Insulation Reference Guide
Spray Foam Insulation R-Value Guide
Complete R-value comparison for spray foam, fiberglass, cellulose, mineral wool, and rigid foam board — plus climate zone requirements per IECC 2021.
Last updated: March 2026
What Is R-Value?
R-value measures a material's resistance to heat flow — the higher the R-value, the greater the insulating effectiveness. It is expressed in units of ft²·°F·h/BTU (square feet × degrees Fahrenheit × hours per British Thermal Unit). When comparing insulation options, R-value per inch of thickness is the key metric because it normalizes performance across materials of different thicknesses. R-value is tested per ASTM C518 (Standard Test Method for Steady-State Thermal Transmission Properties) and reported on insulation product labels as required by the FTC's R-Value Rule (16 CFR Part 460).
Insulation R-Value Comparison Table
The table below compares R-values per inch, typical installation thicknesses, moisture and air sealing properties, and recommended applications for the most common insulation materials used in residential and commercial construction.
| Material | R-Value / Inch | Typical Thickness | Total R-Value | Moisture Barrier | Air Seal | Best For |
|---|---|---|---|---|---|---|
| Closed-Cell Spray Foam | R-6.5 | 2–3" | R-13 to R-19.5 | Yes (class II vapor retarder at 2") | Yes | Crawl spaces, basements, exterior walls, metal buildings |
| Open-Cell Spray Foam | R-3.6 | 6–10" | R-21.6 to R-36 | No | Yes | Attics, rooflines, cathedral ceilings, sound dampening |
| Fiberglass Batts | R-2.9 to R-3.8 | 3.5–6.5" | R-11 to R-21 | No (faced batts have a vapor retarder) | No | Standard wall cavities, attic floors (cost-effective) |
| Blown-In Cellulose | R-3.2 to R-3.8 | 10–14" | R-30 to R-49 | No | Partial (dense-pack only) | Attic floors, retrofitting existing walls |
| Mineral Wool (Rockwool) | R-3.3 to R-4.2 | 3.5–5.5" | R-15 to R-23 | No | No | Fire-rated assemblies, exterior continuous insulation, soundproofing |
| Rigid Foam Board (XPS) | R-5.0 | 1–2" | R-5 to R-10 | Yes | Yes (with taped seams) | Foundation walls, below-grade, exterior sheathing |
| Rigid Foam Board (Polyiso) | R-5.7 to R-6.5 | 1–3" | R-5.7 to R-19.5 | Yes (foil-faced) | Yes (with taped seams) | Roof decks, above-sheathing exterior walls |
Sources: U.S. Department of Energy [1], ASTM C518 testing data [3], Spray Polyurethane Foam Alliance [6]. R-values represent typical installed performance.
R-Value Requirements by Climate Zone (IECC 2021)
The International Energy Conservation Code (IECC) sets minimum insulation R-values based on your climate zone. Indiana spans Climate Zones 4 and 5, requiring R-49 in the attic and R-20 or R-13+5 (cavity plus continuous) in walls.
| Zone | Region | Attic / Ceiling | Wall | Floor | Crawlspace Wall |
|---|---|---|---|---|---|
| 1 | South Florida, Hawaii, US territories | R-30 | R-13 | R-13 | R-10 |
| 2 | Gulf Coast, South Texas, Arizona | R-38 | R-13 | R-13 | R-10 |
| 3 | South Atlantic, Southern California, NM, AZ | R-38 | R-20 or 13+5 | R-19 | R-10 |
| 4 | Mid-Atlantic, Pacific NW, Central Indiana | R-49 | R-20 or 13+5 | R-19 | R-10 |
| 5 | Northern Indiana, OH, PA, NJ, southern NE/IA | R-49 | R-20 or 13+5 | R-30 | R-15 |
| 6 | Northern tier states (MN, WI, MI, VT, NH, ME) | R-49 | R-20+5 or 13+10 | R-30 | R-15 |
| 7–8 | Alaska, northern Minnesota/Wisconsin | R-49 | R-20+5 or 13+10 | R-38 | R-15 |
Source: 2021 International Energy Conservation Code, Table R402.1.2 [2]. "R-13+5" notation means R-13 cavity insulation plus R-5 continuous exterior insulation.
