a man in a hardhat and yellow safety vest installing rigid foam board insulation in the wall.

When considering your insulation options, foam board is bound to come up. Besides the price of insulation, you’re also choosing the best type of insulation for your home based on its R-value. What is the R-value of foam board insulation?

Foam board insulation R-value varies based on type but is anywhere from 3.6 per inch of insulation thickness to 8.0. The types of foam board insulation include polyurethane, polyisocyanurate, extruded polystyrene foam, and expanded polystyrene foam.

In this article, we’ll first discuss the four variations of foam board insulation. Then we’ll jump into the R-value for each, providing this information in succinct chart form. By the time you’re done reading, you’ll know which type of foam board insulation is best for you!

The Types of Foam Board Insulation

Before we can delve into the respective R-values of foam board insulation, we have to talk about the types of foam board. Per the intro, there are four such types. Here’s a recap of each.

Polyurethane

Foam board constructed of polyurethane is known for its sound-insulating properties. This type of insulation can also maintain warmth for fewer temperature fluctuations in the room in which it’s installed.  

Polyurethane comes in several forms. The rigid foam sheets or boards are built to last and are a low-cost means of insulating your home. Spray foam made of polyurethane is a good choice for insulating tight corners where you cannot install rigid boards.

a picture of yellow foam board covered by a metal sheet on the outside
You can order rigid foam boards in many sizes and shapes, with many types of cover materials on it as well to fit your particular build or home.

Closed-cell polyurethane spray foam has a harder texture akin to rigid foam boards. This type of spray foam features sound-absorbing properties, and it’s water-resistant as well.

Since it’s more rigid, closed-cell spray foam insulation is stronger too. It has a higher moisture barrier and a great R-value (more on this to come in the next section).

Opposite that is open-cell polyurethane spray foam. This type of foam insulation is even more adept at sound absorption than the closed-cell variety. It’s low-density but not water-resistant. It’s neither rigid nor very strong, but open-cell spray foam has a low moisture barrier.

Polyisocyanurate

Although it sometimes gets lumped together with polyurethane insulation, polyisocyanurate is its own form of insulation. Unlike polyurethane, polyisocyanurate is only available in one form, rigid foam boards. The cells of the foam are closed, which enhances the strength and rigidity of the boards.

Polyisocyanurate boards are made of a combination of both inorganic and organic materials. The sides have facers bonded to them. The most common places to install polyisocyanurate rigid boards are ceilings, walls, and roofs.

Known for its great thermal properties and its low cost, polyisocyanurate also does well on fire tests. This form of insulation can handle temperatures over 250 degrees Fahrenheit and as low as -100 degrees without bending or warping. If you’re getting a roof system installed that uses hot asphalt, polyisocyanurate insulation can withstand the hot temps.

You can even apply products such as fertilizer, insecticide, or oil-based waterproofing around the insulation without any changes to its effectiveness.

Increasing the thickness of this insulation boosts its R-value in kind. That makes polyisocyanurate a more appealing insulation choice for many homeowners. However, polyisocyanurate is quite expensive compared to mineral wool but polyurethane insulation as well.

Extruded Polystyrene Foam

The next two types of foam board insulation are polystyrene foam, but one is extruded and the other is expanded. We’ll start by talking about extruded polystyrene foam. Comprised of additives and plastic resin that is fed into a die, the foam will expand while it’s cooling.

The resultant product is a form of insulation with closed cells, which increases the R-value of extruded polystyrene foam. Water nor air can easily penetrate. This insulation is also known for its strength due to its rigidity, so one of the most popular forms of extruded polystyrene foam is structural panels.

Expanded Polystyrene Foam

The last type of foam board is expanded polystyrene foam, which is very low-cost. For the cheap prices though, it’s not a very durable form of insulation.

If you’re familiar with packing peanuts, then you understand the texture and feel of expanded polystyrene foam. It’s closed-cell and rigid but lightweight. By increasing the insulation’s compressive strength, it can handle a higher load.

Expanded polystyrene foam won’t develop bacteria, a benefit that extruded polystyrene also has. The high thermal resistance of expanded polystyrene foam is beneficial, as is its stability.

A picture of a person in a white construction outfit installing rigid foam board insulation on a wood wall.

Foam Board Insulation R-Value by Type + Chart

Now that you’re more familiar with each type of foam board insulation, let’s go over the R-value for each.

Type of InsulationR-Value
Polyurethane3.5 for each inch of thickness for open-cell spray foam; 6.0 for each inch of thickness for closed-cell spray foam
Polyisocyanurate7.0 to 8.0 for each inch of thickness
Extruded polystyrene foam4.5 to 5.0 for each inch of thickness
Expanded polystyrene foam3.6 to 4.0 for each inch of thickness

Where Do You Buy Foam Board Insulation?

*While we’re not tied to one brand or another, this section does contain some affiliate links to help support the blog. The links are pointing to what we feel is the easiest way to purchase the foam boards.

While foam board insulation is pretty light in weight, it doesn’t make a ton of sense to have it shipped to you due to its bulkiness. This would really skyrocket any shipping costs.

