Insulation R-Value Chart Per Inch — Every Type Compared

"What's the R-value per inch for spray foam vs fiberglass vs rockwool? I keep seeing different numbers everywhere and I just need one reliable chart that compares everything side by side."

We hear you. Insulation R-values are scattered across dozens of manufacturer spec sheets, building codes, and half-accurate blog posts. Fiberglass batts, blown-in cellulose, closed-cell spray foam, rigid foam boards — each has a different R-value per inch, a different cost, and a different best-use scenario.

That's why we built the most comprehensive insulation R-value chart you'll find anywhere. Below, we consolidate R-values for 15 insulation types, compare them per inch of thickness, break down the 2021 IECC code requirements by climate zone, and include cost-per-R-value data so you can make an informed decision.

Here's what you need to know before we dive in. R-value measures thermal resistance — how well a material resists heat flow. The higher the R-value, the better the insulation. R-value scales linearly with thickness: if a material has R-3.7 per inch, then 3.5 inches of it gives you R-13.

Let's start with the master chart.

Master Insulation R-Value Chart (All Types Compared)

This is the complete R-value per inch comparison for every common residential insulation type. We've included the R-value at standard 2×4 and 2×6 cavity depths, plus the typical cost range and best applications.

Sources: DOE (energy.gov/energysaver/insulation), Owens Corning, Rockwool, PIMA QualityMark, Building Science Corporation, HomeGuide, manufacturer data sheets.

Here's the takeaway from this chart. Closed-cell spray foam wins on R-value per inch at R-6.0 to R-7.0, but it's also the most expensive and requires professional installation. For most homeowners, fiberglass batts and rockwool offer the best balance of performance, cost, and DIY accessibility. Polyiso rigid foam delivers excellent R-value per inch for continuous insulation — but watch out for its cold-weather performance issues discussed below.

R-Value Per Inch By Insulation Type

Each insulation type below follows the same format: a quick spec table, a practical overview, and the best applications. If you're trying to figure out which insulation to use for a specific project, this is where you'll find your answer.

Fiberglass Batt R-Value Per Inch

Fiberglass batt is the most widely used residential insulation in the United States. Standard-density batts come in at R-2.9 to R-3.8 per inch, while high-density batts push up to R-4.3 per inch. The R-value depends heavily on the product's density — a standard R-13 batt at 3.5" delivers R-3.71 per inch, while a high-density R-15 at 3.5" delivers R-4.28 per inch.

One critical rule with fiberglass: never compress the batts. An R-19 batt (rated at 6.25" thickness) compressed into a 5.5" cavity drops to approximately R-18. If you stuff an R-19 into a 3.5" cavity, you'll get roughly R-13 — you've paid for R-19 and gotten R-13.

Fiberglass batts are ideal for standard wall cavities, attic floors, and floors over unconditioned spaces. They're DIY-friendly, widely available, and work well when installed properly with no gaps, compressions, or voids. For sizing your heating system around your insulation levels, check out our heating BTU calculator.

Blown-In Fiberglass Insulation R-Value

Blown-in fiberglass has the lowest R-value per inch of any common insulation at R-2.2 to R-2.7. That means you need roughly 18-19 inches to hit R-49 in an attic. It's lightweight, doesn't settle significantly over time, and resists moisture better than cellulose.

The main advantage of blown fiberglass is speed of installation in attics. A crew can insulate an entire attic floor in a few hours. It's also available for DIY installation with rental blowers from home improvement stores.

Cellulose Blown-In Insulation R-Value

Cellulose delivers noticeably better R-value per inch than blown fiberglass — R-3.2 to R-3.8 versus R-2.2 to R-2.7. That means you need about 13 inches of cellulose to reach R-49 in an attic, compared to 18-19 inches of fiberglass. It's also one of the most eco-friendly insulation options, made from roughly 85% recycled post-consumer paper.

