Rigid Foam vs. Fiberglass Insulation Comparison Guide

When it comes to insulating your home, two of the most popular options are rigid foam insulation and fiberglass insulation. Both have their advantages and disadvantages, so it’s important to understand the key differences between these two insulation types before deciding which is best for your project. This in-depth comparison guide examines the pros and cons of rigid foam vs. fiberglass insulation to help you make an informed decision.

How Rigid Foam and Fiberglass Insulation Work

Rigid foam insulation and fiberglass insulation work in slightly different ways to resist heat flow and keep your home comfortable.

How Rigid Foam Insulation Works

Rigid foam insulation is made from polyurethane, polyisocyanurate, or extruded polystyrene foams blown with hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs). The solid foam contains millions of small cells filled with these gas bubbles. When the small cells trap the gas bubbles, they don’t allow heat to flow through easily, creating an effective insulating barrier.

The closed-cell structure of rigid foam makes it highly resistant to heat flow. It has a high R-value, meaning it offers great insulating power in a relatively thin layer. Rigid foam is also effective at stopping airflow and preventing condensation in wall assemblies when used as a continuous layer.

How Fiberglass Insulation Works

Fiberglass insulation is made from extremely fine fibers of glass which are spun into a texture similar to wool. The fibers criss-cross and overlap to form a material that traps air in small pockets. These trapped air pockets resist heat transfer through the insulation.

Fiberglass insulation relies on the air trapped within the fiber matrix to act as the main insulator. However, the open structure of the material also allows some heat loss through conduction and convection. Fiberglass is not as effective as rigid foam at stopping airflow and moisture movement.

Comparing R-Values: Rigid Foam vs. Fiberglass

A key difference between these two insulation materials is their R-value, or resistance to conductive heat flow. The higher the R-value, the better the insulating performance.

Rigid foam insulation typically has an R-value of R-5 to R-6.5 per inch of thickness. This means just 1 to 1.5 inches of rigid foam provides the same R-value as 3.5 to 5.5 inches of fiberglass.

Fiberglass batt insulation usually provides an R-value of approximately R-3.0 to R-4.0 per inch. To achieve the same R-value, you need about 2 to 3 times the thickness of fiberglass as rigid foam.

Overall, rigid foam offers 2 to 3 times more insulating power inch-for-inch compared to fiberglass batts. This makes it advantageous in applications where space is limited.

Cost Comparison: Rigid Foam vs. Fiberglass Insulation

When comparing insulation materials, it’s also important to consider the costs. Let’s look at how rigid foam and fiberglass insulation compare in terms of price.

Rigid foam insulation costs $0.70-1.30 per square foot on average. This is around 2-3 times more expensive than fiberglass batt insulation, which costs approximately $0.25-0.50 per square foot.

However, since rigid foam provides a higher R-value per inch, you may be able to use a thinner layer and still achieve your target R-value. When you factor in the labor savings from installing thinner insulation, the overall costs for rigid foam may be lower in some applications.

Over the long run, the better insulating performance of rigid foam can also lead to greater energy savings, further offsetting the higher upfront cost of the material.

Moisture Resistance: Rigid Foam vs. Fiberglass

An area where rigid foam clearly outperforms fiberglass is resistance to moisture and water vapor.

Rigid foam has a closed-cell structure that resists bulk water intrusion and effectively blocks moisture vapor. Adding a continuous layer of rigid foam to wall assemblies provides an air barrier and vapor retarder that protects against condensation and moisture damage.

Fiberglass insulation is vapor-permeable, meaning water vapor can readily pass through. In cold climates, this allows moisture to reach the colder areas of wall assemblies, leading to condensation issues. Fiberglass alone does not provide an adequate air or vapor barrier.

If you live in a humid climate or want to minimize moisture risks, rigid foam is generally a better insulation option than fiberglass alone. Rigid foam delivers added protection against rot, mold growth, and structural issues.

