How to Buy Rigid Foam Insulation (With R-Value Chart)

If you’re insulating your garage or basement, you may have settled on using rigid foam insulation. But even if you’ve already narrowed it down, choosing which one to buy can be overwhelming.

Especially if you don’t know what to look for.  

In this article, we’ll cover the basic types of rigid foam insulation, and what kind of R-values you can expect from each. Then we’ll quickly compare them to another popular insulation choice, spray foam insulation, and when you’d use each.


What Are the Types of Rigid Foam Insulation?

When you look at insulation, we measure how effective it is by it’s R-value. The higher the R-value, the better it is at blocking heat transfer.

So it makes sense to go for the highest R-value you’ll save money in the long run.

However, an insulation’s R-value is heavily dependent on how it’s made. Not only that, but using the wrong type of insulation can increase the chances of mold, rot or other damage.

The three primary types of rigid foam insulation are: 

  • Expanded Polystyrene (EPS)
  • Extruded Polystyrene (XPS)
  • Polyisocyanurate (ISO)

Let’s talk about each in more detail.


Expanded Polystyrene (EPS)

EPS, or expanded polystyrene, is similar to what you may find in packing peanuts. It’s the least expensive type of solid insulation you’re likely to find.

Most types of EPS are unfaced, which makes them less durable. You’ll need to be careful when handling EPS foam insulation to avoid damage. 

R-value is low for EPS rigid foam insulation, measuring between R3.6 and R4.0 per inch. Because of this, it has limited use in most homes and garages. It’s best used in attics, and for insulating your garage door. 

Even though it doesn’t have a vapor barrier, like some of the other rigid foam insulation types, it is resistant to moisture. It’s permeable, allowing air to move through it, so moisture dries out quicker. Consequently, there’s less concern about mildew or mold. 

You will typically find expanded polystyrene materials in concrete forms or commercial buildings. It is suitable for ground cover, and you can treat it for insects. In addition, this material is lightweight and will not harbor bacteria, making it a terrific choice for some DIYers. 

Extruded Polystyrene (XPS)

Extruded polystyrene (XPS) rigid foam insulation is commonly used in larger construction projects. It’s typically green, blue, or pink in color and is available in varying thicknesses and profile edges. 

R-value of XPS foam insulation is between R4.5 to R5.0 per inch, making it slightly more efficient than EPS foam insulation.

Additionally, XPS insulation is available in both unfaced and faced options. This allows homeowners to choose whether they want a more breathable insulation, or whether they want to keep moisture out with a vapor barrier.

However, XPS isn’t as environmentally friendly as other options. That makes it less popular with green builders, mindful of their carbon footprint.

It is not ideal to use XPS rigid insulation below grade due to the chances of moisture permeability. If you choose extruded polystyrene insulation for your garage or basement, be sure to use proper wrapping and taping to block air and moisture. 

Finally, this material does lose some of its insulation properties over time, becoming less effective as time passes. 

Polyisocyanurate (ISO)

Polyisocyanurate (ISO) is a closed-cell, insulation normally found in industrial applications. It’s occasionally found in residential homes, even with the high price tag it carries.

The R-value of ISO materials ranges between R7.0 and R8.0 per inch, making it the highest rated insulation on this list. Additionally, it’s usually sold as faced–insulation. It’s reflective foil facing helps sealing and provides additional energy savings by acting as a radiant barrier in addition to insulation

Polyisocyanurate rigid foam insulation is prone to off-gassing over time. This makes it less effective over time, reducing its R-value. Consequently, ISO rigid insulation materials are not ideal for extreme cold weather applications.

Additionally, it can absorb water, so it’s not for ground contact coverage, below-grade applications, or in areas that see regular freezing temperatures. 

This is the most eco-friendly material of the three rigid foam insulation materials. It contains recycled materials and a less-toxic flame retardant than other insulation materials.

Unfortunately, that also means it’s the most expensive option of the three types.

Matador garage door insulation kit close up

R-Values of Rigid Foam Insulation [Per Inch]

As I mentioned earlier, an insulation’s R-value represents the resistance to the transfer of heat.

This number is usually based on a temperature of 75°F, so extreme temperatures one way or the other will alter the R-value.

Here’s a quick chart to compare the R-value of each insulation type. 

Type of InsulationR-Value
Polyurethane Spray Foam (Open Cell)R3.5 for each inch of thickness
Expanded Polystyrene (EPS)R3.6 to R4.0 for each inch of thickness
Extruded Polystyrene (XPS)R4.5 to R5.0 for each inch of thickness
Polyurethane Spray Foam (Closed Cell)R6.0 for each inch of thickness
Polyisocyanurate (ISO)R7.0 to R8.0 for each inch of thickness

Which R-value insulation your project requires will depend on where it is going and your climate zone. Rigid foam insulation can be stacked on top of each other, like most insulation, to increase the R-value.

For a more in-depth analysis of each rating, see below.


R3.5 

Polyurethane spray foam containing open cells provides an R-value of 3.5 per inch. This rating is low but can work in several spaces where you only need a base level of insulation. Open-cell spray foam insulation, as the name suggests, has open bubbles that can collect and retain moisture. 