One important caveat: cellulose settles. In open attic applications, expect 15-20% settling over time. Professional installers compensate by over-blowing to the settled R-value specification. Dense-pack cellulose in wall cavities (installed at 3.5+ lb/cu ft) resists settling and provides excellent air sealing.

Dense-pack cellulose is one of the best retrofit insulation options for existing walls. Installers drill small holes, blow the cellulose to high density, and patch the holes — no drywall removal needed. Better insulation directly reduces your heating costs and your cooling load.

Open-Cell Spray Foam R-Value Per Inch

Open-cell spray foam sits right in the middle of the R-value spectrum at R-3.5 to R-3.7 per inch — comparable to dense-pack cellulose and rockwool. Its big advantage isn't R-value; it's the air seal. Open-cell foam expands to fill every crack and gap, creating a continuous air barrier that batts can't match.

Because open-cell foam is vapor-permeable (~8 perms per inch), it allows moisture to pass through rather than trapping it. This makes it a good choice for cathedral ceilings and interior wall cavities. However, it should not be used below grade or in flood-prone areas — it absorbs water like a sponge.

Open-cell foam requires professional installation. Improper mixing ratios can cause long-term off-gassing issues, so always hire a certified installer. Budget 24 hours for curing before re-occupying the space.

Closed-Cell Spray Foam R-Value Per Inch

Closed-cell spray foam has the highest R-value per inch of any common insulation at R-6.0 to R-7.0. A 2×4 wall cavity filled with 3.5" of closed-cell foam delivers R-21 to R-24.5 — nearly double what fiberglass batts achieve in the same space. At 2" thickness, closed-cell foam also functions as its own vapor retarder (~1 perm), eliminating the need for a separate vapor barrier.

The tradeoff is cost. Closed-cell spray foam runs $1.50 to $4.50 per square foot for a typical 3.5" application — roughly 3-4× the cost of fiberglass batts for the same coverage area. It also requires professional installation with specialized equipment.

Closed-cell foam is the go-to for basements, crawl spaces, rim joists, and exterior walls where moisture resistance matters. It adds structural rigidity to wall assemblies and doesn't absorb water. If your furnace sizing calculations show borderline capacity, upgrading to closed-cell foam can meaningfully reduce your heating load.

EPS Rigid Foam Insulation R-Value (Expanded Polystyrene)

EPS is the most cost-effective rigid foam on a per-R-value basis. Standard Type I EPS delivers R-3.6 per inch; high-density Type IX hits R-4.2 per inch. Unlike XPS and polyiso, EPS does not experience thermal drift — its R-value remains stable for the life of the product because it uses trapped air rather than high-performance blowing agents.

EPS is also the most vapor-permeable rigid foam (~5 perms at 1"), which allows wall assemblies to dry in both directions. This makes it an excellent choice for below-grade applications where EPS needs to handle moisture cycling. Research by the Building Science Corporation confirms that EPS retains more of its R-value than XPS in long-term below-grade moisture exposure.

Best applications: below-grade foundation insulation, ICF construction, SIPs, and budget continuous insulation projects. Not recommended for torch-applied roofing or areas with direct UV exposure (requires a protective covering).

XPS Rigid Foam Insulation R-Value (Extruded Polystyrene)

XPS (the blue, pink, or green foam boards at your local home improvement store) delivers a reliable R-5.0 per inch. It has excellent moisture resistance — less than 1% water absorption by volume — and high compressive strength, making it the standard choice for under-slab insulation and foundation walls.

There's one catch: XPS loses R-value over time. The HFC blowing agents that give XPS its higher initial R-value slowly diffuse out over decades and are replaced by air. Long-term R-value settles closer to R-4.5 per inch. XPS also has a higher global warming potential (GWP) than EPS or polyiso due to those same HFC blowing agents, though manufacturers are transitioning to lower-GWP alternatives.

Best applications: below-grade walls, under concrete slabs, parking decks, and any application requiring high compressive strength plus moisture resistance. For above-grade walls, polyiso offers better R-value per inch in warm climates.

