square meter kelvin per watt
Areal thermal resistanceexact by definition
RSI is the metric areal thermal resistance, in square metres kelvin per watt: the temperature difference in kelvin that one watt per square metre of heat flow will establish across an assembly. It is the SI form used in Canadian and European building codes and on Canadian insulation labels. As the coherent SI combination its factor of 1 is exact.
Watch out: RSI and the imperial R-value differ by a factor of about 5.678, and both are printed on insulation sold in North America. RSI 3.52 is R-20. If someone quotes an unlabelled R of 3.5 they almost certainly mean an imperial R-3.5, which is a thin sheet, while RSI 3.5 is a full 2x6 wall cavity. Reading one as the other is a factor of nearly six.
| 1 | 1 |
The name is simply R with the SI marker attached, coined to keep the metric value visually distinct from the imperial R-value on labels that carry both. Canadian batt packaging typically prints RSI in large type with the imperial R beside it.
About the square meter kelvin per watt
RSI answers the same question as the imperial R-value: how well does one square metre of this assembly resist heat flow? The definition is where is the heat flux in W/m². An RSI 3.5 wall with 25 K across it passes W per square metre.
The arithmetic is pleasant in a way imperial R-values are not. RSI is simply thickness divided by conductivity, both in SI: 100 mm of insulation with W/(m·K) gives m²·K/W. RSI values in series add, and the total heat loss through an assembly is area times temperature difference divided by total RSI. The overall U-value is just the reciprocal, in W/(m²·K), and it is the U-value that building codes usually regulate.
The number to keep in your head is 5.678. , and . The common Canadian wall assemblies convert as follows: RSI 2.11 is R-12, RSI 2.46 is R-14, RSI 3.52 is R-20, RSI 4.23 is R-24, and an RSI 8.8 attic is R-50. Nobody rounds these consistently, which is why the labels print both.
One square meter kelvin per watt in every areal thermal resistance unit
| 1,000,000 | square millimeter kelvin per watt (mm²·K/W) | |
| 10,000 | square centimeter Celsius per watt (cm²·°C/W) | |
| 817.67 | square inch hour Fahrenheit per BTU (in²·h·°F/BTU) | |
| 10 | tog (tog) | |
| 6.45161 | clo (clo) | |
| 5.67826 | hour square foot Fahrenheit per BTU (R (h·ft²·°F/BTU)) | convert → |
| 1.16222 | square meter hour Celsius per kilocalorie (m²·h·°C/kcal) | |
| 1 | square meter kelvin per watt (RSI (m²·K/W))this unit | |
| 1 | square meter Celsius per watt (m²·°C/W) |