Although low R-value material is not ideal in extreme temperatures, it will still provide some insulation properties. It can also help keep a home quieter by providing a sound barrier between walls or floors of two-story homes. 

R3.6 to R4.0

Expanded polystyrene insulation materials are very affordable and can help improve your home’s energy efficiency. These products are easy to use and durable, so even new DIYers can insulate their homes, garages, or other areas of your home.

Because it’s so inexpensive, homeowners often stack several layers to increase the R-value, without breaking the budget. 

R4.5 to R5.0 

Rigid foam with R-values between R4 and R5 is ideal for attics under roofing materials, underneath siding on a home, and over the foundation walls. These are usually mid-grade products, still making it affordable but providing more protection than the cheaper alternatives. 

This is usually closed cell insulation, and highly rigid and durable. Additionally, you’ll also be able to find several options both faced and unfaced. 

R6.0 

The R6.0-value spray insulation is a higher-end insulation product that protects against heat transfer and also adds a soundproofing layer. In addition, it has a high moisture barrier, so you’ll have less problems with water penetration causing mold or mildew build-up. 

Although this insulation is denser and weighs more than lower R-value options, it is still quite manageable and highly durable.

R7.0 to R8.0 

As the top end of the rigid foam insulation R-value chart, polyisocyanurate materials have an R-value of R7.0 to R8.0.

Although it’s more expensive, it provides the highest efficiency insulation for many situations. It also uses reflective foil facing to help minimize heat transfer.

Keep in mind, though, that ISO insulation isn’t recommended in sub-freezing temperatures and will lose effectiveness over time. 


Spray Foam vs Rigid Foam Insulation

If you’re trying to decide between using spray foam or rigid foam insulation, there are some big differences to be aware of.

Although any form of insulation will make your home more energy-efficient, knowing the pros and cons of each will help you make the right choice.

As a general rule: 

  • Use spray foam in hard-to-reach areas or places where you need air-tight seals
  • Use rigid foam insulation when you are on a budget and have ample workspace for installation
Spray foam insulation on a garage ceiling

Benefits of Spray Foam 

Closed-cell spray foam insulation expands to fill tight spaces and cracks that other materials cannot reach. This creates one of the best air-tight seals and help make the best energy-efficient home. 

Because it expands, it can minimize moisture penetration, which leads to mold and mildew in the home. It also helps add a sound barrier between walls, floors, and ceilings. 

Spray foam adheres to the beams and structure materials. This makes it more stable, and less prone to settling or moving as other insulation products do over time. 

It’s also much quicker to install, compared to other types of materials. This makes it a terrific choice for DIYers that don’t want to spend all weekend insulating. 

If you want the highest R-value rating of insulation, using a spray foam product is the best choice. This material has an extremely long lifespan compared to other insulation products in the market. 

Disadvantages of Spray Foam 

Spray foam insulation is almost always more expensive than rigid foam, so there’s a significant price difference when comparing these two products.

For example, a standard cost for spray foam insulation will be approximately $1.50 per square foot, whereas rigid foam is only $0.40 per square foot. 

In some locations with extreme temperature changes, spray foam can shrink and leave gaps that invite moisture build-up. These gaps can reduce its R-value rating, losing its effectiveness over time. 

The application process for spray foam can be complex if you are new to using the product. Areas can be overlooked and leave gaps where you want the foam to expand and seal the space. Consequently, DIYers will need special protective equipment to ensure a safe environment when installing spray foam products. 

The chemical compounds in spray foam must cure, taking approximately two hours. During this time, you’ll need to stay out of the area. This makes it more intrusive for a household than other insulation types. 

Spray foam insulation usually requires an outside contractor. You’ll end up paying more for labor costs with a spray foam application than rigid foam.


Benefits of Rigid Foam 

Rigid foam insulation is easy to install. An amateur DIYer can usually complete an insulation job in a few hours using rigid foam insulation. In addition, this product is easy to cut and shape for customization to any space. 

You can check out my install of the Matador EPS rigid-foam garage door insulation kit here.

On average, rigid foam insulation is less than 1/3 the cost of spray foam products. This makes it the ideal budget solution for homeowners. Even if you hire a contractor to install it, you’ll pay significantly less for labor costs than for spray foam. 

Remember too, it’s easy to layer multiple rigid foam panels to increase the R-value, without spending a lot of money. 

Rigid foam panels are suitable for various applications, making them a versatile insulation product for many homes and spaces. 

Disadvantages of Rigid Foam 

Naturally, there are some disadvantages to using rigid foam insulation products. This material has a low shear-strength. It can suffer cracks or damage if mishandled, altering its R-value and efficiency. 

Rigid foam panels are usually bulky and awkward, making them difficult to ship or transport. Homeowners will need adequate space to haul product home, or cut panels to fit in their vehicle. 

Although some rigid foam products have a moderately good R-value, they are not as high as spray foam insulation. Additionally, it requires you to have open wall space to install the material, making it more labor-intensive for some areas.