Underfloor insulation is key to reducing energy consumption and ensuring home comfort. The right insulation can prevent heat loss and address moisture concerns, especially in crawl spaces, where moisture and temperature control are critical.
Here, we explore nine of the best insulation materials for under floors, with a focus on Rmax’s polyiso solutions that have been designed for energy-efficient construction.
The Importance of Underfloor Insulation
Installing insulation beneath floors helps maintain steady indoor temperatures by reducing heat transfer between the ground and the living space, which directly impacts energy costs and home comfort, especially when the installation is in a crawl space or unfinished basement. Choosing materials suited to the environment can drastically improve the longevity and energy efficiency of a home.
For optimal performance and energy efficiency, selecting the right insulation material is crucial, particularly for above-grade installations where moisture and thermal regulation are key.
9 Best Insulation Materials for Under Floors (Above Grade)
1. Polyiso Rigid Board Insulation (Rmax)
Rmax’s polyiso rigid boards deliver high performance, with an R-value between 5.6 and 8 per inch, which makes them one of the most efficient options for underfloor insulation. Manufactured using eco-friendly processes, including low-GWP blowing agents and recycled materials, polyiso boards provide a sustainable solution for residential and commercial projects. Their closed-cell structure provides excellent thermal resistance and durability to maintain performance even under the pressure of heavy loads.
2. Spray Foam (Icynene)
Spray foam insulation expands to fill gaps and cracks to create an airtight seal that prevents heat loss and moisture infiltration. Icynene, a popular type of spray foam, is made from castor oil and offers a flexible solution for hard-to-reach areas such as crawl spaces with irregular surfaces. With an R-value ranging from 5.5 to 7 per inch, it’s an efficient choice, though proper ventilation is essential to prevent trapped moisture.
3. Rigid Foam (EPS and XPS)
Expanded polystyrene (EPS) and extruded polystyrene (XPS) are widely used for underfloor insulation due to their moisture resistance and compressive strength. They are particularly well-suited for subfloor installations in homes with concrete or wood bases. EPS and XPS offer R-values of 3.6 to 5 per inch, which makes them slightly less efficient than polyiso, but popular for their cost-effectiveness and structural support under slabs.
4. Reflective Foil Insulation
Reflective foil insulation works by directing radiant heat back into the living space, so it is ideal for homes with radiant floor heating systems. Although it has a lower R-value on its own, reflective foil significantly enhances the performance of other insulation types. This insulation is particularly effective in climates where minimizing radiant heat loss is a priority, and it is often used over subfloor materials like plywood and OSB.
5. Mineral Wool (Rockwool)
Mineral wool, or rockwool, provides a durable and fire-resistant insulation solution with soundproofing qualities. Its R-value ranges between 3 to 3.3 per inch and it is commonly used in multi-story homes or noisy areas. Although not highly moisture-resistant, it performs well in dry climates where soundproofing and fire protection are top priorities.
6. Fiberglass Batt Insulation
Fiberglass batt insulation is a cost-effective option, but it has several limitations in high-moisture environments such as crawl spaces. With an R-value ranging from 2.8 to 3.9 per inch, fiberglass works best in dry environments where moisture is less of a concern. When used in crawl spaces, it can absorb moisture that leads to mold and diminished thermal efficiency.
7. Cotton (Denim) Insulation
Made from recycled denim, cotton insulation is an eco-friendly alternative that provides moderate thermal performance, with an R-value of about 3.5 per inch. However, its susceptibility to moisture makes it unsuitable for damp environments like crawl spaces. It is better suited for above-grade installations in homes that emphasize sustainability.
8. Hempcrete
Hempcrete is a carbon-negative insulation material made from a blend of hemp fibers and lime. While its R-value (around 2.4 per inch) is lower than options like polyiso, hempcrete is a sustainable choice due to its carbon-absorbing properties. Hempcrete is breathable for natural mold resistance and to help regulate indoor humidity, which makes it ideal for eco-conscious builders.
