R Value to RSI Converter: Insulation and U Value
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R Value and RSI Converter
Converts between the imperial R value and the metric RSI, adds up the layers of a wall or roof, and gives the U value. Canadian codes are written in RSI while the batt in the shop is labelled R, and the two numbers look nothing like each other.
Jump to a common R value
R value against RSI
| R value | RSI | U value | Typically |
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How to use this tool
Enter a number and say which system it is in. R and RSI both appear, along with the U value in both systems, because a specification will sometimes quote resistance and sometimes quote its reciprocal.
The second layer box adds another R value on top. Thermal resistances in series simply add, so a wall built from several materials is just the sum of its parts. Fill in an area and a temperature difference and it also gives the heat loss in watts and BTU per hour.
R and RSI are the same wall
This is the confusion worth clearing up before anything else. An R 20 wall and an RSI 3.52 wall are not two different specifications. They are the same wall described in two measurement systems.
R value uses square feet, degrees Fahrenheit, hours and BTU. RSI uses square metres, kelvin and watts. Converting between them means converting all three quantities at once: the area by 10.7639, the temperature by 1.8, and the heat by 3.41214. Put those together and the factor comes out at 5.678263.
That is a big number, which is exactly why the mistake is so damaging. Reading an RSI 3.5 specification as though it said R 3.5 makes a properly insulated wall look like a sheet of plywood. Reading it the other way makes a thin wall look like a passive house.
The practical problem in Canada is that the National Building Code and provincial codes are written in RSI, while the batts on the shelf are stamped with R values because the manufacturing is continental. A spec asking for RSI 3.85 in a wall is asking for roughly R 22, and nothing on the packaging will say so.
Resistance adds, but conductance does not
Layers of a wall are in series, so their resistances add. Gypsum board, the insulation, the sheathing and the air films on each face all contribute, and the total is a straight sum.
U values do not add. U is the reciprocal of R, so you have to convert to resistance, sum, and convert back. Adding two U values gives a number with no physical meaning at all, and it is a genuinely easy mistake because U looks like the more natural thing to add.
One more caution that this tool cannot solve for you. A wall is not uniform. Studs conduct far better than the insulation between them, and a 2x6 wall framed at 400 mm centres is roughly a quarter wood by area. The effective R of the assembly is meaningfully lower than the batt rating, which is why continuous exterior insulation is worth so much more than its own R value suggests.
A worked check you can repeat on paper
Take R 20. Divide by 5.678263 and you get RSI 3.5222. Go back the other way, multiplying RSI 3.52 by 5.678263, and you get 19.99, which rounds to the R 20 you started with.
Now build a wall. Interior air film about RSI 0.12, gypsum board 0.08, an RSI 3.52 batt, OSB sheathing 0.11 and an exterior air film 0.03. Add them and the assembly is RSI 3.86, which is R 21.9. The layers other than the insulation contributed about R 2, which is small but not nothing.
The U value is the reciprocal: 1 divided by 3.86 is 0.259 watts per square metre kelvin. For a 30 square metre wall with 30 degrees across it, the heat loss is 30 times 30 divided by 3.86, which is 233 watts.
Mistakes that catch people out
Reading an RSI figure as an R value. The factor is 5.678, so the error is not subtle. If a number in a code document looks impossibly low, it is almost certainly RSI.
Adding U values. Convert to resistance, add those, then take the reciprocal. Adding conductances directly has no physical meaning.
Quoting the batt rating as the wall rating. Framing conducts around the insulation, so the effective assembly value is lower. The gap widens with more framing and narrows with continuous exterior insulation.
Assuming compressed insulation keeps its rating. Squeezing an R 20 batt into a shallower cavity loses part of its value, because the trapped air doing the work has been squeezed out.
R value and RSI conversions
The values people look up most, each opening the converter with its own address.
R value to RSI
RSI to R value
How this was checked
The conversion factor was derived from its three component conversions rather than copied: 1.8 for the temperature, 10.763910 for the area and 3.412142 for the heat, giving 5.678263. That matches the value published in insulation standards.
The headline cases were then checked both directions. R 20 gives RSI 3.5222, and RSI 3.52 gives R 19.99, closing the loop. R 60, a common cold climate attic target, gives RSI 10.5666.
The layered assembly was checked by summing a real wall build up in RSI and converting once at the end, giving RSI 3.86 and R 21.92, with a U value of 0.2591. Summing in R and converting once instead gives the same answer, which it must if resistances are being added correctly.
Related Calculators and Reference
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- Energy Converter for turning a heat loss in watts into kilowatt hours and a running cost.
- Power Unit Converter for watts against BTU per hour when sizing heating equipment.
- Temperature Converter for the degree differences these calculations rest on.
- Area Converter for wall and roof areas in either system.
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- Insulation Calculator: depth and bags of insulation to reach the recommended R value.
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