Rigid Insulation R Value: Chart and Buying Guide

Understanding rigid insulation R value is essential when you are insulating walls, foundations, roofs, or basements with foam board, because R-value tells you how well the material resists heat flow. The higher the R-value per inch, the more insulating power you get in a given thickness. Rigid foam board is prized for delivering high R-value in a thin, moisture-resistant panel, making it ideal where space is limited or where you need to add continuous insulation over framing. But not all foam boards perform the same, and choosing the right type and thickness for your climate and application makes a real difference in comfort and energy bills.

This guide explains R-value, compares the three main rigid foam types, provides an R-value chart, and helps you choose the right board.

What R-Value Means

R-value measures thermal resistance, or how effectively a material stops heat from passing through it. It is expressed per inch of thickness and as a total for a given board. Two inches of a material rated R-5 per inch gives you R-10 total. Higher R-values mean better insulation. When comparing insulation, you compare both the R-value per inch and the total R-value you can achieve in your available space. Rigid foam wins where you need high R-value in a thin package, unlike fluffy batts that need more depth.

The Three Main Types of Rigid Foam

Expanded Polystyrene (EPS)

EPS is the most affordable rigid foam, made of fused foam beads (similar in appearance to a foam cooler). It offers about R-3.6 to R-4.2 per inch. EPS is vapor-permeable, drains and dries well, and is often used below grade, under slabs, and in exterior wall systems. It is the lowest R-value per inch of the three but also the least expensive and does not lose R-value over time.

Extruded Polystyrene (XPS)

XPS is the familiar pink or blue board (Owens Corning Foamular and DuPont Styrofoam). It offers about R-5 per inch and is denser and more moisture-resistant than EPS, making it popular for foundations, basements, and below-grade walls. XPS can lose a small amount of R-value over decades as its blowing agent dissipates, settling toward EPS levels.

Polyisocyanurate (Polyiso)

Polyiso, usually foil-faced, offers the highest R-value at about R-5.6 to R-6.5 per inch at typical temperatures. It is common for roofs and exterior wall sheathing. A quirk of polyiso is that its R-value drops in cold temperatures, so in very cold climates its effective performance can be lower than its rated value, something to weigh for exterior use in northern regions.

Rigid Insulation R-Value Chart

• EPS: about R-3.6 to R-4.2 per inch. 1 inch ≈ R-3.8; 2 inches ≈ R-7.6.
• XPS: about R-5.0 per inch. 1 inch ≈ R-5; 2 inches ≈ R-10.
• Polyiso: about R-5.6 to R-6.5 per inch. 1 inch ≈ R-6; 2 inches ≈ R-12 (rated).

These are typical figures; always check the manufacturer’s rating for the specific product and thickness you buy, since values vary by brand and facing.

Common Thicknesses and Their R-Values

• 1/2 inch: roughly R-2 to R-3 depending on type; used as a thin thermal break or sheathing layer.
• 1 inch: roughly R-3.8 (EPS) to R-6 (polyiso); common for continuous exterior insulation.
• 2 inches: roughly R-7.6 to R-12; used on foundations, basements, and roofs.
• 3 to 4 inches: R-11 up to R-24 or more; used where high total R-value is needed, sometimes in layers.

Choosing the Right Foam for Each Application

• Below-grade and foundation walls: XPS or EPS, both moisture-tolerant; XPS for higher R per inch, EPS for value and drying ability.
• Under slabs: EPS or XPS rated for the compressive load.
• Exterior continuous insulation: Polyiso (foil-faced) for high R-value and as a partial air/water barrier, or EPS/XPS depending on moisture strategy.
• Roofs: Polyiso is the standard for its high R-value per inch.
• Interior basement walls: XPS or foil-faced polyiso, with attention to code-required ignition/thermal barriers.

How Much R-Value Do You Need?