Polyisocyanurate (Polyiso) R-Value Per Inch

Polyiso delivers the highest labeled R-value per inch of any rigid foam board at R-5.6 to R-6.5 (LTTR). It's the most common commercial roofing insulation and is widely used as exterior continuous insulation on residential walls. The foil facing provides a radiant barrier and acts as a Class I vapor retarder (less than 0.05 perms).

However, polyiso has a well-documented weakness: its R-value drops significantly in cold weather. We cover this in detail in the polyiso derating section below. For this reason, the National Roofing Contractors Association recommends using R-5.6 per inch for design calculations rather than the labeled LTTR value. In Climate Zones 6-8, consider using XPS or EPS on the cold side of your assembly instead.

Best applications: above-grade wall sheathing (CZ 1-5), commercial roofing, and any warm-climate continuous insulation application. Polyiso is your best R-per-inch value when the mean temperature across the foam stays above 40°F.

Rockwool (Mineral Wool) Insulation R-Value Per Inch

Rockwool (the brand name that's become the generic term for stone wool or mineral wool insulation) delivers R-3.7 to R-4.2 per inch — about 10-25% higher than standard fiberglass batts. But R-value isn't really why people choose rockwool.

The real selling points are fire resistance and soundproofing. Rockwool can withstand temperatures up to 2,150°F without melting, burning, or producing toxic smoke. It's truly non-combustible per ASTM E136. For sound, rockwool batts achieve STC 43-45 compared to fiberglass at STC 32-35 — a noticeable difference in noise reduction.

Rockwool is also hydrophobic. Water runs right through it without being absorbed, so it won't lose R-value when exposed to moisture. This makes it excellent for exterior applications, basement walls, and any area prone to occasional moisture exposure. For understanding how insulation quality affects your heat pump sizing, rockwool's consistent performance in real-world conditions is a meaningful advantage.

IECC Minimum R-Value Requirements By Climate Zone

The 2021 International Energy Conservation Code (IECC) sets the legal minimum insulation levels for new construction and major renovations across the United States. These values are found in Table R402.1.3 of the 2021 IECC. Understanding your climate zone is critical for properly sizing any HVAC system.

2021 IECC Table R402.1.3 — Minimum Insulation R-Values

ci = continuous insulation (exterior). "R-13+R-10ci" means R-13 cavity insulation plus R-10 continuous exterior insulation. Source: BASC/DOE (basc.pnnl.gov), adapted from 2021 IECC Table R402.1.3.

The biggest change in the 2021 IECC compared to older codes: ceiling insulation went up across the board. Climate Zones 2-3 jumped from R-38 to R-49. Climate Zones 4-8 jumped from R-49 to R-60. Wall insulation also increased significantly — Climate Zone 4+ walls now require either R-30 cavity insulation or a cavity-plus-continuous combination.

ENERGY STAR Recommended R-Values for Existing Home Retrofits

If you're upgrading insulation in an existing home, ENERGY STAR recommendations (based on the 2021 IECC) provide practical guidance:

Source: ENERGY STAR (energystar.gov/saveathome/seal_insulate).

For walls, ENERGY STAR recommends blowing insulation into empty wall cavities whenever siding is replaced, and adding continuous exterior insulation: R-5ci in Zone 3, R-5 to R-10ci in Zones 4-8.

R-Value By Application

Attic Insulation R-Value — What You Need

The attic is where the biggest energy savings are. Heat rises, and an under-insulated attic is the #1 source of heat loss in most homes. The 2021 IECC requires R-30 to R-60 depending on your climate zone, and most of the country (Zones 4-8) needs R-60.

To hit R-49 in an attic, you need approximately 13 inches of cellulose, 15.5 inches of fiberglass batts, or 19 inches of blown fiberglass. For R-60, add roughly 25% more depth. Our heating BTU calculator can show you exactly how much your heating load changes with different attic insulation levels.

