Insulation R-Value Per Inch: Every Material Ranked (2026)
Insulation R-Value Per Inch: Every Material Ranked (2026)
Quick Answer: Closed-cell spray foam delivers the highest R-value per inch at R-6.0 to R-7.0. Polyiso rigid foam follows at R-5.6 to R-6.5 (but drops in cold weather). Standard fiberglass batt sits at R-3.0 to R-3.7 per inch. When space is tight, R-per-inch determines which materials can meet your target — but when you have room, cost per R-value matters more.
Table of Contents
- Complete R-Value Per Inch Ranking
- What Fits in Your Wall Cavity
- When R-Per-Inch Matters Most
- Best R-Value Per Dollar
- Temperature Effects on R-Per-Inch
- Thickness Required for Common R-Values
- Key Takeaways
- FAQ
Complete R-Value Per Inch Ranking
We've worked with every material on this list in the field, and the R-per-inch numbers below match what we see in real installations — with one major caveat for polyiso in cold climates (see temperature section below). R-value data is sourced from manufacturer specifications and DOE insulation fact sheets.
| Rank | Material | R-Value Per Inch | Installed Cost/sq ft | Form | Best Application |
|---|---|---|---|---|---|
| 1 | Closed-Cell Spray Foam | R-6.0 – R-7.0 | $1.50 – $5.00 | Spray | Cavity fill + air barrier, limited-depth assemblies |
| 2 | Polyisocyanurate (Polyiso) | R-5.6 – R-6.5* | $0.70 – $1.50 (material) | Rigid board | Exterior continuous insulation, commercial roofing |
| 3 | XPS Foam Board | R-4.5 – R-5.0 | $0.50 – $1.20 (material) | Rigid board | Below-grade, high-moisture areas |
| 4 | EPS Foam Board | R-3.6 – R-4.4 | $0.35 – $0.90 (material) | Rigid board | Foundation walls, EIFS, ICFs |
| 5 | Mineral Wool / Rockwool | R-3.8 – R-4.3 | $1.00 – $2.10 | Batt / board | Fire-rated assemblies, soundproofing, exterior walls |
| 6 | High-Density Fiberglass Batt | R-3.7 – R-4.3 | $0.40 – $1.70 | Batt | 2×4 walls (R-15 in 3.5"), 2×6 walls (R-21 in 5.5") |
| 7 | Dense-Pack Cellulose | R-3.5 – R-3.8 | $1.50 – $3.00 | Blown (dense) | Wall retrofits, drill-and-fill |
| 8 | Open-Cell Spray Foam | R-3.5 – R-3.8 | $1.00 – $3.50 | Spray | Attic rooflines, cathedral ceilings |
| 9 | Standard Fiberglass Batt | R-3.0 – R-3.7 | $0.30 – $1.50 | Batt | New construction walls, attics, budget projects |
| 10 | Loose-Fill Cellulose | R-3.2 – R-3.8 | $0.60 – $2.30 | Blown (loose) | Attic floors, topping off existing insulation |
| 11 | Blown-In Fiberglass | R-2.2 – R-2.7 | $0.50 – $2.00 | Blown (loose) | Open attics (installed thicker to compensate) |
*Polyiso R-value drops to R-3.5–R-4.5 per inch below 25°F mean temperature. See temperature effects below.
Rigid foam board costs listed are material-only. Add $0.50–$1.50/sq ft for professional installation depending on the application. R-values verified against Owens Corning product data and Rockwool Comfortbatt specifications. For a complete material comparison including fire ratings and moisture performance, see our types of insulation guide. For total R-value data and code requirements, see the R-value insulation chart.
Pro Tip: Blown-in fiberglass ranks last in R-per-inch but don't write it off. In an open attic where depth is unlimited, you simply install it thicker. At 20 inches, it delivers R-49 for $0.50–$2.00/sq ft. R-per-inch only matters when your cavity depth is fixed.
