What's the relationship between R-value and U-value?

R-value and U-value are reciprocals: U = 1/R. R-value measures resistance to heat flow (higher is better insulation). U-value measures thermal transmittance (lower is better insulation). Building codes may specify either depending on region.

How do I add R-values for multiple layers?

R-values add directly for materials in series (heat flows through each layer). A wall with R-13 insulation, R-1 drywall, and R-1 exterior sheathing has total R ≈ 15 (plus air films). This makes R-value convenient for building calculations.

How do I convert SI R-value to US R-value?

Multiply m²·K/W by 5.678 to get ft²·°F·hr/BTU. Example: R-3.5 (SI) × 5.678 = R-20 (US). US R-values are numerically higher than SI R-values for the same insulation due to different unit systems.

What is junction-to-ambient thermal resistance?

In electronics, θ_JA (junction-to-ambient) is the total thermal resistance from a chip's junction to surrounding air, in °C/W. Lower is better. For a transistor with θ_JA = 50°C/W dissipating 2W, junction temperature rises 100°C above ambient.

Résistance thermique Converter

About Thermal Resistance Conversion

Thermal resistance measures how effectively a material or system resists heat flow—the thermal equivalent of electrical resistance. Higher thermal resistance means better insulation, with less heat transferring for a given temperature difference. Just as electrical resistance determines current flow for a voltage, thermal resistance determines heat flow for a temperature difference, following the thermal circuit analogy that makes complex heat transfer calculations manageable.

The SI unit is kelvin per watt (K/W) for total resistance of a specific object, or m²·K/W for resistance per unit area (the metric R-value). In US construction, °F·hr·ft²/BTU (US R-value) is the standard for insulation product ratings. The R-value system revolutionized building insulation by providing a simple additive property—insulation layers simply add their R-values together. Electronics use K/W or °C/W to specify thermal paths from chip junction to heat sink to ambient air.

Our converter handles thermal resistance units used in electronics thermal management, building envelope analysis, and industrial insulation systems.

Common Thermal Resistance Conversions

From	To	Multiply By
K/W	°C/W	1 (equivalent)
°F/W	K/W	0.5556
K/W	°F/W	1.8
m²·K/W	ft²·°F·hr/BTU	5.678
ft²·°F·hr/BTU	m²·K/W	0.1761
K/W	°F·hr/BTU	1.8956
°F·hr/BTU	K/W	0.5275
°C·cm²/W	K/W (for 1 cm²)	0.0001

Thermal Resistance Unit Reference

Kelvin per watt (K/W) – Temperature rise per unit of heat power dissipated. The fundamental unit for component-level thermal resistance in electronics—heat sink specifications list thermal resistance in K/W, and transistor packages are rated for junction-to-case and junction-to-ambient resistance. A heat sink rated at 2 K/W will rise 20 K above ambient when dissipating 10 W. Lower values indicate better thermal performance.

Square meter kelvin per watt (m²·K/W) – The SI R-value, expressing area-normalized thermal resistance. This standardizes comparison between insulation materials regardless of thickness by specifying resistance per unit area. European building codes and international standards use this metric R-value. Dividing by actual area gives K/W for specific applications.

ft²·°F·hr/BTU – The US R-value for building insulation, the standard rating printed on insulation products throughout North America. This unit emerged from the building industry's need for a simple, additive insulation metric. Higher values indicate better insulation—R-38 attic insulation has roughly twice the thermal resistance of R-19. US R-values are about 5.68 times larger numerically than metric R-values.

Degree C per watt (°C/W) – Numerically identical to K/W since temperature differences in Celsius and Kelvin are equal. Common notation in electronics datasheets and thermal management specifications, often preferred because most engineering calculations use Celsius temperatures.