Foam board is a rigid plastic insulation used in construction to reduce heat loss through walls, roofs, foundations, and floors. Sold in large sheets (typically 4-by-8 feet), it serves as a thermal barrier that can be applied to the exterior or interior of a building assembly. Its uses range from wrapping an entire house in continuous insulation to forming the structural core of prefabricated wall panels.
Three Types of Foam Board
Not all foam board is the same material. The three types used in construction differ in cost, insulating power, and where they perform best.
Expanded polystyrene (EPS) is the most versatile and least expensive option. It delivers roughly R-4.2 to R-4.4 per inch of thickness and holds its insulating value over time because it doesn’t absorb significant moisture. EPS is approved for ground contact and below-grade use, and it can be treated to resist insects. It’s the go-to choice for insulated concrete forms and structural insulated panels.
Extruded polystyrene (XPS) is the colorful one, recognizable by its blue, green, or pink tint. It provides about R-5 per inch and costs around 42 cents per square foot for a 1-inch-thick panel. XPS works well in walls and below-grade applications, though it absorbs more moisture over time than the other two types, which means its insulating value can degrade. Unfaced XPS at 1 inch has a vapor permeance of about 1 perm, making it semipermeable: it slows moisture movement without trapping it entirely.
Polyisocyanurate (polyiso) offers the highest R-value at R-5.7 to R-6.5 per inch, depending on the application. It’s the most expensive of the three and is always manufactured with a facing material, often foil or fiberglass. Foil-faced polyiso is essentially impermeable to moisture (0.01 perms), which makes it an effective vapor barrier but also means you need to be careful not to trap moisture inside wall assemblies. Polyiso dominates the commercial roofing market.
Continuous Wall Insulation
The most common residential use for foam board is as continuous exterior insulation on walls. In a standard wood-framed wall, insulation fills the spaces between studs, but the studs themselves conduct heat. Framing members, window frames, door frames, and plates together make up nearly one-fourth of the total wall area, and none of that area is insulated by cavity insulation alone.
Foam board solves this by creating an unbroken thermal blanket on the outside of the framing. The boards are fastened directly over the structural sheathing (or sometimes replace it), covering studs and all. According to the U.S. Department of Energy’s Building America program, this approach blocks heat transfer through studs, reduces drafts, and minimizes the risk of moisture problems by pushing the condensation point outside the framing cavity. Even a single inch of foam board on the exterior of a wall can meaningfully cut energy loss that cavity insulation alone cannot address.
Foundation and Below-Grade Insulation
Concrete foundation walls are notorious heat sinks. Foam board applied to the exterior of a foundation wall, flush against the waterproofing membrane, creates a continuous insulating layer that prevents thermal bridging through the concrete.
Exterior foundation insulation also prevents condensation. In cold climates, an uninsulated concrete wall stays cold while the interior air is warm and humid. That temperature difference causes moisture to collect on the inside surface of the wall, leading to mold and deterioration. With foam board on the outside, the concrete stays closer to interior temperature, and condensation doesn’t form.
The insulation must extend at least 6 inches above grade, and that exposed portion needs protection from sunlight and physical damage. A layer of cement board or gypsum covered with acrylic parging is a common approach. Joints between panels should be taped and sealed to prevent insects from finding gaps.
Interior Basement Walls
When exterior foundation insulation isn’t possible (the house is already built, for instance), foam board can go on the inside of basement walls. The typical approach uses 1 to 2 inches of XPS adhered directly to the masonry, with all joints taped. Wood furring strips are then fastened through the foam into the concrete with power-driven fasteners, and drywall is attached to the furring strips. An alternative is to glue one inch of XPS to the wall and frame a stud wall just in front of it, which allows for additional cavity insulation and easier drywall attachment.
Building codes require the foam to be mechanically fastened, not just glued, and it must be covered with a thermal barrier like half-inch drywall on any surface exposed to living space.
