R-Value, K-Value, C-Value & U-Value — Insulation Units Explained

Updated 10 June 2026 · ASTM C680 / ISO 12241 basis · by the Inzonex engineering team

Direct answer: K-value (k, or λ) is the material's thermal conductivity in W/m·K — lower is better. R-value is the thermal resistance of a given thickness: R = thickness ÷ k (m²·K/W) — higher is better. C-value (conductance) is 1/R, and U-value is the overall heat-transfer coefficient of the whole assembly (1/ΣR). For a 50 mm mineral-wool blanket at k≈0.045 W/m·K, R≈1.1 m²·K/W.

What each value means

K-value (k / λ) — thermal conductivity, W/m·K. The property of the material itself; lower conducts less heat. Rises with temperature.
R-value — thermal resistance of a specific thickness, m²·K/W. R = thickness (m) ÷ k. Higher = better insulator. R-values add up in series.
C-value — thermal conductance, W/m²·K. C = 1 ÷ R = k ÷ thickness. The inverse of R.
U-value — overall heat-transfer coefficient of the whole build-up (including surface films), W/m²·K. U = 1 ÷ (sum of all R-values).

How they relate (worked example)

Take 50 mm of mineral wool with k = 0.045 W/m·K. R = 0.05 ÷ 0.045 ≈ 1.11 m²·K/W. C = 1/1.11 ≈ 0.9 W/m²·K. Double the thickness to 100 mm and R doubles to ≈2.22 — heat loss roughly halves. This is why thickness and k together set the saving, and why our heat-loss calculator solves the full surface energy balance rather than a single R-value.

Typical k-values (W/m·K) by material

Material	k at ~50 °C	Notes
Aerogel blanket	0.015–0.020	lowest k; thinnest for a given R
Mineral wool (Lamella)	~0.040	our ≤220 °C tier; rises with T
Mineral wool (Wired, hi-T)	~0.045	our >220 °C tier, to +600 °C
Calcium silicate	~0.060	rigid, high-temp
Still air (reference)	~0.026	why trapped air matters

k rises with temperature, so high-temperature surfaces need more thickness for the same R — handled automatically in the calculators.

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FAQ

What is the difference between R-value and K-value?

K-value (k or λ) is the material's conductivity (W/m·K) — a fixed property. R-value is the resistance of a chosen thickness of that material: R = thickness ÷ k. A lower k or a greater thickness both raise R and cut heat loss.

How do you calculate R-value?

R = thickness in metres ÷ k-value. Example: 50 mm (0.05 m) of mineral wool at k = 0.045 gives R = 0.05/0.045 ≈ 1.11 m²·K/W. R-values of layers add together.

What is a good R-value for hot industrial insulation?

It depends on temperature and target surface temp. For most hot equipment, 50 mm mineral wool (R≈1.1) holds the surface ≤45 °C up to ~220 °C; above that, 100 mm (R≈2.2) of high-temperature mat. Use the calculator to size for your exact surface.

What is U-value?

U-value is the overall heat-transfer coefficient of the whole assembly including inner and outer surface films, U = 1 ÷ (sum of all R-values), in W/m²·K. Lower U = less heat lost per m² per degree.