Insulation R-Value Calculator
Compute the total thermal resistance (R-value) of an insulation layer from its thickness and material-specific R-value per inch. Useful for meeting energy-code targets in walls, attics, basements, and floors.
Last updated: May 2026
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About this calculator
R-value measures a material's resistance to conductive heat flow per unit area: higher R-value means slower heat transfer, lower utility bills, and better thermal comfort. The formula is R_total = T × R_per_inch, where T is insulation thickness in inches and R_per_inch is the manufacturer's published R-value per inch of thickness for that material. Variables: thickness ranges from less than 1 inch (radiant barriers, rigid foam strips) to 16+ inches (deep attic cellulose blow-ins); R_per_inch ranges from R-2.0/in (loose-fill fiberglass) to R-7+ /in (closed-cell spray polyurethane foam at full cure). Material reference values per ICC and ASHRAE: fiberglass batt R-3.1–3.7/in, loose-fill cellulose R-3.2–3.8/in, mineral wool R-3.0–3.3/in, expanded polystyrene (EPS) rigid R-3.6–4.2/in, extruded polystyrene (XPS) rigid R-5.0/in (de-rated from initial R-5.4 after gas escape), open-cell spray foam R-3.5–3.8/in, closed-cell spray foam R-6.0–7.0/in, polyisocyanurate R-6.5/in (de-rated in cold weather), aerogel R-9.0+/in. Edge cases: R-value is for steady-state conductive flow only — convection through gaps, air infiltration, and thermal bridging through framing can reduce whole-wall effective R-value 20–40% below the insulation's nominal R. Studs in a 2×6 wall with R-19 batts produce a whole-wall effective R around R-14–15 due to framing thermal bridging (studs are R-1.1/in). Temperature dependence: polyiso loses ~20% of R-value below 25 °F, while XPS and fiberglass are largely temperature-independent. The IECC climate-zone R-value table is the regulatory standard; verify your specific jurisdiction's minimum requirements.
How to use
Example 1 — fiberglass-batt wall cavity. Use 5.5-inch thick R-19 fiberglass batt (manufacturer-rated; effective R_per_inch ≈ 3.45). Step 1: R_total = 5.5 × 3.45 = R-19.0. Or simply use the labeled R-19 directly. To meet IECC 2021 zone 4 wall requirement of R-20 cavity + R-5 continuous, you would need either to upgrade the batt to R-21 (high-density fiberglass, 5.5 inches at R_per_inch 3.8) plus 1 inch of XPS continuous (5 × 1 = R-5), giving R-26 effective, beating the requirement. Verify: continuous R-5 + cavity R-21 = R-26 system R-value. Example 2 — attic cellulose blow-in upgrade. Existing attic has 6 inches of cellulose at R-3.5/in = R-21. To reach IECC zone 5 attic requirement of R-49, add: required additional R = 49 − 21 = R-28. At R-3.5/in, additional thickness = 28 / 3.5 = 8 inches. New total: 6 + 8 = 14 inches at R-3.5/in = R-49. Verify: 14 × 3.5 = 49 ✓. Confirm the joist depth and the eave-baffle clearance accommodate 14 inches of insulation without blocking soffit ventilation; if not, switch to a higher-R material like closed-cell spray foam at R-6/in (only 8.2 inches required total) to fit within available depth.
Frequently asked questions
What R-value does the IECC require for walls, attics, and floors in my climate zone?
The 2021 IECC (International Energy Conservation Code) sets prescriptive R-value requirements that vary by climate zone (1 warmest to 8 coldest). For walls (wood-frame): zones 1–2 require R-13 cavity; zones 3 require R-20 or R-13+R-5 continuous; zones 4–5 require R-20+R-5 or R-13+R-10 continuous; zones 6–8 require R-20+R-5 or R-13+R-17 continuous. For ceilings/attics: zone 1 R-30; zone 2 R-49; zones 3–8 R-49 to R-60. For floors over unconditioned space: zones 1–3 R-13–19; zones 4–8 R-19–38. Basement walls and crawlspace walls have their own tables (R-10/13 to R-15/19 continuous/cavity in colder zones). Many jurisdictions adopt the IECC with amendments — California Title 24, Washington WSEC, and Massachusetts Stretch Code are notably stricter. Always check the current edition adopted by your local building department; codes update on a 3-year cycle and amendments vary widely.
