meters
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W/m·°C
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m²
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°C
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Thermal Resistance (R-Value)
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Sources & Methodology
Formulas verified against Engineering Toolbox material data, NIST thermal properties, and ASHRAE Handbook of Fundamentals for building envelope calculations.
1
Engineering Toolbox — Thermal Conductivity of Materials
Reference database for k values of insulation, building materials, metals, and polymers used in all material presets in this calculator.
2
U.S. Department of Energy — Insulation
R-value recommendations by climate zone and building assembly type used in the reference table and FAQ answers.
3
ASHRAE Handbook of Fundamentals — Heat Transfer
Industry-standard reference for thermal resistance calculations, U-value definitions, and building envelope heat transfer methodology.
Methodology: Single layer: R = L / (k × A) in °C/W; heat flow Q = ΔT / R in watts. Multi-layer (series): R_total = ∑(L_i / k_i) / A. U-value = 1 / (R × A). R-value per unit area (RSI) = L / k in m²·°C/W. US R-value (ft²·°F·h/BTU) = RSI × 5.678.
⏱ Last reviewed: March 2026
How to Calculate Thermal Resistance and R-Value
Thermal resistance (R) measures how strongly a material opposes the flow of heat. A high R-value means heat passes through slowly — the material is a good insulator. A low R-value means heat passes through easily. R-value is the single most important number in building energy efficiency calculations.
Single-Layer Thermal Resistance Formula
R = L / (k × A)
R = Thermal resistance (°C/W or K/W)
L = Thickness of material (meters)
k = Thermal conductivity (W/m·°C)
A = Cross-sectional area (m²)
Heat flow: Q = ΔT / R (watts)
Example: 140 mm fiberglass batt (k = 0.040) in a 10 m² wall:
R = 0.14 / (0.040 × 10) = 0.35 °C/W
RSI (per m²) = 0.14 / 0.040 = 3.5 m²·°C/W = approx. R-20 (US)
L = Thickness of material (meters)
k = Thermal conductivity (W/m·°C)
A = Cross-sectional area (m²)
Heat flow: Q = ΔT / R (watts)
Example: 140 mm fiberglass batt (k = 0.040) in a 10 m² wall:
R = 0.14 / (0.040 × 10) = 0.35 °C/W
RSI (per m²) = 0.14 / 0.040 = 3.5 m²·°C/W = approx. R-20 (US)
Multi-Layer (Series) R-Value
R_total = R₁ + R₂ + R₃ + …
For layers in series, simply add individual R-values.
Each layer: RSI_i = L_i / k_i (per unit area)
Example — Typical 2x6 Wood Frame Wall:
Drywall 12 mm (k=0.17): RSI = 0.012/0.17 = 0.071
Fiberglass batt 140 mm (k=0.040): RSI = 0.14/0.040 = 3.50
OSB sheathing 12 mm (k=0.13): RSI = 0.012/0.13 = 0.092
Vinyl siding 10 mm (k=0.16): RSI = 0.010/0.16 = 0.063
Total RSI = 3.73 m²·°C/W ≈ R-21 (US)
Each layer: RSI_i = L_i / k_i (per unit area)
Example — Typical 2x6 Wood Frame Wall:
Drywall 12 mm (k=0.17): RSI = 0.012/0.17 = 0.071
Fiberglass batt 140 mm (k=0.040): RSI = 0.14/0.040 = 3.50
OSB sheathing 12 mm (k=0.13): RSI = 0.012/0.13 = 0.092
Vinyl siding 10 mm (k=0.16): RSI = 0.010/0.16 = 0.063
Total RSI = 3.73 m²·°C/W ≈ R-21 (US)
Thermal Conductivity of Common Building Materials
| Material | k (W/m·°C) | RSI per 25mm | US R per inch |
|---|---|---|---|
| Aerogel blanket | 0.015 | 1.67 | 9.48 |
| Polyisocyanurate (PIR) | 0.022 | 1.14 | 6.46 |
| Extruded polystyrene (XPS) | 0.030 | 0.83 | 4.73 |
| Mineral wool / rockwool | 0.035 | 0.71 | 4.05 |
| Fiberglass batt | 0.040 | 0.63 | 3.55 |
| Expanded polystyrene (EPS) | 0.038 | 0.66 | 3.74 |
| Wood (pine / spruce) | 0.12 | 0.21 | 1.18 |
| Gypsum drywall | 0.17 | 0.15 | 0.83 |
| Brick (clay) | 0.72 | 0.035 | 0.20 |
| Concrete (normal weight) | 1.75 | 0.014 | 0.08 |
| Steel | 50 | 0.0005 | 0.003 |
DOE Recommended R-Values by Location
| Location | Climate Zone 1-2 (Hot) | Zone 3-4 (Mixed) | Zone 5-8 (Cold) |
|---|---|---|---|
| Attic | R-30 | R-38 | R-49 to R-60 |
| Cathedral Ceiling | R-22 | R-30 | R-38 |
| Exterior Wall | R-13 | R-13 + R-5ci | R-20 + R-5ci |
| Floor over crawlspace | R-13 | R-19 | R-30 |
| Basement wall | R-5ci | R-10ci | R-15ci |
💡 U-Value vs R-Value: U-value (W/m²·°C) = 1 / RSI. They are reciprocals. A wall with RSI = 3.5 has U = 0.286 W/m²·°C. US R-value = RSI × 5.678. So RSI 3.5 = US R-20. Windows are typically rated in U-value; walls in R-value in the US.
