Last updated: 2026-07-13

R-Value and U-Value

Insulation is rated by how well it resists heat. The R-value measures that resistance: the higher it is, the slower heat leaks through. Its reciprocal, the U-value, measures how much heat gets through, so a lower U-value is better. Give this calculator a material's thickness and thermal conductivity and it returns both figures, plus the steady-state heat loss through a chosen area and temperature difference.

Where the numbers come from

For a single homogeneous layer the thermal resistance is the thickness divided by the thermal conductivity:

A thicker layer, or a less conductive material, gives a larger $R$. The U-value is simply the reciprocal, and the heat-loss rate follows from it:

Here $A$ is the area facing the flow and $\Delta T$ the temperature difference across the layer. Because a difference is what matters, $\Delta T$ is the same number in degrees Celsius and in kelvin.

R-value units: SI vs imperial

R-value is one of the few quantities quoted in two incompatible units side by side.

The US R-value is about 5.68 times the SI value for the same material: $R_\text{US} = R_\text{SI} \times 5.678$. So an RSI 2.5 layer is roughly a US R-14. A bare "R-30" batt on a hardware-store label is always a US value; the SI equivalent is about RSI 5.3. This calculator shows both, switching with the metric/imperial toggle.

Worked example

Layered walls

Real walls are built up from several layers — cladding, sheathing, insulation, plasterboard. Because thermal resistances in a heat path add up just like electrical resistors in series, the total R-value is the sum of the individual layers:

Compute each layer here, add them, then take $U = 1 / R_\text{total}$ for the whole assembly. Engineers also add small surface film resistances on the inside and outside faces (roughly RSI 0.13 and 0.04), which account for the still-air layers clinging to each surface.

Diminishing returns

Doubling the R-value halves the conductive heat loss, but the savings shrink as R climbs. Going from R-10 to R-20 removes half the loss; going from R-30 to R-40 removes only a small further slice, because each added unit of resistance strips away a smaller share of what remains. This is why building codes target a specific R-value for each climate rather than demanding "as much as possible" — beyond a point the extra material costs more than the energy it saves.

Limitations

This model covers conduction through one homogeneous layer in steady state. It ignores convection and radiation across the air films, thermal bridging through studs and fixings, moisture, air leakage and the warm-up period before temperatures settle. A framed wall's real performance is lower than the insulation's headline R-value because heat short-circuits through the more conductive framing. Use whole-assembly U-values from a full building-physics assessment for code compliance.