What Is Rigid Foam Board Insulation (Complete Guide)

Rigid foam insulation may be a commonly used insulation material in the US, but many people still do not know what it is made of and where to use it for the best insulation results. 

Rigid foam or foam board is a type of continuous insulation most commonly applied to the building’s exterior wall, roof, and foundation. Rigid foam has a closed-cell structure that does not allow air to pass through it. It means that once it’s applied, air cannot seep into cracks or gaps between insulation and the home’s exterior wall making rigid foam a perfect insulation solution for homes.

Continue reading if you want to learn the different types of rigid foam insulation and the answers to the most commonly asked question regarding this insulation. I’ll also look at why you might want to apply the rigid board insulation in your home and how you can use it to decrease your energy costs throughout the year.

What is rigid foam?

When most people think of insulation, they picture fiberglass batts and rolls—and that’s perfectly okay. However, we often forget to consider all our options. There are many different types of insulation available to us. One type that may be unfamiliar to you is rigid foam. 

a room full of rigid foam insulation

This insulation is made up of rigid plastic foam. Generally, it comes in 4-by-8-foot sheets that are ¼ to 2 inches thick. It is lightweight, easy to install, and highly effective compared to other forms of insulation. In short, rigid foam is a perfect alternative to commonly used fiberglass batt insulation.

What is rigid foam insulation made from?

Typically, rigid foam insulation is made of three different types of plastic with different R-values. These slightly different materials give us three different types of rigid foams. 


Polyisocyanurate, or Polyiso (PIR), is a foam used as insulation. Polyiso boasts a high R-value of 6 per inch. The exact amount of polyiso will keep your home or office cooler in summer and warmer in winter than fiberglass insulation. Mainly, PIR consists of an aluminum foil facing surrounding rigid foam, so it is a popular choice for use in roofing or attic spaces because of its reflective efficiency. Polyiso also lasts longer than fiberglass, so you may need to replace it less often.

Expanded polystyrene

You may already be familiar with this type of rigid foam without even knowing!

Expanded polystyrene (EPS) is used chiefly in packaging to prevent the contents from breaking. It’s also used extensively in food and drink containers and coffee cups. It has an R-value of 4 per inch, making it less efficient in thermal insulation than polysio. Nonetheless, it is the cheapest rigid foam insulation available. 

Extruded polystyrene

Extruded polystyrene or XPS comes in blue or pink rigid board form and has an R-value of 5 per inch, making it denser than expanded polystyrene. It has better insulation powers than expanded polystyrene. More importantly, XPS comes with straight edges or tongue-and-groove edges to further minimize air penetration.

What are the benefits of using rigid foam insulation?

Fiberglass is the most common type of home insulation in North America, but rigid foam insulation is gaining popularity as an alternative to traditional insulation solutions. Although it’s not necessarily better than fiberglass, it does have some advantages that can make it worth considering as a replacement to other insulation types. Here are three reasons why you might want to consider using rigid foam insulation in your home:

  1. More energy efficiency by preventing thermal bridging

Rigid foam insulations prevent thermal bridging or, in other words, loss of heat through more conductive materials that come in contact with insulation. It is a massive problem with many homes today. While insulating your home’s interior will ensure you stay warm in winter and cool in summer, those wall studs are just letting heat escape right through them. If you’re currently using fiberglass insulation, consider switching to rigid foam insulation for optimal thermal protection.

Better insulation means less heat escapes through walls and ceilings, which means lower utility bills and a smaller carbon footprint. Using rigid foam insulation is an excellent option because it adds protection without compromising airflow or strength.

  1. Better moisture control because of air barrier qualities

Rigid foam insulation’s air barrier qualities prevent moisture from getting in and damaging your building’s structure. And you can use it for both above-grade and below-grade applications, which gives it an advantage over other common types of insulation.

