes with a conventional, rectilinear design. 

Rigid foam boards can be customized to meet your specific insulation needs. They are available in various shapes, sizes, facings, and thicknesses. You can also cut your rigid foam board panel into any shape or size suitable for your needs. 

Types of Rigid Foam Board Insulation

Depending on how a rigid foam panel was manufactured, it can have slightly different properties. The various manufacturing processes have given rise to three different types of rigid foam board insulation, which are:

Extruded Polystyrene (XPS)

This type of foam board insulation is manufactured through a continuous extrusion process. As a result, a homogenous, closed-cell cross-section is produced, which is highly resistant to moisture, humidity, and heat transfer. 

XPS foam boards are also more durable and maintain their R-values quite well, even during winter’s lower temperature. The moisture-resistant properties of XPS foam boards also make them suitable for kitchen and basement insulation.

XPS foam boards typically have an R-value of about 5 per inch of thickness. They are easily recognizable by their distinctive green, blue, and pink-colored panels. 

Polyisocyanurate (Polyiso) Foam Board

Polyisocyanurate or polyiso foam board insulation is a rigid, closed-cell structure consisting of two face sheets that sandwich a foam core between them.

The manufacturing process begins with liquid foam sprayed against a substrate layer to form a rigid panel. 

The face sheets can be made from paper, foil, and any other organic or inorganic material. These elements come together to form rigid insulation boards that are widely used in residential buildings, particularly for flat roofing applications. 

Polyiso insulation boards cannot be recycled and typically have an R-value of around 5.8 to 6.0 per inch. 

Expanded Polystyrene (EPS)

EPS is the most popular form of rigid foam board insulation and typically offers R-values of around 4.0 to 4.5 per inch of thickness.  

This type of rigid foam board is manufactured by expanding spherical beads in a mold. Then, pressure and heat are applied to fuse the beads together.

EPS foam boards are frequently used to insulate walls, roofs, floors, and attics in residential buildings. They are an exterior insulating sheathing material and can be used both above and below grade. 

Expanded polystyrene is the most cost-effective type of foam board insulation, and it exceeds most energy and building codes. It also does not degrade or settle over time. Depending on your insulation needs, you can purchase EPS foam board panels of varying densities and thicknesses.  

Characteristics of Foam Board Insulation

All types of rigid foam board insulation have several common characteristics or features. These characteristics are:

• Excellent heat-resistant properties. In fact, this is the insulating material with some of the highest R-values currently available. 

• Typically, rigid foam board insulation has an R-value between 4.0 and 6.5 per inch of thickness. 

• Usually, rigid foam board insulation is sold in sheets about 4 feet by 8 feet in size. The thickness of these insulation sheets might vary between 1/4^th of an inch to 2 inches. 

• Rigid foam board insulation is known for being durable and dimensionally stable. It does not expand or contract when subjected to extreme temperatures, moisture, or other external influences. 

• A rigid foam board panel can be custom cut (with a masonry blade or serrated knife) to fit the area in which it will be installed and can be used to insulate rim joists, crawl spaces, exposed basement walls, roof decks, and attic floors. 

• Rigid foam board panels can be fixed in place with the help of screws, silicone caulk, and adhesives. The installation process is pretty straightforward and can be completed within a single DIY weekend. 

• Rigid foam boards are susceptible to sunlight and can be damaged by exposure to ultraviolet radiation. They are also ineffective at sealing gaps and holes in the building envelope. 

A Brief Overview of Fiberglass Insulation 

Fiberglass insulation typically comes in tightly packed rolls of extremely fine glass fibers. It is one of the most common materials used to insulate American homes. 

Some of the raw materials used to manufacture fiberglass insulation are: 

• Limestone
• Silica sand
• Soda ash
• Feldspar
• Calcinated alumina
• Kaolin clay
• Borax
• Magnesite

During the manufacturing process of fiberglass, these ingredients are mixed together and melted at a temperature of more than 2500ºF. The molten raw materials then flow out of the furnace and into a perforated metal container that is cylindrical and is constantly spinning. 

As the perforated container spins, the molten glass flows out of the small holes in its body. When flowing out of the container, the molten glass takes the form of thin, threadlike filaments. 

A strong blast of cold air is then used to separate each filament of molten glass into several short strands. These strands then fall into a conveyor belt, where they are interlaced together to form the fleecy batts of fiberglass insulation available at home improvement stores. 

This complicated manufacturing process gives fiberglass insulation the ability to resist heat flow. The fleecy glass fibers can effectively trap air pockets, helping slow the movement of heat through the material. 

Hence, if properly installed on the walls and floors of your home, fiberglass insulation can help lower your energy bills by preventing the leakage of conditioned air from the building.  