What Fits in Your Wall Cavity
Stud cavity depth determines your maximum R-value. Here's what each material delivers in standard framing:
| Material | R-Value in 2×4 (3.5") | R-Value in 2×6 (5.5") | R-Value in 2×8 (7.25") |
|---|---|---|---|
| Closed-Cell Spray Foam | R-21 – R-24.5 | R-33 – R-38.5 | R-43.5 – R-50.8 |
| Polyiso Board* | R-19.6 – R-22.8 | R-30.8 – R-35.8 | R-40.6 – R-47.1 |
| XPS Board | R-15.8 – R-17.5 | R-24.8 – R-27.5 | R-32.6 – R-36.3 |
| Mineral Wool Batt | R-13.3 – R-15 | R-20.9 – R-23.7 | R-27.6 – R-31.2 |
| High-Density Fiberglass | R-13 – R-15 | R-20.4 – R-23.7 | R-26.8 – R-31.2 |
| Dense-Pack Cellulose | R-12.3 – R-13.3 | R-19.3 – R-20.9 | R-25.4 – R-27.6 |
| Open-Cell Spray Foam | R-12.3 – R-13.3 | R-19.3 – R-20.9 | R-25.4 – R-27.6 |
| Standard Fiberglass Batt | R-11 – R-13 | R-19 – R-20.4 | R-21.8 – R-26.8 |
*Polyiso is rarely installed in stud cavities — it's primarily used as exterior continuous insulation. Values shown for comparison purposes.
A 2×4 wall maxes out at about R-15 with cavity insulation alone (using high-density fiberglass or mineral wool). To exceed that, you need continuous insulation on the exterior. That's exactly why zones 5+ in the 2021 IECC require cavity + continuous insulation — the code writers know that cavity insulation alone can't hit the target in a 2×4 or even a 2×6 wall.
Use our R-value calculator to model your exact wall assembly, including framing, sheathing, and air films.
When R-Per-Inch Matters Most
R-value per inch is the deciding metric in two situations, and in our experience, these are the only situations where it should drive your material selection:
Limited cavity depth. When you're insulating an existing 2×4 wall, you have exactly 3.5 inches to work with. Closed-cell spray foam gives you R-21+ in that space; standard fiberglass gives you R-11. That's a real difference. Same story for cathedral ceilings with 2×8 or 2×10 rafters — you need to hit R-30 or R-38 in limited depth, which rules out low-R-per-inch materials.
Retrofit constraints. Adding insulation to a finished wall without tearing off drywall or siding means working within fixed dimensions. Dense-pack cellulose at R-3.5–R-3.8/inch is the standard retrofit approach — blown through small holes into the cavity. In a 2×4 wall, that yields about R-13. To get more, you'd need to add continuous insulation on the exterior during a re-siding project. See our wall insulation guide for retrofit strategies.
When depth is not constrained — open attics, unfinished basements with framed walls — R-per-inch becomes far less important than cost per R-value.
Pro Tip: If you're re-siding your house anyway, that's the perfect time to add 1–2 inches of continuous rigid foam over the sheathing. It adds R-5 to R-13 depending on thickness and material, breaks thermal bridges, and pays for itself in 5–8 years through energy savings. The marginal cost is low since the siding crew is already there.