Practical Applications
- Building energy audits: Calculate heat loss through each wall, ceiling, and floor assembly to identify where insulation upgrades will give the best return on investment.
- HVAC sizing: Total heat loss through the building envelope determines the heating/cooling load and the size of equipment needed.
- Electronics thermal management: Thermal resistance (°C/W) of heatsinks, thermal interface materials (TIM), and PCB layers determines junction temperature of components.
- Industrial pipe insulation: Pipeline insulation thickness is calculated using R = L/k to minimize heat loss in steam, hot water, and cryogenic systems.
- Code compliance: Building codes in most jurisdictions specify minimum R-values for different building assemblies and climate zones.
Frequently Asked Questions
The thermal resistance formula is R = L / (k × A), where L is material thickness in meters, k is thermal conductivity in W/(m·°C), and A is area in m². The result R is in °C/W. Per unit area (RSI) = L / k in m²·°C/W. Heat flow rate Q = ΔT / R in watts, where ΔT is the temperature difference across the material.
For exterior walls, R-13 (RSI 2.3) is the minimum in most US codes for 2x4 framing. R-20 to R-21 (RSI 3.5) is typical for 2x6 framing with fiberglass batts. High-performance walls target R-30 to R-40+ using continuous insulation plus cavity fill. In cold climates (Zone 6-8), R-20 cavity plus R-5 or more continuous insulation is recommended by DOE.
For layers in series (stacked), total R = R₁ + R₂ + R₃ … Simply add each layer's individual R-value. A typical wall: drywall (R-0.45) + insulation (R-13) + sheathing (R-0.5) + siding (R-0.4) = R-14.35 total. This additive property makes layering insulation always beneficial.
R-value (thermal resistance) and U-value (thermal transmittance) are reciprocals: U = 1/RSI. Higher R = lower U = better insulation. The US building industry uses R-value; Europe and window ratings use U-value in W/(m²·°C). A wall with RSI = 3.5 m²·°C/W has U = 0.286 W/(m²·°C) and US R-20.
Heat loss (watts) = ΔT / R, where ΔT is indoor minus outdoor temperature (°C) and R = RSI / A. For a 20 m² wall (RSI = 3.5) with ΔT = 25°C: Q = (20 × 25) / 3.5 = 143 W of continuous heat loss. Over 24 hours that's 3.4 kWh. Multiply by your electricity or gas cost to get daily heating cost through that wall.
Aerogel blankets top the list at about R-10 per inch (RSI 3.9 per 25mm). Vacuum insulation panels (VIP) reach R-30+ per inch but are expensive and fragile. Closed-cell spray polyurethane foam (ccSPF) delivers R-6 to R-7 per inch. Polyisocyanurate (PIR) rigid boards give R-6 to R-6.5 per inch. Extruded polystyrene (XPS) gives R-5 per inch.
In electronics, thermal resistance (θ, theta) in °C/W describes how much a component's temperature rises per watt of power dissipated. A CPU with θja = 20 °C/W dissipating 50W runs 20 × 50 = 1,000°C above ambient — impossible without a heatsink. A heatsink with θ = 0.5 °C/W reduces junction temperature to 25°C above ambient for the same 50W. Thermal resistances add in series just like wall layers.
Some insulation types do degrade. Polyisocyanurate (PIR) and XPS foam lose R-value as blowing agents slowly escape — PIR can lose 10–20% of rated R-value over years (manufacturers now account for this with "aged" R-values). Fiberglass batts maintain their R-value indefinitely if kept dry and uncompressed. Spray foam is stable long-term if properly applied. Wet insulation of any type loses R-value dramatically — often 50%+ when saturated.
Thermal bridging occurs when a highly conductive material (like steel or wood studs) bypasses insulation, creating a path for heat to flow around rather than through the insulation. A wood-framed wall with R-13 cavity insulation has an "effective" R-value of only about R-10 to R-11 due to thermal bridging through the studs. Continuous insulation (ci) on the exterior eliminates thermal bridges and is why R-13 + R-5ci outperforms R-20 cavity-only.
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