Flat and Low-Slope Roofing
Polyiso is the most widely used insulation product for low-slope commercial roofing. These large flat roofs need high R-values without excessive thickness or weight, and polyiso’s R-5.7 per inch makes it possible to meet modern energy codes in a relatively thin profile. The boards are installed in layers above the roof deck, beneath the waterproof membrane, often with staggered joints to minimize thermal gaps. Both new construction and roof replacement projects rely heavily on polyiso for this application.
In residential construction, foam board is sometimes used above the roof deck in cathedral ceiling assemblies, where there isn’t room for deep cavity insulation. Placing rigid foam above the sheathing keeps the roof deck warm and prevents ice dams in cold climates.
Structural Insulated Panels
Foam board isn’t always just insulation. In structural insulated panels (SIPs), a thick foam core is bonded between two rigid sheathing layers, typically oriented strand board. The resulting panel acts like a structural beam: the sheathing resists loads the way the top and bottom flanges of a steel beam do, while the foam core acts as continuous bracing that prevents buckling. Thicker foam cores produce stronger panels, just as deeper steel beams are stronger.
SIPs handle vertical loads, shear forces, and bending forces, which means they can replace conventional framing entirely. Walls, roofs, and floors can all be built from SIPs, and because the foam core is continuous with no framing interruptions, they deliver excellent thermal performance. EPS is the most common core material for SIPs.
Under-Slab and Floor Insulation
Foam board is placed beneath concrete slabs to prevent heat loss into the ground. EPS and XPS are both used here because they can handle the compressive loads of a concrete pour and the long-term pressure of a building’s weight without losing their shape. The boards are laid directly on prepared gravel beds before the concrete is placed.
In framed floor systems, foam board can be incorporated between or below joists. When foam is used in a floor assembly, building codes require a half-inch wood structural panel on the walking surface and a thermal barrier (like drywall) on the underside if it’s exposed to interior space.
Fire Safety Requirements
Foam board is combustible, and building codes strictly regulate how it must be protected. The International Building Code requires foam plastic to be separated from the interior of a building by an approved thermal barrier, most commonly half-inch gypsum wallboard. This applies to walls, ceilings, and any surface where the foam would otherwise be exposed to living space.
There are exceptions. Foam encased in at least 1 inch of masonry or concrete on each side doesn’t need an additional thermal barrier. In attics and crawl spaces accessed only for utility maintenance, lighter-duty protection is acceptable: a quarter-inch wood structural panel, three-eighths-inch gypsum board, or even 1.5 inches of mineral fiber insulation over the foam will satisfy code.
Moisture and Vapor Control
Beyond thermal insulation, foam board plays an active role in managing moisture within wall and roof assemblies. Each type has a different vapor permeance, which determines how much water vapor can pass through it. Foil-faced polyiso is nearly impermeable at 0.01 perms, making it a vapor barrier. EPS at 1 inch is relatively open at about 3.5 perms, allowing walls to dry through the insulation. XPS falls in the middle at about 1 perm per inch.
Choosing the right type depends on your climate and wall design. In cold climates, a less permeable foam on the exterior keeps warm, moist interior air from reaching cold sheathing and condensing. In mixed or warm climates, a more permeable option like EPS lets walls dry to the outside. Using foil-faced polyiso as exterior sheathing creates an exterior vapor barrier, so it should never be combined with an interior vapor barrier, or moisture will be trapped inside the wall with no way out.
Environmental Considerations
The blowing agents used to manufacture foam board have been an environmental concern. XPS has historically relied on high-global-warming-potential (GWP) HFC blowing agents. As of January 2025, the EPA prohibits the manufacture of foam insulation products using HFC blends with a GWP above 150, and twelve states plus Canada had already adopted similar restrictions. EPS and polyiso manufacturers adopted low-GWP blowing agents decades ago and already comply with these rules. XPS manufacturers have been required to reformulate, and products made before the cutoff dates may still be sold under sell-through provisions until existing stock is installed.