Calcipedia

Thermal Resistance Converter

Convert area-normalised thermal resistance and insulation R-value units between RSI, imperial R, and legacy metric heat-loss reference forms.

Thermal resistance

Convert insulation RSI and imperial R-value units without mixing in whole-assembly K/W

Use this page for area-normalised thermal resistance only, where insulation notes and envelope specs are usually expressed as RSI in SI form or R-value in imperial form.

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Scope note

This converter handles area-normalised thermal resistance and insulation R-values. It does not calculate total thermal resistance in K/W for a specific assembly unless area has already been normalised out of the reported figure.

Enter values Provide a non-negative RSI or R-value to calculate the full conversion sheet.

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Heat and materials

Thermal resistance converter: RSI, imperial R-value, and legacy insulation units explained

A thermal resistance converter helps when the same insulation or envelope property is reported in different regional unit systems. The page keeps area-normalised thermal resistance explicit so RSI, imperial R-value, and older metric reference forms can be compared without confusing them with total assembly K/W.

RSI and imperial R-value describe the same area-normalised property

Thermal resistance in building and insulation work is usually normalised by area. In SI form that is written as square metre kelvin per watt, often shortened to RSI. In US customary references it is commonly written as square foot hour degree Fahrenheit per BTU, usually called R-value.

Because both forms describe the same area-normalised resistance, they can be converted directly. That is why RSI 2.5 and an imperial R-value a little above 14 describe the same level of resistance even though the numbers and symbols look very different.

1 m²·K/W ≈ 5.678 ft²·h·°F/BTU

Direct conversion between the common SI RSI form and the common imperial R-value form.

1 m²·K/W = 1 m²·°C/W

Kelvin and Celsius intervals have the same size, so the area-normalised resistance is numerically unchanged between those two SI interval labels.

Why this page does not treat every K/W value as interchangeable

A whole-assembly thermal resistance written simply as K/W depends on the actual area of the part, wall, exchanger, or component being analysed. Insulation and envelope references often remove that area term and report the normalised property instead, which is why RSI and R-value can be compared directly.

Keeping that distinction visible matters. Mixing area-normalised R-values with total K/W figures can create a real sizing or compliance mistake because the underlying dimensions are not the same.

When legacy metric forms still appear

Some older engineering references and translated materials still report thermal resistance as square metre hour degree Celsius per kilocalorie. It is the same physical property, just expressed through an older heat-flow convention.

That is useful when a spec sheet, retrofit note, or archived standard gives one legacy form while the current project needs RSI or imperial R-value for comparison.

Frequently asked questions

Is RSI the same thing as R-value?

They describe the same area-normalised thermal resistance but in different unit systems. RSI uses SI units such as m²·K/W, while the common US R-value uses ft²·h·°F/BTU.

 Why does this page avoid whole-assembly K/W conversion?

Because a whole-assembly K/W value depends on area. This page is intentionally limited to area-normalised thermal resistance so the conversions remain dimensionally correct.

 Does a higher thermal resistance number always mean better insulation?

Within the same unit family, yes. Higher thermal resistance means less heat flow through the assembly for a given temperature difference.

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