A 2-inch foam board is a rigid insulation panel that slows heat flow through trapped gas cells, delivering roughly R-8 to R-13 depending on whether it is EPS, XPS, or polyiso.
Two inches of the wrong foam is a wasted trip to the lumber yard. The thickness is identical on every panel, but the R-value stamped on the plastic facing is not, and the gap between the best and worst 2-inch board is wider than most people expect. That single number decides whether a basement wall or a slab edge hits its target — and it decides how many panels you need to buy. Here is what the board actually is, how it stops heat, and which numbers are real.
What A 2-Inch Foam Board Actually Is
A 2-inch foam board is a rigid panel of plastic foam, two inches thick, sold in standard sheet sizes and made from one of three materials: EPS (expanded polystyrene), XPS (extruded polystyrene), or polyiso (polyisocyanurate). Each one insulates the same way — millions of small gas pockets trapped inside the plastic — but the plastic and the gas differ, so the R-value differs too.
Owens Corning’s FOAMULAR line is XPS, and the company’s product data lists it at R-5.0 per inch at a 75°F mean temperature. That works out to R-10 on a 2-inch panel. The 2-inch size is also a standard stocked dimension: FOAMULAR NGX CodeBord is sold as 2 in x 24 in x 96 in at R-10, and FOAMULAR NGX C-300 carries the same 2-inch, R-10, 24-by-96 spec with an under-slab rating. Same thickness, same brand family — but still XPS, not EPS or polyiso.
How A Rigid Foam Board Slows Heat
Rigid foam works by putting a continuous layer of resistance between warm air and cold air, and its gas cells block conductive heat flow far better than the air inside a hollow wall cavity does. Higher R-value means more resistance, which is why two inches beats one inch every time within the same product.
Material matters as much as thickness. Published figures put 2-inch EPS near R-8.0, 2-inch XPS near R-10.0, and 2-inch polyiso in the R-12.0 to R-13.0 range at standard test temperatures. That spread is the whole reason a “2-inch board” is not a single product.
There is a catch with polyiso specifically. Its rating is measured at a warm mean temperature, and its performance can drop in cold conditions — so the R-12 or R-13 number on the label may not reflect what the panel does through a northern winter. Owens Corning’s own data sheet shows FOAMULAR holding R-5.0 per inch at both 75°F and 40°F mean temperature, a steadier curve. The real lesson: installed performance can land below the label, so plan for the cold-case number, not just the brochure figure.
| Foam Type | 2-Inch R-Value | Typical Use |
|---|---|---|
| EPS (expanded polystyrene) | About R-8.0 | Walls and interiors where cost matters |
| XPS (extruded polystyrene) | About R-10.0 | Basements, foundation walls, under slab |
| Polyiso (polyisocyanurate) | R-12.0 to R-13.0 | Roofs and walls needing high R per inch |
| FOAMULAR 150 (XPS) | R-10 at 2 in | General rigid insulation, R-5 per inch |
| FOAMULAR NGX CodeBord (XPS) | R-10 at 2 in x 24 in x 96 in | Sheathing and continuous exterior layers |
| FOAMULAR NGX C-300 (XPS) | R-10 at 2 in x 24 in x 96 in | Under-slab applications |
| Polyiso in cold weather | Below its warm-temperature rating | Any cold-climate install |
Where 2-Inch Boards Get Used
Because the panels are stiff, moisture-resistant, and dimensionally stable, 2-inch foam boards show up in the places that need a continuous thermal break rather than fluffy batt insulation. Product listings place them in basements, below-grade and foundation walls, under slabs, walls, crawlspaces, roofs, and interior walls — with the exact application tied to the foam type and the facing on the panel.
Under slab is unforgiving: once the concrete is poured, the board is buried for good, so the XPS ratings like FOAMULAR NGX C-300’s under-slab listing matter more than a few dollars of price difference. On a roof, where thickness squeezes against other layers, polyiso’s higher R per inch often wins the space trade. In a basement, where cost and moisture dominate, EPS or XPS usually makes more sense.
If you are pricing panels for a specific job, a tested roundup of 2 foam board insulation options can save you a spreadsheet, since the R-value, thickness, and sheet size all vary between brands.
The Number That Decides Your Purchase
Buy on R-value per inch, not on thickness alone. A 2-inch EPS board and a 2-inch polyiso board are both “2-inch foam board,” yet one delivers roughly R-8 and the other R-12 or better — a 50% swing in thermal performance from two products that look identical on a shelf. Owens Corning’s FOAMULAR documentation sets R-5 per inch, and the brand’s 2-inch boards land at R-10 across several product lines.
Two other facts change the math. And foam board is not a one-type product: a brand name never tells you the material. FOAMULAR is XPS. It is not EPS, and it is not polyiso, and buying it by name alone will not get you a polyiso panel’s cold-weather behavior.
Before you cut, fasten, or cover any foam board, pull the manufacturer’s official installation instructions and check your local building code. The data sheets cover thermal performance thoroughly, but they are not installation or safety manuals, and code requirements vary by application and region.
FAQs
Is 2-inch foam board the same as 2-inch XPS?
No. Two inches is only the thickness. The material can be EPS, XPS, or polyiso, and each carries a different R-value. XPS lands around R-10 at two inches, EPS around R-8.0, and polyiso between R-12.0 and R-13.0 at standard test temperatures. Check the panel’s label for both the foam type and the R-value before buying.
Does a thicker foam board always mean a higher R-value?
Within the same product, yes. R-value rises with thickness, and Owens Corning lists FOAMULAR at R-5.0 per inch at a 75°F mean temperature, so a 2-inch panel reaches R-10 while a 1-inch panel reaches R-5. Across different foam types, though, thickness tells you nothing — a 2-inch polyiso board can out-insulate a much thicker EPS panel.
Why might polyiso underperform in cold weather?
Polyiso’s published R-value is measured at a warm mean temperature, and its performance can drop when conditions turn cold. That means the R-12 to R-13 figure on a 2-inch polyiso label may not match what the panel delivers through a northern winter. If you build in a cold climate, size the insulation using cold-weather expectations rather than the warm-temperature rating.
References & Sources
- Owens Corning. “FOAMULAR 150 Product Data Sheet.” Supports R-5.0 per inch at 75°F and 40°F mean temperature and the stocked 2-inch dimension.
- Owens Corning. “FOAMULAR CI-C Product Data Sheet.” Supports the FOAMULAR product line specifications and R-value per inch figures cited.
- Alaska Housing Finance Corporation. “Building Manual.” Supports cold-climate insulation performance expectations, including polyiso behavior in low temperatures.