Wall Insulation R-Value — 2×4 vs 2×6 Walls

For a 2×4 wall (3.5" cavity), your options max out at:

• Fiberglass batt: R-13 to R-15
• Rockwool batt: R-15
• Closed-cell spray foam: R-21 to R-24.5

For a 2×6 wall (5.5" cavity), you can reach:

• Fiberglass batt: R-19 to R-21
• Rockwool batt: R-23
• Closed-cell spray foam: R-33 to R-38.5

The 2021 IECC effectively mandates continuous exterior insulation for Zones 4+ on top of cavity insulation. This eliminates thermal bridging through the studs and is a major shift from older codes that only required cavity insulation. The combination of cavity insulation plus continuous exterior insulation is now the standard approach for energy-efficient wall assemblies.

Floor, Basement, and Crawl Space Insulation R-Value

Floors over unconditioned spaces (above garages, over crawl spaces) require R-13 to R-38 depending on climate zone. Fiberglass batts or rockwool batts are the standard choice, supported between floor joists with wire supports or stapled to the framing.

Basement walls need R-10 to R-19 in Zones 3-8. Rigid foam board (XPS or EPS) directly against the concrete is the most common approach, often with a stud wall and fiberglass batts in front. Closed-cell spray foam applied directly to the basement wall is the premium option — it insulates, air seals, and vapor retards in one step.

Duct Insulation R-Value Requirements

The 2021 IECC requires R-8 minimum for supply and return ducts (3" diameter and larger) located outside conditioned space, and R-6 for ducts smaller than 3". Ducts in ventilated attics must be insulated to R-8 and surrounded by at least R-19 of ceiling insulation.

For more on how duct configuration affects system efficiency, see our guide on cold air return vents and AFUE ratings.

Rigid Foam Comparison: XPS vs EPS vs Polyiso

This is one of the most common questions in building science: which rigid foam board should I use? Here's the head-to-head comparison.

The bottom line: Use polyiso for above-grade walls and roofing in Climate Zones 1-5. Use XPS for below-grade, under-slab, and high-moisture applications. Use EPS when budget matters, when you need long-term R-value stability, or for below-grade in moderate climates. Understanding how your building envelope affects heating and cooling loads ties directly into your gas vs electric heating analysis.

Polyiso Cold-Temperature R-Value Derating

This section matters. If you're building in a cold climate and planning to use polyiso as your exterior continuous insulation, you need to understand this phenomenon before specifying your wall assembly.

What Happens to Polyiso Below 50°F

Research by the Building Science Corporation (BSC Info-502) and confirmed by multiple independent studies has shown that polyiso's R-value drops significantly when the mean temperature across the foam falls below 50°F. While most insulation materials actually improve at lower temperatures (less conduction, less convection, less radiation), polyiso does the opposite.

Sources: BSC Info-502, GreenBuildingAdvisor, Energy Vanguard, GAF Building Science.

At a mean temperature of 25°F — which represents a common winter condition in Climate Zones 5-7 — polyiso can lose 30-45% of its labeled R-value. At that point, XPS actually outperforms polyiso despite having a lower labeled R-value.

Why Does This Happen?

The hypothesis, supported by BSC research, is that the blowing agent gas trapped in polyiso's closed cells condenses at temperatures below approximately 50°F. When the gas condenses, it coats the cell walls with a conductive liquid, dramatically increasing heat transfer through the foam. XPS and EPS use different blowing agents (or air) that don't condense at these temperatures, which is why their performance improves in the cold.

What To Do About It

If you're building in Climate Zones 5-8, you have three practical options:

1. Use XPS or EPS instead of polyiso for exterior continuous insulation. XPS performs better than its label in cold weather, not worse.

2. Design a hybrid assembly with polyiso on the warm side and XPS or EPS on the cold side. This keeps the polyiso above its derating threshold while getting the benefit of its higher warm-weather R-value.

3. Oversize the polyiso by using R-5.0 per inch rather than R-6.0 for your design calculations. The NRCA has recommended R-5.6 per inch for design purposes since 1987.

