Insulation Sheets: Foam Board Types and Uses

Rigid foam board has quietly become one of the most useful materials on a job site, and most homeowners still don’t know the difference between the three types stacked at the store. Insulation sheets come in three main flavors: expanded polystyrene, extruded polystyrene, and polyisocyanurate. They look similar, ranging from crumbly white to dense pink and blue to foil-faced tan, but they insulate at different rates, handle moisture differently, and cost different amounts. Pick the right sheet for the location and you build a warmer, drier assembly; pick wrong and you can trap water where you least want it.

The Three Types at a Glance

Every rigid foam board you’ll find in a home center is one of these three:

• EPS (expanded polystyrene): the white beadboard, about R-3.6 to R-4.2 per inch, the cheapest
• XPS (extruded polystyrene): the pink or blue board, about R-5 per inch, strong moisture resistance
• Polyiso (polyisocyanurate): the foil-faced board, about R-6 per inch, highest R but weaker in cold

That per-inch R-value is the number that should drive your buying decision, along with where the sheet will live. A board that performs beautifully on an above-grade wall can fail against wet soil, so location matters as much as R-value.

EPS: Budget Board That Drains

Expanded polystyrene is the white board made of fused foam beads. It is the least expensive rigid sheet at roughly $0.40 to $0.90 per square foot for 1 inch, and it gives about R-4 per inch. People dismiss it as the cheap stuff, but EPS has a real advantage: it dries out well and does not trap water the way people fear, which makes it a legitimate choice below grade when it is rated for that use. It is also the core of most insulated concrete forms and structural insulated panels. The trade-offs are lower compressive strength and a slightly lower R per inch, so where thickness is tight or loads are high, you step up to XPS.

XPS: The Moisture-Resistant Middle

Extruded polystyrene is the familiar pink and blue board with a smooth skin on both faces. At about R-5 per inch and $0.95 to $1.60 per square foot for 1 inch, it is the middle of the price range and the go-to for anything involving moisture or ground contact. Its closed-cell structure absorbs well under 0.3 percent water by volume, so it holds R-value in damp conditions. Use XPS under slabs, on foundation walls, and as exterior continuous insulation. It comes in compressive-strength grades from 25 psi up to 100 psi, so for a garage slab or driveway edge you can specify a board tough enough to carry the load without crushing.

Polyiso: Highest R, With a Cold-Weather Catch

Polyisocyanurate is the foil-faced board and the R-value champion at about R-6 per inch, sometimes advertised higher. At $0.70 to $1.40 per square foot for 1 inch, its price is competitive given the extra R. The foil facing doubles as a radiant barrier and a vapor barrier, which makes it excellent behind siding and on roofs. The catch every installer needs to know: polyiso’s R-value drops in cold temperatures, so its real-world winter performance in a frigid climate can fall below its rated number. It also wicks water, so it must stay above grade and dry. Never put polyiso against soil or under a slab.

Common Thicknesses and What They Deliver

Sheets come in standard 4×8 dimensions and a predictable set of thicknesses. Rough R-values for XPS, the middle performer, run like this:

• 1/2 inch — around R-2.5, a thin thermal break or sheathing layer
• 3/4 inch — around R-3.75, common under siding
• 1 inch — around R-5, the everyday continuous-insulation board
• 1.5 inch — around R-7.5, foundation and basement walls
• 2 inch — around R-10, cold-climate exteriors and slab edges

Scale those up for polyiso, which runs about 20 percent higher R per inch, and down for EPS. To hit a target like R-10 of continuous exterior insulation, you’d use 2 inches of XPS, roughly 1.7 inches of polyiso, or about 2.5 inches of EPS.

Where Each Sheet Belongs

Here is the quick reference I keep in my head on the job:

• Under a concrete slab: XPS or rated EPS, never polyiso
• Exterior of a foundation wall: XPS or rated EPS
• Interior basement wall, glued to concrete: XPS is the safe default
• Continuous exterior insulation behind siding: polyiso for max R, XPS where some moisture is expected
• Low-slope roof over the deck: polyiso is the industry standard
• Interior sheathing where budget rules and it stays dry: EPS

Installing Foam Sheets Correctly

Cutting is simple. Score with a utility knife and snap thin board, or run a fine-tooth handsaw or track saw for clean thick cuts. The details that separate a good install from a leaky one are seams and fasteners. Tape every joint with a compatible foam tape to build a continuous air and thermal layer, and stagger seams when you double up so no gap runs straight through both layers. Use cap-head fasteners or plastic washers long enough to bite the framing through the full foam thickness, and on exterior walls follow the manufacturer’s furring guidance so the cladding has solid backing.

Two safety rules are non-negotiable. All three foams are combustible and must be covered by an approved thermal barrier, usually 1/2-inch drywall, in any living space; never leave foam exposed in a finished room. And keep the boards away from prolonged high heat and petroleum solvents, which can attack the foam. Follow those and rigid board is about as forgiving as building materials get.

Facings, Foil, and What They Do

Foam sheets come plain or with a facing, and the facing changes how the board behaves in an assembly. Foil-faced polyiso reflects radiant heat, which is why it earns a small performance bonus when it faces an air gap, such as the space behind vented siding or above an attic floor. The foil also acts as a vapor barrier, so on a wall in a mixed or cold climate you want to think about which side that barrier lands on, because a vapor barrier trapped on the wrong side can hold moisture in the wall. Plain XPS and EPS are semi-permeable and more forgiving in this regard, letting a wall dry in one direction. Some EPS and XPS boards come with a plastic facing or a fiberglass mat that improves durability or adhesion for stucco and cladding systems.

The practical rule is to match the facing to the job. Behind vented cladding where you want maximum R and a radiant boost, foil-faced polyiso shines. On a wall that needs to dry to the inside, an unfaced or semi-permeable board is safer. Under a slab, facing is irrelevant, so buy the plain board rated for the compressive load. When in doubt, a semi-permeable unfaced board is the least likely to create a moisture trap.

Calculating How Many Sheets You Need

Estimating a foam order is straightforward. A single 4×8 sheet covers 32 square feet. Divide the wall or slab area by 32, then add roughly 10 percent for cutting waste and around penetrations. For a basement wall 8 feet tall and 40 feet long, that is 320 square feet, or about ten 4×8 sheets, plus one or two extra for waste. If you are doubling up to reach a higher R-value, double the count and remember to stagger the seams between layers so no gap runs straight through. Budget for tape too; a wall that size needs one or two rolls of compatible foam tape to seal every joint into a continuous air and thermal layer.

One more cost consideration: thicker single sheets often cost less per R than stacking two thinner ones, and they install faster with fewer seams to tape. If you need R-10, a single 2-inch XPS board usually beats two 1-inch boards on both price and labor, so buy the thickest sheet that fits your fastening and cladding plan rather than layering thin board unnecessarily.

Buying Smart

Before you load a cart, decide two things: how much R-value you need for the surface, and whether that surface will ever get wet or touch the ground. Those two answers point straight to the right board. If it’s dry and above grade and you want max R in minimum thickness, reach for polyiso. If moisture or soil is anywhere in the picture, reach for XPS or rated EPS. Choosing your insulation sheets by matching material to location, rather than grabbing whatever is on sale, is the difference between a wall that stays warm and dry for decades and one that quietly grows a moisture problem behind the drywall.