Understanding Rigid Insulation: Types, Uses, and Best Practices

When building or renovating a high-performance home, insulation plays a vital role in keeping the interior warm, dry, and energy efficient. While batt insulation and spray foam are commonly discussed, rigid insulation boards are equally important. They’re used in exterior wall assemblies, below-grade foundations, and along basement interior walls.

So, what exactly is rigid insulation, and how do you choose the right type for your project?

What Is Rigid Insulation?

Rigid insulation refers to solid foam panels, typically sold in 4-by-8-foot sheets and available in a range of thicknesses from half an inch to over four inches. Unlike batt insulation that’s tucked between studs, rigid insulation is installed in continuous layers. This helps prevent air leaks and limits thermal bridging—preventing heat escaping through wooden studs and other framing materials.

Types of Rigid Insulation

There are several kinds of rigid insulation, each suited to specific applications:

Expanded Polystyrene (EPS) is the most budget-friendly option. This white or gray lightweight foam has an R-value around R-3.6 to R-5 per inch and can be used above or below grade. When treated for moisture, it’s suitable for exterior walls or beneath basement slabs. It has a lower R than other materials, but this is offset by the fact that the R value remains constant through all temperatures and over the materials lifespan.

Extruded Polystyrene (XPS) - often recognized by its blue or pink color—offers a higher R-value of approximately R-5 per inch. It’s more durable and moisture-resistant than EPS, making it a popular choice for foundation walls and under concrete slabs. However, it’s more expensive and less eco-friendly due to the blowing agents used in its manufacturing. It also loses a small amount of R value as it ages, due to offgassing.

Polyisocyanurate (Polyiso) stands out for its high R-value of R-6 to R-6.5 per inch and is commonly faced with foil, which serves as both a vapor barrier and radiant barrier. It has the highest R value per inch but can lose effectiveness in very cold climates.

Mineral Wool Boards, also known as rigid rock wool, are made from spun rock fibers. They offer an R-value of about R-4.2 per inch and are fireproof, sound-absorbing, and moisture-resistant. Though heavier and pricier than foam boards, mineral wool is a great choice for fire-resistant assemblies or walls that need to remain breathable.

Where Rigid Insulation Is Used in Homes

Exterior above-grade walls benefit greatly from a layer of rigid insulation applied over the sheathing. This reduces heat loss through studs and significantly improves energy efficiency in cold climates like Renfrew County. A minimum of 1”, R5 continuous insulation is required by code for most wall assemblies. A typical assembly might include plywood or OSB, one or two inches of rigid insulation, vertical strapping, and then siding. This can be added when installing new siding on an existing home, making a big difference in comfort and heating bills.

Interior basement walls often use rigid insulation to provide a vapour barrier and prevent any thermal bridging. In a retrofit scenario, two inches of EPS or XPS is attached directly to the concrete and sealed at the joints. A stud wall can be added in front for wiring, drywall and to contain additional mineral wool insulation.

Under basement slabs, rigid foam such as XPS or treated EPS provides both insulation and a vapor barrier. It’s an effective way to improve comfort and energy performance from the ground up.

Final Thoughts

Rigid insulation is an essential component in creating warm, dry, and energy-efficient homes. Whether you’re tackling a new build or upgrading an existing structure in Renfrew County, selecting the right type of insulation—and placing it in the right location—makes a big difference in performance and comfort.

At Quality Homes & Renovations, we guide clients through insulation choices tailored to their specific needs, climate, and budget. Whether you're insulating a Golden Lake cottage or improving a basement in Barry’s Bay, our team ensures the materials used will stand up to the local climate and keep your home comfortable all year long.