Why does the effective R-value of a wall differ from the rated R-value of the insulation alone?
Insulation R-value is measured on a uniform sample at controlled lab conditions, but a real wall has wood framing (studs, plates, headers) that acts as a thermal bridge. Wood is roughly R-1.1/in, so a 2×4 stud (3.5 inches deep) has R ≈ 3.9 versus the R-15 fiberglass batt in the same cavity — a 4× thermal short-circuit. A typical 2×6 wood-frame wall is about 23% framing by area, dragging the whole-wall effective R-value from R-19 cavity to about R-14 effective. Steel framing is much worse: a 2×6 steel-stud wall with R-19 batt has effective R-value around R-7 due to steel's R-1/inch-equivalent conductivity. Continuous exterior insulation (rigid foam outside the studs) breaks the thermal bridge and recovers most of the lost R-value, which is why IECC adds the continuous R-5 to R-17 requirement in cold zones. Air leakage adds further losses beyond conductive R-value.
How do different insulation materials compare for R-value, cost, and other properties?
Fiberglass batt: R-3.1–3.7/in, $0.40–0.80/sq ft installed, easy DIY, mold-resistant but loses R-value if compressed. Loose-fill cellulose: R-3.2–3.8/in, $0.50–1.20/sq ft installed, eco-friendly recycled paper, settles 10–20% over time. Mineral wool batt: R-3.0–3.3/in, $0.80–1.50/sq ft installed, fire-resistant to 1,000+ °F, sound-dampening, moisture-tolerant. Closed-cell spray foam: R-6.0–7.0/in, $1.20–2.50/sq ft installed, also air seals, water-resistant, but high-cost and HFC global-warming concerns. Open-cell spray foam: R-3.5–3.8/in, $0.50–1.00/sq ft installed, air seals but absorbs water. XPS rigid foam: R-5/in (post-blowing-agent escape), $0.80–1.50/sq ft, good for below-grade and exterior. Polyiso rigid: R-6–6.5/in, $0.70–1.50/sq ft, foil-faced, best above-grade but loses R below 25 °F. Aerogel and vacuum-insulated panels reach R-9–25/in but cost 5–10× more — used only where space is critical.
What are common mistakes when planning insulation R-values?
The most common mistake is compressing insulation to fit thinner cavities — fiberglass loses R-value linearly with compression, so a 6.25-inch R-19 batt jammed into a 3.5-inch 2×4 cavity gives only about R-11. Another error is ignoring thermal bridging through studs; the rated R-value on the label is not the whole-wall R-value. Mixing imperial R (hr·ft²·°F/Btu) and SI RSI (m²·K/W) values causes 5.68× scaling errors — R-30 = RSI-5.28. Forgetting to derate XPS and polyiso for long-term thermal resistance (LTTR) over-states actual installed R by 5–15%; manufacturers publish initial R that includes blowing-agent contribution that escapes over years. Installing insulation without an air seal (no caulk, foam, or gaskets at penetrations) lets air leakage bypass the insulation entirely, sometimes negating 30–50% of the rated R-value. Finally, ignoring moisture and vapor management — installing insulation against a cold surface where it can condense — leads to mold and structural damage that makes the energy savings irrelevant.
When should I NOT use this calculator?
Skip the simple thickness × R/in formula for whole-wall, whole-roof, or whole-house performance calculations — use the U-factor (1/R) and assembly-level thermal modeling tools (THERM, WUFI, or ASHRAE 90.1 compliance software) that account for framing fractions, air films, fasteners, and thermal bridging. Do not use it for cold-climate basement walls where moisture and thermal gradient effects require careful vapor-management design beyond R-value alone. Avoid it for radiant barriers, which work primarily by reflecting infrared (not conduction) and are rated in reflectivity, not R-value. The formula is also inappropriate for vacuum-insulated panels (VIPs) where edge effects and air pressure dominate, or for phase-change materials (PCMs) that store thermal energy rather than just resisting flow. For commercial building envelopes, use ASHRAE 90.1 envelope tradeoff or performance paths instead of prescriptive R-values. Finally, never use R-value alone to size HVAC equipment — load calculations require fully accounting for air leakage, windows, doors, occupancy, internal gains, and orientation, which the R-value alone cannot capture.