R-Value and Thermal Resistance Converter
Same value in every unit
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The same insulation gets two very different numbers
US R-value and metric RSI measure the same thing — how strongly a layer resists heat flow — on different scales, and the figures are not interchangeable. One RSI unit (m²·K/W) is worth about 5.68 US R units, because the US unit ft²·°F·h/BTU equals 0.1761 m²·K/W. A US R-13 wall batt is therefore about RSI 2.29, and a board sold as RSI 3 is roughly US R-17. The common mistake is putting an American "R-13" next to a European "R-3" and concluding the American product is four times better. It actually has about three-quarters the resistance.
Working out which scale a label uses
US labels are in ft²·°F·h/BTU. European, Australian and New Zealand datasheets are in m²·K/W. Canadian packs often print both, which gives a quick check: if two R numbers on the same product differ by a factor of about 5.7, that is one insulation quoted twice, not two options. Note also that m²·°C/W and m²·K/W are identical — a kelvin and a degree Celsius are the same size as an interval, so the factor is exactly 1.
R-value belongs to the product, not the material
Thermal conductivity (k, or lambda, in W/(m·K)) is a material property. Resistance is not: R = thickness ÷ k. Double the thickness of the same material and the R-value doubles. That is why "the R-value of mineral wool" means nothing without a thickness attached, and why two products only compare fairly at equal thickness, or once you work back to k. A labelled figure also describes the insulation layer on its own — a framed wall performs worse than its batt rating, because the studs bridge the insulation. In metric practice the U-value of an assembly is 1 ÷ total RSI, so a build-up totalling RSI 5 gives U = 0.2 W/(m²·K).
Tog and clo
Tog and clo apply the same physics to textiles. One tog is exactly 0.1 m²·K/W, and it is the unit UK duvets are sold in. One clo is exactly 0.155 m²·K/W, defined as roughly the clothing insulation a resting person needs to stay comfortable at about 21 °C in still air — near enough an ordinary indoor outfit.
- US R-13 ≈ RSI 2.29; US R-19 ≈ RSI 3.35; US R-30 ≈ RSI 5.28
- RSI 1 ≈ US R-5.68; RSI 2.5 ≈ US R-14.2
- 1 clo = 1.55 tog ≈ US R-0.88
- Common UK duvet ratings: 4.5 tog (summer), 10.5 tog (spring and autumn), 13.5 tog (winter) — RSI 0.45, 1.05 and 1.35
- 0 clo is a naked person; the still air layer at the skin surface is counted separately, not inside the clothing figure
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Frequently Asked Questions
No. American R-13 is about RSI 2.29 in metric m²·K/W units, because one metric RSI unit is worth roughly 5.68 US R units. Read the other way, a genuine metric R-13 (RSI 13) is about US R-74 — nothing like a US R-13 stud batt. Always establish which system a label uses before putting the two numbers side by side.
RSI 1 is about US R-5.68, since the US unit (ft²·°F·h/BTU) equals 0.1761 m²·K/W. Going the other way, divide a US R-value by 5.68 to get RSI, or multiply it by 0.1761. So RSI 2.5 is roughly US R-14.2 and RSI 5 is roughly US R-28.4.
Because R-value depends on thickness, not just on what the material is. The material property is thermal conductivity k, in W/(m·K), and resistance is thickness divided by k, so a 100 mm board has twice the R-value of a 50 mm board of the same material. Quoting an R-value for a material without stating a thickness is meaningless, and two products only compare fairly at the same thickness.
One clo is 1.55 tog, because a clo is exactly 0.155 m²·K/W and a tog is exactly 0.1 m²·K/W. Clo is a clothing insulation unit from thermal comfort work, where 1 clo is roughly what a resting person needs to stay comfortable at about 21 °C in still air — near enough a normal indoor outfit. Zero clo is bare skin, and the still air layer at the skin is counted separately, outside the clothing figure.
They are the same unit. A kelvin and a degree Celsius are identical in size as an interval, and thermal resistance depends on a temperature difference rather than an absolute temperature, so the conversion factor is exactly 1. Only the zero points of the two temperature scales differ, and that never enters the calculation.
In metric, U is simply 1 divided by the total RSI of the assembly, counting every layer plus the surface resistances. A build-up adding up to RSI 5 m²·K/W gives U = 0.2 W/(m²·K). If you are starting from a US R-value, convert it to RSI first, otherwise the U-value comes out wrong by a factor of about 5.68.