Recommended insulation levels depend on your climate zone. The U.S. Department of Energy publishes recommended R-values by zone; colder northern zones call for higher total R-values in walls, basements, and roofs than warm southern zones. When adding rigid foam as continuous insulation over framing, building codes in cold climates specify a minimum foam R-value to keep the sheathing warm enough to avoid condensation. Check your local code and climate zone to size the foam correctly.

Continuous Insulation and Thermal Bridging

One of the biggest advantages of rigid foam is that it provides continuous insulation across studs, which are themselves poor insulators. Wood and steel framing create thermal bridges that let heat bypass the cavity insulation. A layer of rigid foam over the exterior wraps the framing in an unbroken blanket, cutting these losses and boosting the whole-wall R-value well beyond what cavity insulation alone provides. This is why continuous rigid foam is increasingly required and recommended in energy codes.

Rigid Foam vs. Fiberglass and Spray Foam

Homeowners often weigh rigid foam board against other insulation types, and the right choice depends on the job. Fiberglass batts are cheaper and easy to install in open stud cavities, but they only reach about R-3.1 to R-4.3 per inch and do nothing to stop thermal bridging across the framing or air leakage on their own. Rigid foam shines precisely where batts fall short: as a continuous layer over studs or on the exterior, where it wraps the framing in unbroken insulation and, when seams are taped, helps control air movement.

Spray foam offers similar or higher R-values per inch and seals air leaks in place, but it is more expensive and must be professionally applied for large jobs. Rigid board occupies a useful middle ground: higher R-value per inch than batts, moisture resistance for below-grade and exterior use, and DIY-friendly installation for the handy homeowner. In many well-built assemblies, the best results come from combining materials, such as cavity insulation between studs plus a continuous layer of rigid foam over the sheathing, which pairs the strengths of each.

Installing Rigid Foam Correctly

Getting the rated R-value in the real world depends on proper installation. Boards should be cut to fit snugly with minimal gaps, and the seams between panels should be sealed with the appropriate tape or spray foam so air cannot leak through them and rob you of performance. When installing multiple layers to reach a high total R-value, stagger the seams between layers, like bricks, so no joint runs straight through both layers; this reduces both thermal bridging and air leakage at the seams.

Fastening matters too. Use fasteners rated for the board thickness and the substrate, and follow spacing guidelines so the foam stays flat and secure. On exterior walls, integrate the foam with the home’s water-resistive barrier and flashing so water drains properly and does not get trapped behind the board. Finally, respect fire-safety code: most building codes require rigid foam in living spaces to be covered by a thermal or ignition barrier, commonly half-inch drywall, because the foam is combustible. Following these steps ensures the foam delivers its full rated R-value safely and for the life of the building.

Common Mistakes to Avoid

• Ignoring temperature effects on polyiso: Its R-value drops in cold; account for this in northern climates.
• Assuming XPS keeps its rated R-value forever: It can drift down slightly over decades.
• Wrong foam for moisture conditions: Use moisture-tolerant EPS or XPS below grade, not unfaced polyiso.
• Skipping the ignition barrier: Code usually requires a thermal or ignition barrier like drywall over foam in living spaces.
• Not taping seams: Unsealed board seams leak air; tape them to maximize performance.

Choosing the Right Board

Weigh R-value per inch, moisture resistance, cost, and your available thickness. Polyiso gives the most R-value per inch and suits roofs and above-grade walls; XPS balances high R-value and moisture resistance for foundations; EPS offers the best value and reliable long-term performance, especially below grade. Layer boards and stagger and tape the seams to reduce thermal bridging and air leakage, and always confirm the specific product’s rated R-value rather than relying on general figures.

Getting the rigid insulation R value right for your climate and application is the key to a comfortable, energy-efficient home. Match the foam type to the location, use enough thickness to hit your target total R-value, add it as continuous insulation to beat thermal bridging, and follow code for barriers and moisture. Do that, and rigid foam board will deliver strong, lasting insulating performance in a remarkably thin, durable package.