Types of Fiberglass Insulation

There are two main types of fiberglass insulation that typically have distinct applications and offer their own advantages and drawbacks. They are:

Loose-Fill Fiberglass

Loose-fill fiberglass insulation consists of tiny particles of glass fiber that can be blown into attics and finished wall cavities. A mechanical blower must be used to install loose-fill fiberglass insulation. 

Loose-fill fiberglass is popular with architects and building contractors because it is non-combustible, fire-resistant, and inorganic. Being inorganic also means that this insulation material is resistant to the growth of mold, mildew, and fungus. 

Increasing the R-value of loose-fill fiberglass insulation is pretty easy since you only need to increase the number of fiberglass particles being blown into a given space. In addition, loose-fill fiberglass does not absorb moisture and is not prone to rotting, settling, or deteriorating over time. 

Fiberglass Batts

Fiberglass batts or rolls have long been used to insulate residential and commercial spaces. These batts are usually between three to six inches in thickness. They can be cut into various shapes and sizes and installed through a relatively simple DIY process. 

Fiberglass batts are produced by placing layers of molten glass filaments on top of each other on a conveyor belt until the desired thickness is achieved. The thicker the batt, the higher its R-value. Typically, fiberglass batts have R-values between 2.9 to 3.8 per inch of thickness. 

Differences Between Rigid Foam Board and Fiberglass Insulation

Fiberglass and rigid foam board insulation are pretty different in terms of their structure and properties.

Application 

Fiberglass is one of the most common types of insulation for homes due to its versatility and effectiveness.

Fiberglass batts are commonly applied in attics and exterior walls, as they are good at preventing heat loss through the ceiling and wall panels. Loose-fill fiberglass is generally used to insulate curved or irregular architectural elements. 

Rigid foam boards are more moisture-resistant than fiberglass. Therefore, they are frequently used to insulate basement walls, kitchen areas, and other parts of homes prone to humidity. However, rigid foam boards are not flexible, so they are not well-suited to insulate irregular surfaces such as domes or arches.  

Cost

Fiberglass is more affordable than rigid foam boards. When installing fiberglass batts in your home, you will usually not have to pay any more than $0.88 to $1.64 per square foot of insulation, including labor. The exact price will depend on the R-value of the insulation and the type of fiberglass used.

Rigid foam boards are much more expensive since they cost about $2 to $2.75 per square foot of insulation, including labor costs. Therefore, you may have to pay almost double the price to insulate the same space with rigid foam boards instead of fiberglass batts.

While loose-fill fiberglass is slightly more expensive, it is still more affordable than rigid foam insulation.

R-Value

Rigid foam boards undoubtedly have better thermal resistance capabilities than fiberglass. Consequently, rigid foam board insulation will usually have a much higher R-value. 

Loose-fill fiberglass provides an R-value of about 2.2 to 2.9 per inch of thickness, whereas fiberglass batts have R-values of about 2.9 to 3.8 per inch. On the other hand, rigid foam board insulation boasts R-values ranging from 4.0 to 6.0. 

However, the R-value of loose-fill fiberglass can be easily increased by simply blowing more of the loose fibers into a given space. Rigid foam board panels, on the other hand, have a fixed R-value that cannot be easily changed. 

Installation Process

Rigid foam insulation is – as the name suggests – rigid and unbending, while fiberglass is quite flexible. As a result, foam boards can be easily cut or sliced into any required size or shape. However, they cannot be bent to fit around wires, electrical outlets, vents, and other obstructions.

On the other hand, fiberglass batts are very flexible and easily fit around vents, wires, and electrical outlets. However, loose-fill fiberglass cannot be installed without the help of professional insulation technicians. You will also need more safety gear – such as gloves, masks, and goggles – when installing fiberglass insulation.  

You can see the major differences between the two types of insulation outlined below. 

Criteria	Rigid Foam Board	Fiberglass
Form	It is typically available in the form of rigid panels.	Available in loose particles or solid batts.
R-value	Higher R-value, usually between 4.0 to 6.5 per inch.	Relatively low R-value, between 2.2 and 3.8 per inch.
Cost	It is one of the most expensive types of insulation.	Fiberglass insulation is highly cost-effective.
Raw Materials	Made from polymers like polyurethane and polystyrene.	Made from raw materials like recycled glass, silica sand, and limestone.
Application	Used to insulate basement walls and rim joists.	Used to insulate exterior walls, floors, and attics.

Conclusion

These are some of the factors you must consider when deciding whether to install fiberglass or rigid foam insulation in your living space. In addition, you might want to consult a trusted architect, builder, or insulation technician, to get a better idea of which type would be best suited to your home.