Open-Cell vs. Closed-Cell Spray Foam: Detailed Comparison
The two main types of spray polyurethane foam (SPF) differ in cell structure, which determines their physical properties. Open-cell foam has cells that are not fully encapsulated, creating a soft, spongy material that allows moisture vapor to pass through. Closed-cell foam has fully encapsulated cells filled with a blowing agent (typically HFC-245fa or HFO-1233zd), creating a rigid, dense material that acts as both an air barrier and a class II vapor retarder. Open-cell foam (R-3.6 per inch) requires greater thickness to achieve equivalent R-values but costs significantly less per board foot ($0.35–0.45 vs. $1.25–1.40 for closed-cell). Closed-cell foam (R-6.5 per inch) adds structural rigidity, resists moisture intrusion, and meets code requirements at thinner installations — making it the standard for below-grade, crawl space, and exterior wall applications. Open-cell foam excels in attic and roofline applications where its superior sound dampening, lower cost, and ability to be sprayed in thick lifts make it the practical choice.
Open-Cell Spray Foam
- R-Value per inch
- R-3.6
- Density
- 0.5 lb/ft³
- Cost per board foot
- $0.35–$0.45
- Vapor permeability
- Permeable (10+ perms)
- Air barrier
- Yes (at 3.5"+)
Closed-Cell Spray Foam
- R-Value per inch
- R-6.5
- Density
- 2.0 lb/ft³
- Cost per board foot
- $1.25–$1.40
- Vapor permeability
- Class II retarder (at 2"+)
- Air barrier
- Yes (at 1"+)
Frequently Asked Questions
What R-value do I need for my attic in Kokomo?
Kokomo falls in Climate Zone 5 per the IECC. The code requirement is R-49 in the attic. With open-cell spray foam at R-3.6 per inch, this requires approximately 14 inches of foam. Many homeowners achieve R-38 (about 10.5 inches) as a cost-effective balance between performance and budget.
Does spray foam R-value degrade over time?
Open-cell spray foam maintains its R-value indefinitely because it uses air (not a gas) as its insulating medium. Closed-cell foam experiences a small amount of thermal drift as the blowing agent slowly diffuses — typically losing 1–2% of R-value over the first 1–2 years before stabilizing. The aged (long-term) R-value for closed-cell is approximately R-6.0 to R-6.5 per inch, which is the value manufacturers report per ASTM C518 testing at 180 days.
Is higher R-value always better?
Not necessarily. Insulation performance follows the law of diminishing returns — the first inches of insulation provide the most significant energy savings, and each additional inch provides less incremental benefit. Beyond code-minimum R-values, the energy cost savings may not justify the added insulation cost. Air sealing (which spray foam provides inherently) often matters more than raw R-value because air leakage can account for 25–40% of a home's heating and cooling energy loss.
What is the R-value per inch of closed-cell spray foam?
Closed-cell spray polyurethane foam has an R-value of approximately R-6.5 per inch at initial installation, stabilizing to R-6.0 to R-6.5 per inch at the aged (long-term) value. This is the highest R-value per inch of any commonly available insulation material, which is why it is specified when space is limited (thin wall cavities, rim joists, crawl spaces).
How does spray foam compare to fiberglass in real-world performance?
While spray foam and fiberglass batts may have similar R-values per inch on paper (R-3.6 vs. R-3.7 for open-cell vs. high-density fiberglass), spray foam significantly outperforms fiberglass in real-world conditions. Spray foam creates a monolithic air seal that eliminates convective heat loss — a major weakness of fiberglass. Studies by Oak Ridge National Laboratory (ORNL) found that fiberglass batts installed in real buildings achieved only 60–70% of their rated R-value due to gaps, compressions, and convective loops, while spray foam maintained its full rated performance.
Does spray foam insulation add structural strength?
Closed-cell spray foam adds meaningful structural rigidity. Studies have demonstrated that closed-cell SPF applied to wall sheathing can increase racking strength by up to 300%. This is one reason it is popular in metal building and pole barn applications. Open-cell spray foam is too soft to provide structural benefit.
Sources
- [1] U.S. Department of Energy — Insulation Types. https://www.energy.gov/energysaver/insulation. Accessed March 2026.
- [2] 2021 International Energy Conservation Code (IECC) — Table R402.1.2, Insulation Minimum R-Values. https://codes.iccsafe.org/content/IECC2021P7. Accessed March 2026.
- [3] ASTM C518 — Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. https://www.astm.org/c0518-21.html. Accessed March 2026.
- [4] Oak Ridge National Laboratory (ORNL) — Whole-Wall Thermal Performance. https://web.ornl.gov/sci/buildings/tools/whole-wall/. Accessed March 2026.
- [5] FTC R-Value Rule — 16 CFR Part 460, Labeling and Advertising of Home Insulation. https://www.ecfr.gov/current/title-16/chapter-I/subchapter-D/part-460. Accessed March 2026.
- [6] Spray Polyurethane Foam Alliance (SPFA) — Technical Resources. https://sprayfoam.org/technical-resources. Accessed March 2026.
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