Thermal Resistance Converter

Thermal Resistance in Building Insulation and Electronics

Thermal resistance measures how strongly a material or assembly opposes the flow of heat. The higher the thermal resistance, the better its insulating ability. The SI unit is kelvin per watt (K/W) or, for area-specific thermal resistance, square metre kelvin per watt (m²·K/W — the RSI value). In North America, the familiar US R-value (hr·ft²·°F/BTU) is used for building insulation rating.

Thermal resistance is used in two distinct contexts: in building science, it rates the insulation value of walls, roofs, and windows; in electronics, it characterises the heat dissipation path from a component's junction to the ambient environment. Both applications use the same fundamental equation: temperature difference = heat flow × thermal resistance (ΔT = Q × R), but operate on very different scales.

US R-Values and Building Codes

The US R-value system rates insulation by how many BTU of heat pass through one square foot of material per hour per degree Fahrenheit of temperature difference across it. A higher R-value means better insulation. The unit is hr·ft²·°F/BTU, usually written simply as R-13, R-19, R-30, etc. The US Department of Energy (DOE) recommends R-49 to R-60 for attic insulation in cold climate zones (zones 5–8), and R-13 to R-21 for exterior walls in most of the country.

IRC (International Residential Code) and IECC (International Energy Conservation Code) prescribe minimum R-values for each building component in each climate zone. A new home in Chicago (zone 5) must achieve at least R-49 in the attic, R-20 in above-grade walls, and R-10 under the foundation slab. These minimums are set to achieve acceptable heating and cooling loads for energy efficiency.

UK RSI Values and Part L

The UK uses the RSI (metric R-value) system, where the unit is m²·K/W. UK building regulations Part L (Conservation of Fuel and Power) specify maximum U-values (W/(m²·K)) for new construction elements. U-value is the inverse of total RSI: U = 1/RSI. A typical UK external wall with cavity fill insulation might have U = 0.28 W/(m²·K), corresponding to RSI = 3.57 m²·K/W or US R-value ≈ 20.3.

The conversion between US R-value and RSI is: RSI = US R × 0.1761. Conversely, US R = RSI × 5.678. An architect specifying double-glazed windows with US R-3 glazing translates that to RSI 0.53 m²·K/W for UK regulatory compliance submissions. The RSI value is also used in ASHRAE and ISO thermal standards.

Electronic Heat Sinks: K/W and °C/W

In electronics, thermal resistance is expressed in kelvin per watt (K/W) or equivalently °C/W (numerically identical since K and °C intervals are equal). A microprocessor has a junction-to-case thermal resistance (θ_jc) specified in °C/W by the manufacturer. If θ_jc = 0.5 °C/W and the chip dissipates 100 W, the junction temperature will be 50 °C above the package case temperature.

Heat sink selection requires adding thermal resistances in series: θ_j-ambient = θ_jc + θ_c-s (case-to-sink contact resistance) + θ_s-a (heat sink-to-ambient). If θ_s-a = 0.8 °C/W and ambient is 35 °C, the junction reaches 35 + (0.5 + 0.1 + 0.8) × 100 = 175 °C — at or above the maximum junction temperature for many processors. US manufacturers (Intel, AMD) and UK chipmakers (ARM, Imagination) publish θ_jc in °C/W; converting to K/W is trivial (the values are numerically equal).

Practical Conversion Reference

Engineers regularly encounter both US R-values and RSI in the same project. An international retrofit project might have US insulation products rated in R-values and a UK-origin mechanical specification using RSI. The relationship is simple: 1 US R-value = 0.1761 RSI. For component-level thermal resistance: 1 K/W = 1 °C/W (numerically equal). For area-specific: 1 RSI (m²·K/W) = 5.678 US R-value. These conversions must be accurate to avoid significant errors in energy modelling and compliance calculations.