This is a critical consideration when calculating your heat pump sizing or furnace sizing — if your wall assembly delivers less R-value than designed, your heating load will be higher than expected.

Cost Per R-Value Comparison

When comparing insulation types, cost per square foot doesn't tell the whole story. What matters is cost per R-1 per square foot — how much do you pay for each unit of thermal resistance?

On a pure cost-per-R basis, rigid foam boards are the most cost-effective insulation you can buy. EPS leads the pack at $0.06 to $0.14 per R-1 per square foot. Fiberglass batts and cellulose offer good value for cavity insulation. Rockwool costs more per R-value, but you're paying for fire resistance and soundproofing that other materials can't match.

Spray foam looks expensive in this chart, but remember: spray foam is simultaneously insulation, air barrier, and (for closed-cell) vapor retarder. If you'd otherwise need to buy separate air sealing products and vapor retarders, the total installed system cost gap narrows considerably. Better insulation directly reduces your cost to run electric heaters and all other heating equipment.

Worked Examples

Example 1: Insulating an Attic in Climate Zone 5

Scenario: You have a 1,500 sq ft attic floor in Indianapolis (Climate Zone 5) with 4 inches of existing fiberglass (approximately R-11). The 2021 IECC requires R-60 for ceilings in Zone 5, and ENERGY STAR recommends R-49 for retrofits with existing insulation.

Solution: You need to add approximately R-38 to R-49 on top of your existing R-11. Using blown-in cellulose at R-3.5 per inch, you'd need 11 to 14 inches of additional cellulose. At $1.00 to $1.50 per sq ft installed, the project would cost approximately $1,500 to $2,250 for the 1,500 sq ft attic.

Best choice: Blown-in cellulose. It's affordable, conforms to irregular surfaces, fills around existing batts, and delivers better R-value per inch than blown fiberglass. This upgrade would bring your attic to R-49 to R-60, matching or exceeding code requirements.

Example 2: Choosing Wall Insulation for a New Build in Climate Zone 4

Scenario: New construction in Nashville, Tennessee (Climate Zone 4). 2×6 framing. The 2021 IECC requires R-30 for wood-frame walls, or R-20+R-5ci, or R-13+R-10ci.

Solution: The most practical path is R-20 cavity insulation + R-5 continuous exterior insulation. Fill the 2×6 cavities with R-20 fiberglass batts or R-23 rockwool batts, then add 1" of polyiso (R-6.0) on the exterior. Total wall R-value: R-26 to R-29 (cavity) + R-6 (continuous) = R-32 to R-35, which comfortably exceeds code.

Best choice: Rockwool R-23 batts in the cavity (for fire and sound performance) plus 1" foil-faced polyiso on the exterior. Nashville's mild winters mean polyiso cold-weather derating isn't a concern here.

Example 3: Upgrading 1970s Home Insulation

Scenario: 2,200 sq ft ranch home built in 1972 in Chicago (Climate Zone 5). Original insulation: R-11 fiberglass batts in 2×4 walls, R-19 in the attic. Current code requires R-60 ceiling and R-30 wall (or equivalent).

Solution: Two priorities — attic first, then walls during any siding project.

Attic: Blow 15 inches of cellulose on top of the existing R-19 attic insulation. This adds approximately R-52, bringing the total to roughly R-71. Cost: approximately $2,200 to $3,300.

Walls (during siding replacement): Dense-pack cellulose into the existing 2×4 cavities (bringing them to full R-13 to R-15) plus add 2" of XPS rigid foam (R-10) under the new siding. Total wall R-value: approximately R-23 to R-25. Not quite R-30, but a massive improvement over R-11 and a reasonable retrofit target.

Example 4: Insulating a Detached Garage

Scenario: 24×24 ft detached garage in Denver (Climate Zone 5) that you want to use as a workshop. No existing insulation. 2×4 walls, exposed ceiling joists.

