Rigid foam board insulation delivers more thermal resistance per inch than almost any other common insulation, which is why builders use it for exterior sheathing, foundation walls, and unvented roofs. Polyisocyanurate, extruded polystyrene, and expanded polystyrene boards each carry a different R-value per inch, and those differences change how thick a wall assembly must be to meet code. A comparison of spray foam, fiberglass, cellulose, and rigid foam R-values shows where foam board fits in the insulation family, and this article covers the numbers that matter for material selection.
R-value measures resistance to heat flow, and higher numbers mean better insulation. Foam boards earn their place in wall and roof assemblies by packing R-3.6 to R-6.5 into each inch of thickness, several times the per-inch performance of batt insulation. The trade-offs involve water absorption, compressive strength, and cost.
How R-Value Works for Rigid Foam
R-value is the measure of thermal resistance a material provides, expressed in square-foot-degree-Fahrenheit-hours per BTU. For a given material, total R-value is the per-inch value multiplied by thickness, so a two-inch board at R-5 per inch delivers R-10, and a four-inch board delivers R-20. This linear relationship lets the designer pick thickness to hit a target assembly value.
Compared with other insulation families, foam board wins on space efficiency. Fiberglass batts deliver about R-3.2 per inch, loose-fill cellulose about R-3.7, and spray polyurethane foam anywhere from R-3.5 to R-6.5 per inch depending on cell structure. The per-inch comparison of fiberglass, cellulose, spray foam, and rigid foam insulation shows why a 2×4 wall filled with batts reaches roughly R-13 while the same cavity detailed with foam can reach R-20 or more.
Energy codes set minimum R-values by climate zone, and the required numbers have climbed steadily. Current codes in cold zones call for R-20 cavity insulation plus R-5 continuous insulation or better, and foam board is the practical way to add that continuous layer without changing the wall depth.
R-value per inch at a glance
| Material | R-Value per Inch | Notes |
|---|---|---|
| Fiberglass batt | 3.0 to 3.8 | Compresses easily; loses value when wet |
| Cellulose | 3.2 to 3.8 | Settles over time; dense-pack performs best |
| EPS foam board | 3.6 to 4.2 | Low cost; vapor permeable |
| XPS foam board | 5.0 | Resists moisture well |
| Polyiso foam board | 5.6 to 6.5 | Highest per inch; derates in cold weather |
| Closed-cell spray foam | 5.5 to 6.5 | Air-seals and insulates at once |
R-value is not the whole story
Two boards with identical R-values can perform very differently in a real wall. Air leakage bypasses insulation entirely, so a board with taped seams outperforms the same board with gaps. Thermal bridging through studs and fasteners also reduces the effective value of any insulation, which is why continuous exterior foam is a standard upgrade over cavity-only insulation.
EPS, XPS, and Polyiso Compared
The three common foam boards share a plastic foam base but differ in manufacturing, performance, and price. Expanded polystyrene (EPS) is molded from beads, extruded polystyrene (XPS) is pushed through a die as a continuous sheet, and polyisocyanurate (polyiso) is a rigid foam core faced with foil on both sides. Those differences produce the numbers that matter on a job site.
| Property | EPS | XPS | Polyiso |
|---|---|---|---|
| R-value per inch | 3.6 to 4.2 | 5.0 | 5.6 to 6.5 |
| Water absorption | Higher; needs protection | Low | Low with foil facers |
| Compressive strength | 10 to 60 psi | 15 to 60 psi | 16 to 25 psi typical |
| Cold weather performance | Stable | Stable | Derates below about 40°F |
| Relative cost | Lowest | Mid | Highest |
EPS costs the least and handles below-grade use well when protected, but it absorbs more water over time if left exposed. XPS resists moisture and holds its R-value in wet soil, which makes it the traditional choice for foundation insulation. Polyiso offers the highest per-inch value for roofs and wall sheathing, with one catch: its foil-faced core loses rated performance in cold weather, so designers derate it for unheated assemblies.
Whatever board you choose, the seams and edges deserve as much attention as the board itself. Gaps around pipes, wires, and framing members let air and moisture bypass the insulation, and injection and spray foam products are often used to fill those voids. BuildingGreen published a practical list of tips for getting injection and spray foam right, covering nozzle technique, material temperature, and cavity prep for any foam-based retrofit.
Choosing by climate and exposure
Climate decides which trade-offs you can accept. In cold climates, per-inch R-value matters because wall depth is limited, which favors polyiso above grade and XPS below grade. In mixed and hot climates, cost per R often wins, which pushes projects toward EPS.
Below-grade and wet locations
Below-grade foam must survive soil pressure, groundwater, and insects. XPS is the classic choice because it holds R-value when wet and shrugs off modest crushing loads. EPS works below grade when the soil is well drained and the boards are protected with a coating or drainage mat. Polyiso is rarely specified below grade because moisture and cold both hurt it.
