Rigid foam insulation is a lightweight, solid board material used to reduce heat transfer through walls, roofs, foundations, and floors. Unlike the pink fiberglass batts most people picture when they think of insulation, rigid foam comes in flat panels, typically 4 by 8 feet, that deliver significantly more insulating power per inch of thickness. That compact performance makes it especially useful where space is tight or where you need a continuous layer of insulation over an entire surface.
The Three Main Types
Rigid foam boards fall into three categories, each made from different plastics and suited to slightly different jobs.
Expanded polystyrene (EPS) is the least expensive option. It’s essentially the same material as a disposable coffee cup, formed into dense, sturdy boards. EPS offers roughly R-3.8 to R-4.6 per inch of thickness, depending on density. It’s the most vapor-permeable of the three types, which can be an advantage or a drawback depending on where you use it.
Extruded polystyrene (XPS) is denser and more moisture-resistant than EPS. You’ll recognize it by its color: most manufacturers dye it pink, blue, or green. XPS delivers about R-5 per inch. Its closed-cell structure absorbs very little water (around 0.1% by weight), making it a popular choice for below-grade foundation walls and under concrete slabs where contact with damp soil is constant.
Polyisocyanurate (polyiso) is the highest-performing option, rated at roughly R-6.2 per inch when new. Boards typically ship with foil or fiber-glass facings on both sides, which also act as a near-total vapor barrier. Foil-faced polyiso has a vapor permeance as low as 0.01 perms, meaning virtually no moisture passes through it. That combination of high R-value and built-in vapor control makes polyiso a go-to for commercial roofing and exterior wall sheathing.
How R-Value Compares Across Types
R-value measures a material’s resistance to heat flow. Higher numbers mean better insulation. For rigid foam, the numbers per inch stack up roughly like this:
- EPS: R-3.8 to R-4.6
- XPS: R-5
- Polyiso: R-6.2
These values matter because rigid foam is often chosen specifically for tight spaces. If you need R-10 on an exterior wall, you can get there with about 2 inches of XPS or roughly 1.6 inches of polyiso, compared to over 3 inches of fiberglass batts. That difference adds up fast when you’re finishing a basement or insulating a cathedral ceiling where every inch of depth counts.
Where Rigid Foam Gets Used
Rigid foam boards work in places where other insulation types struggle. The most common applications include foundation walls (both interior and exterior), underneath concrete basement slabs, along the exterior of framed walls as continuous sheathing, and on flat or low-slope roofs. The Department of Energy notes that when installed continuously over framing, rigid foam blocks “thermal short circuits,” meaning it stops heat from conducting through wood studs and metal joists that would otherwise bypass your cavity insulation.
For interior installations, building codes require rigid foam to be covered with at least a half-inch of drywall or another fire-rated material, because the foam can release toxic gases if it burns. Exterior applications need weatherproof facing or cladding over the foam.
Moisture Resistance Varies Significantly
One of the biggest reasons people choose rigid foam over fiberglass or cellulose is moisture performance, but the three types behave very differently with water.
Foil-faced polyiso is essentially waterproof. It absorbs 0% water and blocks nearly all vapor transmission. XPS sits in the middle: its closed-cell structure limits water absorption to about 0.1%, and it has a moderate vapor permeance of around 1 perm per inch. EPS is the most open of the group, absorbing up to 3% of its weight in water and allowing 3.5 perms of vapor through per inch.
This matters most in below-grade applications. If you’re insulating a basement wall that contacts soil, XPS or foil-faced polyiso will hold up far better than EPS over decades. For above-grade walls in dry climates, EPS works fine and costs less. Choosing the wrong type for a wet location can lead to waterlogged insulation that loses its thermal performance and encourages mold.
Polyiso Loses R-Value Over Time
Polyiso boards contain a gas trapped inside their closed cells that insulates better than still air. Over the years, that gas slowly leaks out and is replaced by regular air, a process called thermal drift. The result is a gradual drop in R-value.
Research from Oak Ridge National Laboratory led to a standardized test called long-term thermal resistance (LTTR), which estimates a 15-year average R-value. But real-world testing suggests even LTTR may be optimistic. A 2015 study by RDH Building Science tested three-year-old polyiso panels removed from a roof in British Columbia and found R-values had already dropped up to 25% at low temperatures compared to when the boards were new.
Longer-term data tells a similar story. Samples aged 21 to 28 years measured between R-4 and R-5.5 per inch, well below the R-6.2 printed on the label of new boards. This doesn’t make polyiso a bad choice, but it means you should plan for a realistic aged R-value closer to R-5 or R-5.5 per inch rather than the initial number. In cold climates, where polyiso performance dips further at low temperatures, some builders use XPS below grade and reserve polyiso for warmer applications like walls and roofs.
Compressive Strength and Load-Bearing Use
Rigid foam can support weight, which is why it works under concrete slabs, beneath paving, and in commercial roofing assemblies where foot traffic and equipment loads are a concern. Compressive strength varies widely by product. Polyiso boards commonly range from 16 to 25 PSI, with high-density versions rated up to 140 PSI. XPS boards designed for below-slab use typically fall in the 15 to 60 PSI range depending on density.
For a residential basement slab, a standard 25 PSI board is more than adequate. The concrete distributes loads across a wide area, so the actual pressure on the foam is a fraction of what it could handle. Higher-density boards come into play under commercial floors, parking decks, or cold-storage facilities.
Environmental Considerations
Rigid foam’s environmental footprint has historically been a weak point. XPS in particular relied on HFC-134a as a blowing agent, a gas that traps 1,430 times more heat than carbon dioxide over a 100-year period. For a product designed to save energy, that chemical footprint was hard to justify.
Manufacturers have been transitioning to a newer class of blowing agents called HFOs, which have a global warming potential near 1, essentially the same as CO2 itself. Owens Corning and other major producers now offer XPS made with these low-impact gases. EPS has always used pentane as its blowing agent, which has a relatively low climate impact compared to HFCs. If the environmental profile matters to you, look for products specifically labeled as using HFO-blown formulations.
How It Compares to Other Insulation
Rigid foam isn’t always the right call. Fiberglass batts and blown cellulose cost less per R-value and work perfectly well inside standard wall cavities. Spray foam delivers similar or better R-values and seals air leaks in irregular spaces, but costs significantly more per square foot.
Rigid foam fills a specific niche: situations where you need high R-value in minimal thickness, continuous coverage without gaps at framing, reliable moisture resistance, or structural support under loads. A common approach in high-performance homes is to combine rigid foam on the exterior of the wall sheathing with fiberglass or cellulose in the stud cavities, getting the benefits of both continuous insulation and cost-effective cavity fill.