Solution: Walls: R-15 fiberglass or rockwool batts in the 2×4 cavities. Ceiling: blown cellulose to R-49 (14 inches). The 2021 IECC requires R-24 ceiling and R-13 wall for heated garages in Zone 5 — we're exceeding that for the ceiling to match the comfort of the attached house.

Cost estimate: Walls (576 sq ft): $290 to $865 with fiberglass batts. Ceiling (576 sq ft): $575 to $865 with blown cellulose. Total: approximately $865 to $1,730 for a well-insulated workshop. This insulation level significantly affects your heating cost calculations for maintaining a comfortable 65°F workshop temperature.

Example 5: Duct Insulation Selection

Scenario: Replacing flex ductwork in an unconditioned attic in Charlotte, NC (Climate Zone 4). The 2021 IECC requires R-8 minimum for ducts in unconditioned spaces.

Solution: Standard R-8 insulated flex duct meets code minimum. However, in a hot attic that can reach 140°F in summer, we'd recommend upgrading to R-8 duct insulation plus ensuring the ducts are buried under at least R-19 of attic insulation (per 2021 IECC Section R403.3). This reduces the temperature differential the duct insulation has to manage and improves system efficiency. Proper duct insulation pairs with understanding your MERV rating chart for complete HVAC system optimization.

FAQ — Insulation R-Values

What Insulation Has the Highest R-Value Per Inch?

Closed-cell spray foam at R-6.0 to R-7.0 per inch is the highest R-value common residential insulation. Aerogel insulation reaches R-10.0 per inch but costs $5+ per square foot and is typically reserved for industrial and space-constrained applications. Among rigid boards, polyiso leads at R-5.6 to R-6.5 per inch (LTTR), but remember its cold-temperature derating.

What R-Value Insulation Do I Need for My Attic?

The 2021 IECC requires R-30 (Zone 1), R-49 (Zones 2-3), or R-60 (Zones 4-8) for ceilings/attics. Most of the US falls in Zones 4-8, which means R-60 is the target for new construction. For retrofits, ENERGY STAR recommends R-49 if you already have 3-4 inches of existing insulation. Check your local building department — your jurisdiction may not have adopted the 2021 IECC yet.

Does Polyiso Lose R-Value in Cold Weather?

Yes. Building Science Corporation testing confirms that polyiso's R-value drops from a labeled R-6.0 to R-6.5 per inch down to approximately R-3.5 to R-4.5 per inch when the mean temperature through the foam drops to 25°F. This is caused by the condensation of blowing agent gases within the closed cells. XPS and EPS do not exhibit this behavior — they actually improve slightly in cold weather. In cold climates (CZ 6-8), consider XPS or a polyiso/XPS hybrid assembly.

What Is the R-Value of a 2×4 Wall With Insulation?

A standard 2×4 wall with fiberglass batts delivers R-13 to R-15 in the cavity. With closed-cell spray foam, you can reach R-21 to R-24.5 in the same 3.5" cavity. Add continuous exterior insulation and the total assembly reaches R-18 to R-35 depending on the insulation type and thickness. The 2021 IECC requires cavity plus continuous insulation in most climate zones for new construction.

Is R-30 or R-38 Better for an Attic?

R-38 is better, and the 2021 IECC actually requires R-49 to R-60 for most of the United States. R-30 only meets code in Climate Zone 1 (southern Florida, Hawaii, territories). If you're adding attic insulation, go to at least R-49 — the incremental cost of going from R-30 to R-49 is modest compared to the energy savings over the life of the home. The attic is the single most cost-effective place to add insulation.

What Is the Difference Between XPS, EPS, and Polyiso?

All three are rigid foam boards, but they differ in R-value, moisture resistance, fire behavior, and cost. Polyiso has the highest R-value (R-5.6 to R-6.5/inch) but loses performance in cold weather. XPS offers R-5.0/inch with excellent moisture resistance, ideal for below-grade. EPS provides R-3.6 to R-4.2/inch at the lowest cost with stable long-term R-value. See our full rigid foam comparison table for the detailed breakdown.