Thermal Conductivity Converter

Thermal conductivity measures how quickly heat moves through a material. The SI unit is the watt per meter-kelvin, W/(m·K). A high number means heat races through (metals); a low number means the material holds heat back (insulators). The gap between the two is enormous.

Key conversions:

• 1 kW/(m·K) = 1,000 W/(m·K)
• 1 BTU/(ft·h·°F) = 1.7307 W/(m·K)
• 1 cal/(cm·s·°C) = 418.68 W/(m·K)

Where common materials sit:

Copper conducts heat roughly 15,000 times better than still air, which explains a lot of everyday design. Pots and heat sinks are aluminum or copper; insulation works by trapping air inside foam or fiber so it can’t circulate and ferry heat away. Notice that no ordinary insulation beats still air by much. The whole game is stopping the air from moving, because a moving gas carries heat by convection far faster than it conducts it.

R-value is conductivity plus thickness

Insulation is normally sold by R-value, which is resistance rather than conductance, and it folds in how thick the layer is:

R = thickness ÷ conductivity

In SI that gives m²·K/W. American products quote the imperial R-value instead, in ft²·°F·h/BTU, and the two differ by a factor of 5.678. An R-30 American batt is R-5.3 in SI units. Quoting one where the other is expected is the single most common mistake in this corner of building work, and it is a factor of nearly six, not a rounding.

U-value is simply the reciprocal, U = 1 ÷ R, and is what European window and wall specifications use. Lower is better for U, higher is better for R, which is another thing worth saying out loud before someone buys the wrong product.

The BTU per foot-hour-Fahrenheit unit is the one to watch, since US building and HVAC work still runs on it. Comparing an American insulation spec against a metric one without converting is asking for a mistake. A thick layer of a mediocre insulator can easily beat a thin layer of a good one: 6 inches of fiberglass at 0.039 W/(m·K) outperforms 2 inches of polyisocyanurate at 0.024, despite the foam being the better material per inch.