Best R-Value Per Dollar
When space isn't the constraint, your wallet is. We typically recommend that homeowners start with cost-per-R-value before considering R-per-inch — especially for attic projects where depth is unlimited. Here's how the materials stack up on cost efficiency, ranked by approximate R-value per dollar per square foot:
| Material | Installed Cost/sq ft | R-Value Delivered (Typical) | Approximate R per Dollar | Value Rank |
|---|---|---|---|---|
| Standard Fiberglass Batt | $0.30 – $1.50 | R-13 (3.5") / R-19 (5.5") | R-13 to R-43 per $ | 1st |
| Blown-In Fiberglass | $0.50 – $2.00 | R-38 (at 16") | R-19 to R-76 per $ | 2nd |
| Loose-Fill Cellulose | $0.60 – $2.30 | R-38 (at 11") | R-17 to R-63 per $ | 3rd |
| EPS Foam Board | $0.35 – $0.90 (material) | R-4/inch | R-4.4 to R-11.4 per $ | 4th |
| Mineral Wool Batt | $1.00 – $2.10 | R-15 (3.5") / R-23 (5.5") | R-7 to R-15 per $ | 5th |
| XPS Foam Board | $0.50 – $1.20 (material) | R-5/inch | R-4.2 to R-10 per $ | 6th |
| Open-Cell Spray Foam | $1.00 – $3.50 | R-13 (3.5") | R-3.7 to R-13 per $ | 7th |
| Polyiso Board | $0.70 – $1.50 (material) | R-6/inch | R-4 to R-8.6 per $ | 8th |
| Dense-Pack Cellulose | $1.50 – $3.00 | R-13 (3.5") | R-4.3 to R-8.7 per $ | 9th |
| Closed-Cell Spray Foam | $1.50 – $5.00 | R-21 (3.5") | R-4.2 to R-14 per $ | 10th |
The clear winners on pure value: fiberglass batts for DIY new construction, blown-in cellulose or fiberglass for attic floors. These materials have been around for decades, and the market keeps them affordable.
Spray foam costs 3–5× more per R-value than blown-in options, but it delivers air sealing, moisture control, and structural reinforcement that cheaper materials don't. You're paying for those additional functions, not just R-value. Check our insulation cost calculator to compare project-specific pricing.
Temperature Effects on R-Per-Inch
R-value per inch isn't constant for all materials across all temperatures. The FTC tests everything at 75°F mean temperature, but real buildings see much wider temperature swings.
| Material | R/Inch at 75°F | R/Inch at 25°F Mean | R/Inch at 40°F Mean | Temperature Sensitivity |
|---|---|---|---|---|
| Polyiso | R-5.6 – R-6.5 | R-3.5 – R-4.5 | R-4.5 – R-5.5 | High — loses up to 25% below 50°F mean |
| XPS | R-4.5 – R-5.0 | R-5.0 – R-5.5 | R-4.8 – R-5.2 | Slight improvement in cold |
| EPS | R-3.6 – R-4.4 | R-3.6 – R-4.4 | R-3.6 – R-4.4 | Most stable — minimal change |
| Fiberglass | R-3.0 – R-4.3 | R-3.0 – R-4.3 | R-3.0 – R-4.3 | Stable across range |
| Mineral Wool | R-3.8 – R-4.3 | R-3.8 – R-4.3 | R-3.8 – R-4.3 | Stable across range |
| Cellulose | R-3.2 – R-3.8 | R-3.2 – R-3.8 | R-3.2 – R-3.8 | Stable across range |
The practical takeaway: if you're in climate zones 5–8 and using polyiso on the exterior, derate your R-value calculations by 15–25%. We've seen this firsthand on blower door tests where polyiso-clad walls in Minnesota underperformed the calculated U-factor. Building Science Corporation's research on rigid foam temperature performance documents this extensively. Alternatively, layer polyiso with EPS — the EPS performs at full R-value in cold, offsetting the polyiso derating. Some builders in northern climates have switched to EPS or XPS entirely for exterior continuous insulation to avoid the cold-weather R-value penalty.
Pro Tip: Building Science Corporation research shows the best cold-climate strategy for exterior rigid foam is "polyiso sandwich" — EPS on the outside (cold side) with polyiso on the inside (warm side). The polyiso stays warmer and maintains its R-value; the EPS handles the cold exposure. It's a niche detail but it can deliver 10–15% more real-world R-value than polyiso alone in zones 6+.