Fire Safety: Rigid Foam vs. Fiberglass

When comparing insulation materials, fire safety is another important consideration. Fiberglass and rigid foam insulation have notably different fire safety characteristics.

Fiberglass insulation is inherently fire-resistant and does not easily ignite. It’s considered a non-combustible material. During a fire, fiberglass insulation can help slow the spread of flames.

Most rigid foam insulation products contain hydrofluorocarbons as blowing agents. These gases are flammable and can enable the foam to burn rapidly when ignited. Special flame-retardant additives are added to improve fire performance.

To address fire concerns, building codes often require rigid foam insulation to be covered by a thermal barrier, such as gypsum wallboard. Adhering to fire safety codes helps ensure rigid foam is used safely.

Overall, fiberglass offers better fire resistance, while rigid foam requires protective measures to achieve adequate fire safety.

Soundproofing Ability: Rigid Foam vs. Fiberglass

In addition to insulating against heat transfer, both rigid foam and fiberglass offer sound dampening properties that can help reduce noise transmission through walls and ceilings.

Rigid foam insulation provides moderate soundproofing ability on its own. The closed-cell structure doesn’t easily allow sound waves to pass through. Layering rigid foam under drywall can help block exterior noise like traffic.

Fiberglass insulation contains millions of tiny air pockets that effectively absorb and dissipate sound waves. This gives fiberglass excellent noise dampening qualities. Fiberglass insulation is commonly used as soundproofing material in walls between living spaces.

For maximum soundproofing, the best solution is to use both insulation types – rigid foam on the exterior to block sound, coupled with fiberglass in interior wall cavities to absorb interior noise. The layered approach creates an effective sound buffer.

Ease of Installation: Rigid Foam vs. Fiberglass

When it comes to installation, rigid foam and fiberglass insulation have very different handling considerations.

Rigid foam insulation boards are lightweight and easy to cut with simple tools like a utility knife or table saw. Installation is straightforward, with foam boards glued or fastened to wall studs or roof rafters. Many homeowners can install rigid foam insulation themselves.

Fiberglass batt installation requires fitting the flexible batts into wall cavities or joist spaces. The insulation tends to be bulky and less wieldy. Loose fiberglass can be blown into attics or other hard to reach spaces. Protective clothing and masks are recommended when handling fiberglass to avoid skin and respiratory irritation.

Overall, rigid foam offers easier installation for DIY-ers. Fiberglass batt installation has more specialized handling considerations and may benefit from professional installation in some cases.

Durability and Longevity

When properly installed, both rigid foam and fiberglass insulation solutions can last for decades, offering long-term thermal protection and energy savings.

Rigid foam’s closed-cell structure resists moisture damage, retaining its insulating performance over time. Properly taped seams also prevent air leakage. Undamaged rigid foam does not degrade or lose R-value.

Fiberglass insulation relies heavily on the intact matrix of glass fibers to retain its insulating properties. Compression due to improper installation or excess moisture can reduce effectiveness over time. Periodic inspection and minor repairs may be needed.

Overall, rigid foam offers more durable long-term thermal performance. Fiberglass requires proper installation and maintenance of fiber matrix to achieve optimal product lifespan.

Environmental Impact

Today’s homeowners are increasingly considering environmental impact when selecting insulation. Here’s how rigid foam and fiberglass compare:

Rigid foam production and use does release greenhouse gases associated with climate change, including blowing agents and carbon dioxide from manufacturing. However, energy savings during use offsets some of these impacts. Rigid foam also contains recycled material in many cases.

Fiberglass contains high recycled glass content, giving it an advantage on embodied energy. Off-gassing of formaldehyde during curing does have some environmental and health impacts. Fiberglass is more likely to end up in landfills during remodeling.

Overall, fiberglass insulation may have a slight edge currently on sustainability. Manufacturers are working to develop more eco-friendly rigid foam products with reduced impacts.

Choosing the Right Insulation Type

When comparing rigid foam vs. fiberglass insulation, there are clear trade-offs to consider for your specific project requirements.

