Why Choose Rigid Foam Insulation?

The best insulation for your home

Insulation plays a vital role in maintaining a comfortable, energy-efficient home. Among the popular choices for home insulation are foam boards. But why choose rigid foam insulation over the other options?

Aside from blocking heat, moisture, and airflow, these energy-saving solutions offer many benefits. In this post, we’ll delve into the advantages of rigid foam insulation, exploring why it stands out as a great insulation option in some instances. We’ll also discuss the drawbacks and different types, providing you with all the information you need to make an informed decision. 

Why Choose Rigid Foam Insulation?

Rigid foam insulation, also known as foam board or sheathing, is a popular choice for insulators, all for good reasons. Aside from saving energy, they provide structural support, making them a good investment. Not to mention, they’re easy to install!

Let’s discuss the perks of rigid foam insulation in further detail!

Save Energy

Globally, heating, ventilation, and air conditioning systems (HVAC) account for more than a third of a buildings’ energy consumption. Despite that, domestic energy consumption in the UK is 26.8% lower than that in the early 2000s, all thanks to insulation!

Regardless of its type and thickness, rigid foam insulation is energy-efficient. It can help homeowners lower their energy consumption. But how?

That’s because these boards undergo polymeric reactions. They’re made from small chemicals that cross-link together, forming a larger compound. This process produces a closed-cell structure.

As a result, it prevents heat conduction and airflow, reducing the load on your HVAC systems. So they don’t need much energy to reach the desired heating or cooling.

Control Moisture

As mentioned earlier, rigid foam sheathing has a compact structure, making it semi-impermeable. Since the boards contain little to no pores, moisture doesn’t pass through them and reach the walls.

Additionally, manufacturers combine water repellants into the mix to prevent dampness. Still, this insulation isn’t 100% waterproof.

While it can prevent moisture penetration and doesn’t absorb water, some rigid foam insulation, like EPS, can get wet. However, the process isn’t quick—the boards only absorb around 5% of their weight after being submerged in water for one year.

In practice, no house is completely submerged in water for that long. The problem is that excessive moisture exposure can damage the foam boards in the long run and cause mildew.

Enhance Thermal Resistance Rating

Building codes require home materials to meet specific U-value ranges. For those wondering, the former is a measurement of heat loss or gain through various building structures, such as floors, walls, and windows. Lower values mean better insulation.

For instance, floors should have a U-value of  0.13 W/(m²·K), but what does that have to do with rigid foam insulation?

You see, these boards have a mid-to-high R-value, between 3.6 and 8, depending on the material. R-values measure heat flow resistance. The higher the value, the better the insulation properties.

Additionally, as the R-value increases, the U-value decreases. So, you can use these boards to enhance thermal resistance and, thus, prevent energy loss.

Provide Structural Stability

Aside from its energy-saving properties, rigid foam sheathing can also provide some structural strength.

Sure, the boards aren’t as heavy or sturdy as other building materials, like plywood. Still, the foam is essential for structural integrity, thanks to its insulation properties.

As you might know, most contractors install rigid foam boards on the interior side of external walls right before studs.

That layer provides air and moisture sealing. As a result, it protects the wood and other building materials from rot, among other damaging environmental conditions—all of which support structural stability.

Plus, that added thickness can provide sound-proofing benefits, reducing noise transmission and creating a quieter indoor environment!

Long-Term Savings

Although the initial cost of rigid foam insulation may be higher than other options, the boards can help you save money in the long term. For starters, insulation can help you cut up to 35% or more off your energy bills.

Additionally, rigid foam sheathing is durable, resisting water and building chemicals like adhesives. If kept under proper conditions, the foam can last for 50 years or more! So you don’t have to pay for replacements or repairs, which can be costly.

Ease of Installation and Versatility

If you’re an avid DIYer, you may be pleased to hear that rigid foam boards are easy to install. Unlike other insulation, such as spray foam, you don’t need any fancy tools to attach those boards to the walls—a simple insulation adhesive or masonry screws will get the job done.

However, cutting those boards so that they fit together requires some technical skills. That’s especially true for tough spots, like windows or around pipes. You may also need to apply tape or expanding foam to seal any small gaps.

Aside from ease of installation, rigid foam insulation is versatile. Whether it’s above or below grade, these boards thrive in many environments! So, you can use them in basements, roofs, attics, and even the foundation!