Thickness Required for Common R-Values
How many inches of each material do you need to hit standard R-value targets? This is the question every installer asks when specifying material quantities.
| Target R-Value | Fiberglass Batt | Blown Cellulose | Blown Fiberglass | Mineral Wool | Closed-Cell Spray Foam | Open-Cell Spray Foam | XPS Board | Polyiso Board |
|---|---|---|---|---|---|---|---|---|
| R-13 | 3.5" | 3.5"–4" | 5"–6" | 3"–3.5" | 2" | 3.5" | 2.6"–2.9" | 2"–2.3" |
| R-19 | 6.25" | 5"–5.5" | 7"–8.5" | 4.5"–5" | 2.75"–3.2" | 5"–5.5" | 3.8"–4.2" | 2.9"–3.4" |
| R-30 | 9.5"–10" | 8"–9" | 11"–14" | 7"–7.9" | 4.3"–5" | 7.9"–8.6" | 6"–6.7" | 4.6"–5.4" |
| R-38 | 12"–12.5" | 10.5"–11" | 14"–17" | 8.8"–10" | 5.4"–6.3" | 10"–10.9" | 7.6"–8.4" | 5.8"–6.8" |
| R-49 | 15"–16" | 13.5"–14" | 18"–22" | 11.4"–12.9" | 7"–8.2" | 12.9"–14" | 9.8"–10.9" | 7.5"–8.8" |
| R-60 | 18"–19" | 16"–17" | 22"–27" | 14"–15.8" | 8.6"–10" | 15.8"–17.1" | 12"–13.3" | 9.2"–10.7" |
Cellulose thickness assumes settled density — install 20% deeper to account for settling over time.
For complete product-specific thickness data (like which fiberglass batt SKU fits which cavity), see our insulation thickness chart.
Key Takeaways
- Closed-cell spray foam leads at R-6.0–R-7.0 per inch, making it the go-to for space-constrained applications.
- R-per-inch matters when your cavity depth is fixed (existing walls, cathedral ceilings). When depth is unlimited (open attics), cost per R-value matters more.
- Fiberglass and cellulose deliver the best R-value per dollar — 3–5× more cost-effective than spray foam in open spaces.
- Polyiso loses up to 25% of its labeled R-per-inch in cold climates. Derate it in zones 5+ or pair it with EPS.
- A 2×4 wall maxes out at about R-15 with cavity insulation. Exceeding that requires continuous exterior insulation.
- Hitting R-49 for your attic takes about 14 inches of cellulose, 16 inches of fiberglass batts, or 7–8 inches of closed-cell spray foam.
- Don't pick insulation by R-per-inch alone — factor in cost, fire safety, moisture resistance, sound performance, and your specific application.
FAQ
What insulation has the highest R-value per inch?
Closed-cell spray foam at R-6.0 to R-7.0 per inch. Polyiso rigid board is close behind at R-5.6 to R-6.5, but its real-world performance drops in cold weather. For a complete ranking of all materials, reference the chart above or see our full R-value insulation chart.
How much R-value can I fit in a 2×4 wall?
A 2×4 stud cavity is 3.5 inches deep. The maximum cavity R-values are: R-21 to R-24.5 with closed-cell spray foam, R-15 with high-density fiberglass, R-15 with mineral wool, R-13 with standard fiberglass or dense-pack cellulose. To exceed R-15 without spray foam, add continuous rigid foam insulation on the exterior of the wall.
Does R-value per inch change over time?
For most materials, no. Fiberglass, mineral wool, and EPS maintain their R-per-inch indefinitely if they stay dry and uncompressed. XPS loses some blowing agent over decades and its long-term R-value settles from R-5.0 to about R-4.5 per inch (ASTM C578 accounts for this with LTTR testing). Cellulose doesn't lose R-per-inch, but it settles ~20%, reducing total R-value unless it was initially over-installed.
Is higher R-per-inch always worth the extra cost?
No. In open attics, blown-in cellulose at R-3.5/inch installed to 14+ inches is far cheaper than closed-cell spray foam at R-6.5/inch for the same total R-value. High R-per-inch materials are worth the premium only when space is limited or when you need their secondary properties (air sealing, moisture barrier, structural reinforcement). Energy Star's insulation recommendations focus on total R-value targets, not R-per-inch. Run the numbers for your specific project using our insulation cost calculator.