What Is Foam Board Insulation?
Foam board insulation consists of rigid panels designed to provide thermal resistance in walls, foundations, basements, crawlspaces and other compatible areas. Unlike flexible batt and roll insulation, rigid foam maintains its shape and can be installed as a continuous layer of insulation.
Common types of rigid foam insulation include expanded polystyrene (EPS), extruded polystyrene (XPS) and polyisocyanurate (polyiso). Each offers different characteristics, so consider where you’re installing the insulation as well as the required R-value, thickness and moisture resistance.
Types of Foam Board Insulation
Here are the most common types of foam board insulation you may need for your project.
Expanded Polystyrene (EPS)
Expanded polystyrene, or EPS foam, is a lightweight rigid foam made from expanded polystyrene beads. It’s available in a range of densities, thicknesses and R-values and is commonly one of the more economical foam board options.
EPS can be used for walls, roofs, foundations and other applications when the specific product is rated for the intended use. Check the product specifications for moisture resistance, compressive strength and approved applications.
Extruded Polystyrene (XPS)
Extruded polystyrene, or XPS foam, has a closed-cell structure that provides good moisture resistance and thermal performance. XPS rigid foam insulation is commonly available for walls, basements, crawlspaces, foundations and other above- and below-grade applications.
Some shoppers refer to rigid polystyrene insulation as Styrofoam insulation or Styrofoam board. Technically, STYROFOAM™ is a brand name associated with certain XPS products, rather than a general name for all foam board insulation.
Polyisocyanurate (Polyiso)
Polyisocyanurate, commonly called polyiso, is a rigid foam insulation that typically provides a high R-value per inch. This can make polyiso useful when you need greater insulating performance but have limited space for thick foam board.
Polyiso is often available with foil or other facings. Check the product specifications to make sure the board is appropriate for your application.
EPS vs. XPS vs. Polyiso Foam Board
| Foam Board Type | Key Characteristics | Common Applications |
| EPS | Lightweight, economical and available in multiple densities and thicknesses | Walls, roofs and foundations when product rated |
| XPS | Good moisture resistance, durable and easy to cut | Basements, crawlspaces, foundations, walls and floors when product-rated |
| Polyiso | High R-value per inch and commonly available faced | Walls, roofs and other above-grade applications when product rated |
Always check the manufacturer’s specifications. Products of the same foam type can have different R-values, moisture characteristics and approved applications.
How to Choose Foam Board Insulation
R-Value
- R-value measures an insulation material’s resistance to heat flow. A higher R-value indicates greater insulating performance.
- The R-value of foam board varies by material, product and thickness. Rather than selecting insulation based only on foam type and recommended application, check the labeled R-value of the specific product.
- The R-value your project requires depends on factors such as your climate zone, where the insulation will be installed and local building codes.
Foam Board Thickness
- In general, increasing the thickness of the same foam board product increases its total R-value. Foam insulation board is available in a variety of thicknesses, allowing you to select a product that fits the available space and provides the required thermal performance.
- Check both the thickness and labeled R-value when comparing products.
Board Dimensions
- Foam board comes in several panel sizes, with 4-foot-by-8-foot sheets being common. Smaller panels are also available and may be easier to transport or use for smaller projects.
- Measure the installation area before shopping to determine the panel size and number of sheets you’ll need.
Faced vs. Unfaced Foam Board
- Rigid foam insulation may be faced or unfaced. Faced insulation includes a layer of material on one or both sides that can affect durability, water vapor control and other performance characteristics.
- Don’t assume that any facing automatically functions as the vapor retarder or air barrier your project requires.
- Check the manufacturer’s instructions and local building codes when choosing between faced and unfaced insulation.
Where Can You Use Foam Board Insulation?
Depending on the product, rigid foam insulation can be used in several areas of a home:
- Basements: Foam board can be used as part of an insulation system for basement and foundation walls.
- Crawlspaces: Certain foam boards are suitable for crawlspace walls and other moisture-prone locations.
- Foundations: Products rated for below-grade or ground-contact applications can provide continuous insulation around foundations.
- Exterior Walls: Rigid foam can be installed as continuous insulation to help reduce heat transfer through wall framing.
- Garages: Foam board may be suitable for garage walls, ceilings and some garage doors, depending on the product and assembly.
- Floors and Ceilings: Certain products are rated for use under floors, above ceilings and with other horizontal structures.
Not every foam board is approved for every application. Some foam insulation must also be protected by an approved thermal or ignition barrier when installed inside a building. Follow the product instructions and local building codes.
How Much Foam Board Insulation Do You Need?
- Start by calculating the square footage of the area you plan to insulate. Multiply the length by the height or width of each surface, then add the areas together.
- Next, check the coverage provided by each insulation board. For example, a 4-foot-by-8-foot sheet covers 32 square feet before accounting for cuts and waste.
- Plan for some additional material when the project includes numerous corners, openings or irregular shapes.
How to Install Foam Board Insulation
- Installation methods vary by product and application, but the basic process typically includes measuring the area, cutting the boards to fit and securing them according to the manufacturer’s instructions.
- Measure carefully and use an appropriate cutting tool for the insulation you’re installing. Fit panels closely together and around openings and penetrations. Depending on the insulation system, seams, edges and gaps may need to be sealed with compatible tape, sealant or spray foam.
- Use adhesives and fasteners specifically approved for the foam board and application. Some adhesives can damage foam insulation.
Foam board can be part of an air-sealing strategy, but simply placing rigid insulation against a surface doesn’t necessarily create a complete air barrier. Follow the manufacturer’s installation instructions for sealing seams, edges and penetrations.
How Much Does Foam Board Insulation Cost?
The cost of rigid foam insulation depends on the foam type, R-value, thickness, dimensions, facing and quantity you need. Higher-R-value and thicker foam board generally costs more per sheet, but price alone shouldn’t determine which insulation you choose.
Compare products based on the R-value and performance required for your application. Shop foam board insulation at Lowe’s or ask a red-vest associate for help finding the right insulation board for your project.