Here are some key factors to help determine which insulation type is right for your home:

  • Climate – In hot or mixed climates, focus on maximum R-value with rigid foam. In cooler climates, moisture control and air sealing become more critical.
  • Space constraints – If you need to maximize R-value in a tight space, rigid foam is the better solution.
  • Budget – Rigid foam has a higher upfront cost, but offers long-term energy savings.
  • Noise control – Use both insulation types for maximum sound dampening ability.
  • DIY installation – Rigid foam is generally easier to install without professional help.
  • Fire safety – Fiberglass does not require additional fireproofing like rigid foam.
  • Moisture management – Rigid foam provides superior vapor and water resistance.
  • Sustainability – Fiberglass edges out rigid foam on recycled content currently.

Carefully weighing these factors against your own needs helps determine when to choose rigid foam or fiberglass insulation for optimal home comfort and energy efficiency.

Rigid Foam vs. Fiberglass Insulation: Which is Better for You?

Deciding between rigid foam and fiberglass insulation ultimately depends on your climate, project requirements, and personal priorities.

Rigid foam insulation offers superior insulating value per inch, moisture resistance, and durability. It excels when space is limited or maximum energy efficiency is needed. Rigid foam works well as exterior wall insulation and in other applications where vapor control is critical.

Fiberglass insulation provides solid R-value at a lower cost. It offers excellent noise reduction and fire resistance. Fiberglass insulation is a smart choice when moisture risks are low and budget is a primary concern. It can be combined with rigid foam to optimize energy efficiency and soundproofing.

There is no universally “better” option – choose the insulation type that best aligns with your climate, building needs, and project goals. With an understanding of the trade-offs between rigid foam vs. fiberglass insulation, you can make the ideal choice for your home.

Frequently Asked Questions

Is rigid foam better than fiberglass insulation?

Rigid foam has a higher insulating value per inch compared to fiberglass and excels at minimizing moisture risks. However, fiberglass costs less and has good sound dampening qualities. There is no blanket answer on whether rigid foam or fiberglass is better overall – the right insulation depends on your specific project requirements.

Does rigid foam replace fiberglass insulation?

Rigid foam can replace or supplement fiberglass insulation. Used alone, rigid foam often requires thicker insulation depth to achieve the same R-value as fiberglass. Rigid foam is commonly combined with fiberglass to provide moisture control, air sealing, and soundproofing that fiberglass alone cannot match.

Should I use rigid foam and fiberglass?

Using both rigid foam and fiberglass insulation offers performance advantages over using either one alone. The layered approach provides excellent thermal insulation, air sealing, moisture resistance, sound dampening, and fire protection when properly installed. Combining these two materials gives you the best of both insulation worlds.

Is rigid foam safe for indoor use?

Rigid foam insulation products made with polyisocyanurate or polyurethane can off-gas potentially harmful VOCs indoors if left exposed. Ensure rigid foam used inside is certified low-VOC and always cover indoor rigid foam with a thermal barrier like drywall. With proper safety precautions, indoor use of rigid foam is safe and effective.

Does rigid foam need a vapor barrier?

Rigid foam insulation itself acts as a vapor barrier, resisting diffusion of water vapor. When installed correctly with taped seams on the cold side of wall or ceiling assemblies, rigid foam eliminates the need for a separate vapor barrier. The exceptions are colder climates where an interior vapor barrier is also recommended.


Rigid foam and fiberglass both offer effective insulating properties, but with key performance differences. Rigid foam provides superior R-value per inch, moisture resistance, and sound blocking, while fiberglass offers affordability and fire safety. By understanding the trade-offs between these two popular insulation types, you can determine the right solution for your climate, budget and project needs. Combining rigid foam and fiberglass layers gives you the best of both worlds. With this comparison guide, you have the knowledge to make an informed, performance-driven choice between rigid foam vs. fiberglass insulation.