When should I use thermal resistivity vs thermal conductivity?

Use thermal conductivity (k) when calculating heat flow rates: Q = kAΔT/L. Use thermal resistivity (ρ = 1/k) when calculating R-values for insulation: R = ρL. Both describe the same material property—just inverse perspectives. Conductivity emphasizes heat transfer; resistivity emphasizes insulation.

What materials have the highest thermal resistivity?

Aerogel: ~40-100 K·m/W. Vacuum panels: effective ~100+ K·m/W. Polyurethane foam: ~35-50 K·m/W. Fiberglass: ~25-30 K·m/W. Still air: ~40 K·m/W. Aerogel is among the best practical insulators; vacuum panels achieve highest values but are fragile.

How does thermal resistivity relate to building R-values?

R-value = thermal resistivity × thickness. If insulation has ρ = 25 K·m/W and thickness is 0.15 m (6 inches), R = 3.75 m²·K/W (metric) or about R-21 (US). This linear relationship makes it easy to calculate R-values for any thickness from a single resistivity value.

Why do insulators trap air or gas?

Still air has high thermal resistivity (~40 K·m/W) because gases are poor heat conductors. Insulation materials trap air in small pockets, preventing convection while exploiting air's natural resistance. Smaller air gaps reduce convection further, which is why fine-celled foams outperform coarse materials.

Thermal Resistivity Converter

About Thermal Resistivity Conversion

Thermal resistivity measures a material's intrinsic resistance to heat flow per unit thickness—the inverse of thermal conductivity. Where thermal conductivity describes how easily heat flows through a material, thermal resistivity describes how effectively it blocks heat. High thermal resistivity means the material is a good insulator, resisting heat transfer; low resistivity means it conducts heat well and is a poor insulator.

The SI unit is kelvin meter per watt (K·m/W). The key advantage of thermal resistivity over thermal conductivity is that it directly calculates R-values: R = ρ × thickness. This makes it especially useful for comparing insulating materials, calculating composite wall thermal resistance, and specifying insulation requirements. Building scientists often prefer resistivity because adding another inch of insulation simply adds to the R-value proportionally—an intuitive relationship that conductivity doesn't provide directly.

Our converter handles thermal resistivity units used in building science, insulation engineering, and thermal design applications.

Common Thermal Resistivity Conversions

From	To	Multiply By
K·m/W	°C·m/W	1
K·m/W	K·cm/W	100
K·cm/W	K·m/W	0.01
K·m/W	°F·ft·h/BTU	0.5778
°F·ft·h/BTU	K·m/W	1.731
K·m/W	°C·cm/W	100
K·m/W	°F·in·h/BTU	6.933
°F·in·h/BTU	K·m/W	0.1442
K·mm/W	K·m/W	0.001
K·m/W	K·mm/W	1,000

Thermal Resistivity Unit Reference

Kelvin meter per watt (K·m/W) – The SI unit for thermal resistivity, representing the temperature gradient (in kelvins) needed to conduct 1 watt of heat through 1 square meter of cross-sectional area per meter of thickness. Good insulators like fiberglass have values around 25-30 K·m/W, while metals have very low values (copper: ~0.0026 K·m/W). To get R-value, multiply by thickness in meters.

Degree Fahrenheit foot hour per BTU (°F·ft·h/BTU) – US engineering unit widely used in American building industry and HVAC specifications. This is the reciprocal of thermal conductivity in BTU/hr·ft·°F. 1 °F·ft·h/BTU ≈ 1.731 K·m/W. When multiplied by thickness in feet, yields the US R-value directly.

Kelvin centimeter per watt (K·cm/W) – Convenient unit for thin insulation layers, thermal interface materials, and electronics applications where thicknesses are measured in centimeters or millimeters. 1 K·cm/W = 0.01 K·m/W. Thermal interface materials are often specified in K·cm²/W which incorporates area.

Degree Celsius meter per watt (°C·m/W) – Numerically identical to K·m/W since temperature differences in Celsius and Kelvin are equal. Some engineering references prefer this notation when working with Celsius temperatures throughout a calculation.