Not only does rigid foam keep cold drafts from seeping into your home, but it also stops any that already exist. Like fiberglass and cellulose, traditional insulation tends to absorb moisture from the surrounding air. So, if you’ve ever seen a wet wall in a poorly insulated home, you know what I mean. Since the rigid foam is fully sealed, moisture can’t get in or out. It means you stay warmer and drier no matter what time of year it is outside.

  1. Fire safety by added flame retardants

One of the biggest problems with traditional fiberglass insulation is its tendency to burn. On the other hand, manufacturers of rigid foam for home insulations have come up with fire-resistant foam boards. The addition of flame retardants to rigid foam insulation makes it less likely to spread any fires.

When to use rigid foam insulation?

There are various times when you might want to consider using rigid foam insulation. The main benefit of using rigid foam insulation is that it is easier to work with than other types of insulation. So, rigid foam insulation can be applied wherever thermal protection is needed. In commercial and residential constructions, it’s used to protect against heat loss. 

Rigid foam insulation can also be used as an electrical insulator behind electric wires because of its resistance to fire and thin size. Be sure to ask your local building supply store which brand has passed flame-resistance tests so you know what type of material will best meet your needs.


What is the difference between pink and blue board foam?

There’s not a lot of difference between pink foam and blue foam rigid insulation. They have similar insulation efficiency. The only significant difference is that they are made by different companies and are available with specific brand names.

Is rigid foam insulation waterproof?

Rigid foam insulation can be highly water-resistant, and that’s good enough for most applications. When installed properly, water cannot get through rigid foam insulation. If you plan to build a home in a flood-prone area, make sure to use higher-density rigid foam boards for below-grade installation.

Is rigid foam insulation good for walls?

You can use rigid foam insulation anywhere in your house, but it is mainly a better choice for basement walls. Basement walls are more moisture-prone, so fiberglass must not be used on basement walls because of its moisture-retaining properties. Although the rigid foam doesn’t fit snuggly into space between studs because of its rigidity, it is the best insulating solution for a vapor-heavy basement wall.

Unlike fiberglass, rigid foam insulation does not absorb water and has a smooth surface that prevents mold build-up. Therefore, it is better to use rigid foam insulation between the studs. In addition, low-expanding spray foam can be used as a sealant to seal off the gaps between studs and rigid foam panels for a perfect insulation barrier.

Can you put fiberglass insulation over rigid foam?

It is not a good idea if you want to put fiberglass over rigid foam insulation on the basement walls. I don’t recommend using fiberglass in the basement at all. In any other house area, you can put fiberglass batt over rigid board and then cover it with drywall. However, for basement walls, closed-cell foam spray is the better option to use with rigid foam boards.

Can rigid foam insulation be left exposed?

The simple answer is, No!

The building codes require covering all rigid foam insulations with a 15-minute thermal barrier. It means that all rigid foams should be covered with a material that doesn’t let the temperature increase more than 120 degrees Celsius after 15 minutes of exposure to fire. In other words, putting these barriers over rigid foams improves safety and prevents the spread of fire.

How long does rigid foam insulation last?

Although rigid foam insulation will easily last for over 100 years, it may need to be replaced if a hole is drilled in your home’s foundation or wall during construction. Holes can compromise your home’s insulation; you could also experience damage from rodents or other pests entering your home through a small hole.

Also, if left uncovered for more than sixty days in the open air, rigid foam deteriorates and loses its insulating properties.

Wrap up

If you’re looking for a way to make your home more energy-efficient, then rigid foam insulation may be proper for you. There are excellent reasons why rigid foam insulation is worth considering as part of your overall energy efficiency plan. Rigid foam insulation prevents heat loss, has a long life, and the additional fire retardants reduce the chances of a fire hazard. By adding this insulation on basement walls with more chances of moisture seeping through, your home will become much safer from water damage.

Charles John

Experienced HVAC technician with 8 years of experience in the industry. Capable of handling all sorts of heating and cooling equipment as well as proficient in operational management, construction-related techniques such as preventative maintenance, electrical troubleshooting and AutoCAD

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