9. Recycled Plastic Insulation
Recycled plastic insulation, made from post-consumer plastic bottles, provides decent thermal performance with an R-value of around 3.1 per inch. It is a sustainable option for those looking to reduce plastic waste. While it doesn’t offer the same moisture resistance as polyiso or spray foam, it’s suitable for above-grade applications or in areas with low moisture exposure.
As insulation demands differ significantly for below-slab installations, it is essential to choose materials that can withstand ground contact and heavy loads while also providing lasting moisture protection.
Insulation for Below-Slab Applications
When insulating below a concrete slab, the demands are more rigorous than standard subfloor or crawl space insulation. Below-slab insulation must handle ground contact, support heavy loads from the concrete, and resist moisture over time. Using the wrong material can lead to moisture penetration, slab cracking due to freeze-thaw cycles, or diminished thermal performance, which compromises the foundation’s durability and energy efficiency.
To meet these challenges, only certain insulation materials are suitable for below-slab applications. These options provide both compressive strength and moisture resistance, ensuring long-term protection and performance:
Polyiso Rigid Board (Rmax)
Polyiso rigid boards, such as those from Rmax, are highly effective below-slab due to their superior compressive strength and low moisture absorption. The closed-cell structure of polyiso provides a high R-value per inch to maintain thermal performance even under the weight of a concrete slab. Its resistance to ground moisture makes it a reliable choice for residential and commercial projects.
Extruded Polystyrene (XPS)
XPS is another common option for below-slab insulation that is known for its durability and moisture resistance. It has high compressive strength to support the weight of concrete without losing its insulating properties. XPS is especially useful in high-moisture environments where maintaining a thermal barrier is critical.
Expanded Polystyrene (EPS)
EPS, though slightly less moisture-resistant than XPS, is a cost-effective option for below-slab use. It provides decent compressive strength and thermal insulation at a lower price point. However, EPS is more prone to moisture absorption, which makes it best suited for drier environments or when used with a vapor barrier.
Closed-Cell Spray Foam
Closed-cell spray foam can be an effective alternative for below-slab insulation, especially on uneven ground surfaces. Its ability to expand and fill voids creates a tight seal to prevent air and moisture infiltration. This insulation type also offers high compressive strength, though care must be taken to ensure even application to avoid weak points in the slab.
Among the various insulation options, Rmax polyiso stands out due to its exceptional combination of thermal efficiency, moisture resistance, and sustainability, which makes it a top choice for both above-grade and below-slab applications.
How Rmax Polyiso Insulation Excels
Compared to other insulation options, Rmax polyiso stands out for its superior thermal resistance, high moisture protection, and environmental sustainability:
Polyiso’s closed-cell structure makes it an excellent choice for below-grade applications, where moisture resistance is critical.
Its high R-value per inch allows for thinner insulation layers to reduce material costs and installation time.
Rmax’s commitment to eco-friendly insulation also supports sustainable building practices and helps projects meet LEED and other energy efficiency certifications.
To fully optimize the performance of underfloor insulation, proper installation techniques are just as important as selecting the right material.
Installation Tips for Underfloor Insulation
To maximize the effectiveness of your underfloor insulation, consider these best practices:
Seal all gaps and joints: Ensure that insulation boards are tightly fitted, leaving no gaps that could allow heat to escape.
Choose the right thickness: Select insulation with the appropriate R-value for your climate zone. Rmax polyiso, for example, offers excellent performance with thinner boards.
Install moisture barriers: In areas prone to dampness (such as under slabs, crawl spaces and unfinished basements, use vapor barriers to protect insulation from moisture damage and potential mold growth.
For more detailed installation advice, Rmax provides technical resources that outline best practices for installing various types of insulation.
Explore Energy-Efficient Solutions with Rmax
Rmax offers a broad range of polyiso insulation products designed for both residential and commercial projects. Whether you’re insulating a subfloor, crawl space, or below-grade area, Rmax’s polyiso solutions deliver top-tier performance with sustainability in mind.
Contact us today for more information!