Matching Foam Board to the Application
Application drives the selection. Exterior continuous insulation on walls and roofs favors polyiso for its high per-inch value, foundation and below-grade work favors XPS or protected EPS for moisture resistance, and interior basement walls often use EPS because it is inexpensive and easy to handle. A rundown of rigid foam insulation types and their applications can help narrow the choice before you price boards.
Exterior continuous insulation
Continuous insulation outside the framing breaks thermal bridging through studs, which can cut a wall’s effective R-value by 20 percent or more in cold climates. Code pathways such as the International Energy Conservation Code’s R-value and U-factor options reward this approach, and foam board is the cheapest way to add it. Boards are attached with cap nails or approved adhesives, seams are staggered and taped, and cladding attaches through furring strips or long fasteners.
Slab insulation follows the same logic. A heated slab loses heat through its edges and downward into the soil, and a layer of XPS or protected EPS under and around the slab cuts that loss substantially. Sealed joints also make the board a capillary break against ground moisture.
Roof and cathedral ceiling assemblies
Unvented roof assemblies use foam board above or between the rafters to reach high R-values without deepening the roof structure. Polyiso is the usual pick because per-inch value matters most where the rafter depth is fixed. The foil facer also serves as a vapor retarder, which simplifies the assembly in most climates, though ventilation details still follow the local code.
Thickness, Cost, and Code Compliance
Total R-value scales with thickness, so meeting a code target is an arithmetic exercise. A wall requiring continuous R-10 needs 2 inches of XPS, about 2.5 inches of EPS, or just over 1.5 inches of polyiso. The technical details of EPS, XPS, and polyiso boards for exterior sheathing, foundation, and continuous insulation applications matter most at the transitions, where air barriers, flashings, and fasteners meet.
Board prices vary by region, but EPS typically runs 30 to 50 percent less than XPS at the same thickness, and polyiso sits between them on a per-inch basis when compared by R-value. Buying by R rather than by board thickness changes the comparison: a thicker, cheaper EPS stack often matches a thinner XPS stack at lower cost, at the expense of build time and wall depth.
Estimating your thickness
- Walls with continuous insulation: divide the code-required continuous R-value by the board’s per-inch value.
- Cathedral ceilings and unvented roofs: use polyiso to hit high R-values without deepening the rafter.
- Slab edges and foundations: use XPS or protected EPS sized to the local frost depth requirements.
- Add one inch of extra R-value where the assembly will see thermal bridging or cold corners.
Long-term performance
Foam boards hold their R-value for decades when installed correctly, but moisture and physical damage are the two things that shorten their life. Keep above-grade boards protected from UV, keep below-grade boards drained and covered, and tape all seams so air cannot move behind the insulation.
Installation Details That Protect Performance
Air sealing determines how much of a foam board’s rated R-value actually reaches the building. Seams between boards, penetrations for pipes and wires, and the gap between the board and the framing all leak if left untreated. Many crews tape the seams with manufacturer-approved tape and fill the small voids around penetrations with canned or injected foam. The complete technical guide to open-cell and closed-cell spray polyurethane foam systems covers the air-sealing side of this equation for crews that want more depth.
- Check the substrate for flatness; bowed framing crushes the board and reduces effective R-value.
- Cut boards with a straightedge and sharp knife, and measure each piece rather than trusting the layout.
- Stagger the seams between layers and from course to course.
- Tape every seam and corner with the tape the manufacturer specifies.
- Fasten boards with cap nails or screws sized for the substrate, typically six fasteners per board.
- Protect exposed edges with trim, flashing, or cladding within the time frame the manufacturer allows for UV exposure.
Working with foam board safely
Foam boards release dust and particles when cut, and the solvents used by some adhesives are flammable during application. Cut boards outdoors or with a shop vacuum at the saw, wear a respirator rated for particulates, and keep ignition sources away from solvent-based adhesives. Follow the manufacturer’s storage guidance, since boards stored in direct sun can warp before they ever reach the wall.
Sizing Up a Foam Board Retrofit
A retrofit project starts with a target: decide what R-value the wall, roof, or slab needs, then pick the board that meets it at the right price. For slabs, the insulation goes below the concrete, and the guide to foam board insulation and installation under a radiant slab covers the placement and protection details that matter before the pour.
Costs should be compared per R, not per board. A 2-inch sheet of XPS at R-10 can cost the same as a 3-inch sheet of EPS at roughly R-12, so the thicker EPS sheet wins on performance for the same money. Where wall depth is fixed, the higher per-inch products earn their premium.
The formula is arithmetic plus discipline. Pick the board type that fits the moisture and temperature conditions, do the thickness math to hit the target R-value, and install with taped seams and protected edges. Foam board is one of the few insulation products where the rated value is achievable in practice, when the crew treats every seam as part of the thermal system. Measure twice, tape everything, and the assembly will perform to the number on the label.