Thermal Conductivity of Fibreglass Batt Insulation

k_{\mathrm{fg}} = 0.045\ \text{W/(m}{\cdot}\text{K)}

Value	45 mW/(m·K) (0.045 W/(m·K)) Use in the calculator →
Status	Typical value: varies by about ± 5 mW/(m·K) in practice
Source	Energy Renovations: Insulation (Building America Best Practices Series Vol. 17, PNNL-20972, prepared by PNNL and ORNL for the US Department of Energy, 2012), Table 2.2: fibreglass batt R-3.2 per inch, which is k = 0.1442 / 3.2 = 0.045 W/(m·K); Table 3.2: high-density batts R-3.7 to R-4.3
Categories	Material Properties Engineering & Trade

Thermal Conductivity of Fibreglass Batt Insulation in every thermal conductivity unit

Unit	Value
milliwatt per meter-kelvin	45 mW/(m·K)
BTU inch per hour square foot Fahrenheit	0.31200623 BTU·in/(h·ft²·°F)
watt per meter-kelvin	0.045 W/(m·K)
BTU per hour-foot-Fahrenheit	0.026000519 BTU/(h·ft·°F)
watt per meter-Celsius	0.045 W/(m·°C)
kilocalorie per hour meter Celsius	0.038718929 kcal/(h·m·°C)
watt per centimeter-kelvin	0.00045 W/(cm·K)
calorie per second centimeter Celsius	0.00010755258 cal/(s·cm·°C)
kilowatt per meter-kelvin	0.000045 kW/(m·K)

Learning zone

Convert with care: R-value in North America is expressed in h·ft²·°F/BTU per inch, and 0.045 W/(m·K) corresponds to about R-3.2/in, so a standard 3.5 in cavity batt is about R-11; the denser batts sold as R-13 and R-15 reach R-3.7 to R-4.3 per inch. SI R-values (m²·K/W) are 5.68 times smaller for the same product, so an R-13 batt is RSI-2.3: a factor that has caused more than one badly under-insulated wall.

The laboratory number is measured on a flat, uncompressed sample at a mean temperature of 24 °C. Installed performance is usually worse. Compressing an R-19 batt into a 2×4 cavity gives roughly R-13, not R-19. Gaps, voids and wiring cavities that leave even 5 % of the area uninsulated can cost 20–30 % of the assembly performance, and framing members bridging the insulation typically drop a nominal R-19 wall to an effective R-13 or so. Air movement through low-density batt degrades it further. Treat this figure as the material's best case and compute assembly U-values with the framing factor included.

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Heat Conduction Rate

P = \tfrac{k A \Delta T}{d}

Revised 8 November 2026

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formula.expert. (2026, November 8). Thermal Conductivity of Fibreglass Batt Insulation. https://formula.expert/constants/thermal-conductivity-fibreglass-batt