What Are the Disadvantages of Rigid Foam Insulation?

Although rigid foam insulation offers many benefits, it still has some drawbacks, including:


Rigid foam insulation can cost between £20 and £50 per square meter. It can even be costlier depending on several factors, including material, house area, and project size.

For instance, expanded polystyrene (EPS) foam boards are more affordable than the other rigid foam types. Likewise, roof or loft insulation’s total cost will generally be less than external wall insulation projects.


As the name implies, rigid foam sheathings are sturdy. That can be a double-edged sword. On the one hand, it prevents air and moisture infiltration as well as provides structural support.

On the other hand, such an inflexible structure makes it harder to attach to nooks and crannies.

Not to mention, the foam is solid; it won’t seal cracks. Unlike expandable foam, you need to fix the former issue before installing the boards. Otherwise, the insulation will lose its energy efficiency.


Plastics remain one of the most non-environment-friendly materials, and rigid foam insulation is no different! These compounds don’t have any nutrients. So mould, bacteria, and other microorganisms can’t biodegrade them.

If disposed of improperly, rigid foam boards can leach toxic chemicals into the soil and nearby water sources.

Additionally, foam plays a role in the global climate crisis. During manufacture, blowing agents, known as chlorofluorocarbons (CFCs), are used to create the bubble structure, which provides insulating properties.

The problem is that these chemicals deplete the ozone layer, allowing harmful UV radiation to enter the earth. That negatively impacts plants, animals, and humans, causing several health and environmental problems.

UV Sensitivity

Although rigid foam insulation is durable, it’s not immune to UV exposure. This radiation breaks down the polymer’s bonds, causing degradation. Some of the visible damage includes yellowing and dusty residue. If left unprotected, the board will lose its thickness.

Of course, that causes several issues. For starters, the boards will lose their thermal resistance, allowing heat to flow. Water can also seep into your home, increasing the risk of mould growth.


Unless the label says it’s fire-resistant, most plastics are flammable. Sure, manufacturers add fire retardants to most insulation. However, that only slows down flare-ups; it won’t make them fire-proof.

Under significant heating, rigid foam boards can ignite and burn rapidly, producing intense flames. Not only that, they emit toxic gases like carbon monoxide.

Inhaling these fumes can cause carbon monoxide poisoning, breathing difficulties, nausea, and drowsiness, among other symptoms. Plus, hazardous fumes and gases pollute the air.

Rigid Foam Insulation Types

Now that you know the pros and cons of rigid foam insulation, let’s discuss its types. The former comes in three materials: expanded polystyrene, extruded polystyrene, and polyisocyanurate.

Here’s a detailed explanation of each option:

Expanded polystyrene (EPS)

Expanded polystyrene (EPS) is a rigid, closed-cell plastic that can be melted and recast multiple times. As the name implies, polystyrene is the main component of this foam. The process involves heating small polystyrene beads and using pentane gas for expansion.

That air-filled structure provides great thermal insulation, reaching an R-value of 3.6-4.6. Compared to other rigid foam boards, EPS is the most versatile, as it’s suitable for roofs, walls, foundations, and other geofoam applications. Plus, it’s affordable!

Extruded polystyrene (XPS)

Like EPS, extruded polystyrene (XPS) is also made from polystyrene and has a closed-cell structure. However, instead of using expanding gas, the plastic is melted, pushed through a small opening, and pressed into sheets.

XPS is stronger and has better mechanical properties than EPS. Additionally, it has an R-value of 5, but it can drop to 4.6 over time. This foam costs more than EPS and is mostly used in walls and below grade.

Polyisocyanurate (Polyiso, ISO)

Out of the three rigid foam options, polyisocyanurate is the most expensive, thanks to its impressive insulation properties. It has an R-value of 5.8-6.8 and can maintain such performance for a long time due to its durability.

However, polyiso is typically used in roofing applications, making it less versatile than the other two options.

Rigid foam insulation conclusions

When it comes to insulation choices, there’s a strong case for why to choose rigid foam insulation. These sturdy boards prevent moisture and air infiltration, saving energy. 

Additionally, they can add structural support. Despite some downsides, like cost and inflexibility, its durability and long-term savings outweigh the cons.

With options like EPS, XPS, and polyisocyanurate, you can find the ideal match for your requirements, creating a comfortable, energy-efficient property.!

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