Luckily, your local Home Depot or Lowes (these links take you to their foam board category pages) usually carries this in stock, so you can pick up whatever you need. What’s nice also is that they have different thicknesses of board, depending on what kind of R-value or material you want.

The sheets are usually in 4x8ft cutouts rectangle sizes (pretty big) – but bring or buy a basic utility knife with you to cut it up in the store if you need to fit small pieces in your car.

Here are some common in-stock (usually) foam boards that you can pick up for yourself. You can pick the thickness based on your specific application and where it’s being installed, and the R-value will differ slightly also –

ProductDetailsRough Cost
RMAXPro Select R-Matte Plus-3, 3/4 in. x 4 ft. x 8 ft. R-5.0 Foam Insulation Board3/4 inch thick R-5$17
FOAMULAR 150 1 in. x 4 ft. x 8 ft. R-5 Scored Square Edge Rigid Foam Board Insulation Sheathing1 inch thick R-5$25
R-Tech 2 in. x 4 ft. x 8 ft. R-7.7 Rigid Foam Insulation2 inch thick R-7.7$24
FOAMULAR 1/2 in. x 4 ft. x 8 ft. R-3 Square Edge Rigid Foam Board Insulation Sheathing1/2 inch thick R-3$19
R-Tech 1 in. x 4 ft. x 8 ft. R-3.85 Insulating Sheathing1 inch thick R-3.85$14

Foam Board Insulation vs. Other Types of Insulation: Which Has the Highest R-Value?

You’re uncertain if you’ll proceed with foam board insulation. Is it one of the most insulating materials you can select or do other types of insulation boast a higher R-value?

Rigid foam board insulation indeed has some of the best R-values of all insulation materials. Polyisocyanurate features an R-value up to 8.0, which other insulating materials cannot touch. Spray foam insulation, especially the closed-cell kind, has an R-value of 6.0 per inch, sometimes as high as 6.5.

Compare that to fiberglass insulation, including loose-fill, blankets, or batts. Although popular for insulating unfinished ceilings, walls, and floors, the R-value of fiberglass is between 3.1 and 4.3 per inch. That’s not a super low R-value, but it’s not particularly high either.

We’ve compared many other types of insulation as well if you’d like to check those articles out. Here you can see the comparison between Rockwool (mineral wool) and cellulose, and also spray foam vs. traditional fiberglass.

Cellulose insulation has an even lower-performing R-value. Loose-fill insulation, per inch, has an R-value of 3.2 to 3.9. Its low cost offsets its equally low R-value.

How to Choose the Right Type of Foam Board Insulation

You’re pleased with foam board insulation’s high R-value, and you think this could be the right choice for your home. With four types of foam board insulation to choose from, how do you select one? Here are some factors to keep in mind.

A man on scaffolding installing polyurethane insulation in a wall of a house
You can get all types, sizes, and shapes of rigid insulation foam board. You can see that it actually installs pretty easy, and could be a good DIY project to save some money.

Price

Is insulating your home one step of many to make it more eco-friendly? In that case, you might not have a huge budget to dedicate to insulation. For other homeowners, their budget might be more sizable.

However much money you have set aside will guide your choice in which type of foam board insulation you’ll use. Expanded polystyrene foam is by far the cheapest of those materials we discussed earlier, but it also has the lowest R-value.

If you need a higher R-value than what expanded polystyrene foam offers, you can always try extruded polystyrene foam. It’s only marginally more expensive.

Longevity

The time and expense of insulating your home shouldn’t be recurring anymore often than necessary. Closed-cell foam board has the greatest longevity and durability, which means you might consider closed-cell polyurethane spray foam or polyisocyanurate foam boards. Once again, extruded polystyrene foam could make a good materials choice as well.

Ease of Application

Crafty homeowners who want to insulate their own attics or other unfinished rooms in their home with foam board insulation can usually do so themselves. Unlike fiberglass, which can feel very abrasive against the skin, foam board isn’t nearly as injurious.

The easiest type of foam board insulation to use is undoubtedly polyurethane spray foam, especially the open-cell kind. The lack of rigidity means spraying this foam is quick and easy. Closed-cell polyurethane spray foam works in a very similar fashion.

Having to install foam boards, such as you do with extruded or expanded polystyrene and polyisocyanurate is more time-intensive and difficult.

Type of Application

Depending on what kind of insulation job you’re working on, certain types of foam board insulation are better than others.

For example, if you’re covering an unfinished wall or ceiling with insulation, then any rigid board foam will work. That gives you lots of options, everything from polyisocyanurate to expanded polystyrene for a truly low-cost project.

For tight corners, narrow passageways, and other areas of your attic that are strangely shaped, polyurethane spray foam works best. Open-cell spray foam can cover those areas with unique curves and shapes while rigid spray foam easily fills all those narrow passageways in your attic.

Conclusion

The R-value of foam board insulation varies on its type but can be as high as 8.0 in the case of polyisocyanurate. Outside of the foam board’s insulating qualities, you have to consider its price, ease of application, and durability as well.

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