R-Value to U-Value: Converting Insulation Ratings Between Metric and Imperial

U-value is one over R-value, and imperial R is 5.678 times metric RSI. How to convert between insulation ratings, and why a product R-value is not an assembly U-value.

Introduction

Insulation is rated two incompatible ways. North America quotes R-value, where higher is better. Europe quotes U-value, where lower is better. They are reciprocals of each other, and to make matters worse the metric and imperial versions of R-value are not the same unit despite sharing the name. Comparing a product sold in one system against a specification written in the other needs both conversions.

The Two Quantities

R-value measures thermal resistance — how well a material stops heat passing through it. More resistance means less heat lost, so higher is better.

U-value measures thermal transmittance — how much heat passes through. It is simply the reciprocal:

U = 1 ÷ R and R = 1 ÷ U

So a wall assembly with RSI 5.0 has a U-value of 0.20 W/(m²·K). The two describe the same wall from opposite directions.

The Unit Trap: R-Value Is Not RSI

This is where most conversion errors happen. R-value exists in two unit systems that are both called R:

Imperial R-value: ft²·°F·h/BTU — the "R-30" on an American batt
Metric R-value, written RSI: m²·K/W

1 RSI = 5.678 imperial R

Imperial R to RSI: divide by 5.678
RSI to imperial R: multiply by 5.678

That constant is not arbitrary. It falls out of converting each unit in the expression: a square metre is 10.764 square feet, a kelvin interval is 1.8 Fahrenheit degrees, and a watt is 3.412 BTU per hour. Multiply the first two and divide by the third and you get 5.678.

The consequence is that "R-5" means very different things in the two systems. An imperial R-5 is a thin sheet of foam board; RSI 5 is a well-insulated modern wall.

Converting Between Them

To go from an American R-value to a European U-value, do it in two steps: divide by 5.678, then take the reciprocal.

R-13 → RSI 2.29 → U = 0.437 W/(m²·K)
R-19 → RSI 3.35 → U = 0.299
R-30 → RSI 5.28 → U = 0.189
R-49 → RSI 8.63 → U = 0.116
R-60 → RSI 10.57 → U = 0.095

Going the other way, take the reciprocal then multiply by 5.678. A U-value of 0.18 W/(m²·K) is RSI 5.56, or imperial R-31.5.

Why You Cannot Compare a Batt to a Wall

The most important caveat in the whole subject, and the one that makes naive comparisons misleading.

An R-value printed on a product describes that product. A U-value in a building regulation describes the whole assembly — every layer, plus the thin films of still air on each face, and reduced by thermal bridging where studs or joists interrupt the insulation.

Resistances in series simply add, which makes assembly R easy to build up:

Interior air film: about R-0.68 (RSI 0.12)
Plasterboard, sheathing, cladding: each contributes its own R
Insulation: the largest single term
Exterior air film: about R-0.17 (RSI 0.03)

Then U is the reciprocal of the total. But timber studs conduct far better than the insulation between them, so the assembly's real performance sits below the sum — which is why a wall built with R-13 batts does not achieve a U-value of 0.437 in practice.

A U-value quoted in a regulation already accounts for this. An R-value on a packet does not.

Working From Thermal Conductivity

If you know a material's conductivity rather than its rating, the relationship is:

RSI = thickness (m) ÷ λ

where λ (lambda) is thermal conductivity in W/(m·K). Typical values:

Mineral wool: λ ≈ 0.038 — 100 mm gives RSI 2.63, or R-14.9
EPS: λ ≈ 0.036 — 100 mm gives RSI 2.78, or R-15.8
PIR board: λ ≈ 0.022 — 100 mm gives RSI 4.55, or R-25.8
Dense concrete: λ ≈ 1.4 — 100 mm gives RSI 0.07, essentially nothing

This is the calculation that tells you how thick to build. To reach RSI 5.0 needs 190 mm of mineral wool but only 110 mm of PIR, which is why PIR is used where depth is constrained despite costing more.

The concrete figure is worth noting: structural mass is not insulation. Seven metres of it would be needed for RSI 5.

Diminishing Returns

Because U is the reciprocal of R, equal increases in insulation do not buy equal savings.

R-10 to R-20 cuts heat loss through that element by 50 percent
R-20 to R-30 cuts it by 33 percent
R-30 to R-40 by 25 percent
R-40 to R-50 by only 20 percent

Each added R-10 costs roughly the same and delivers steadily less. This is why insulation standards specify a level rather than "as much as possible", and why beyond a point air-tightness and glazing matter more than another layer in the loft.

Quick Reference Conversion Table

R-11 = RSI 1.94 = U 0.516
R-13 = RSI 2.29 = U 0.437
R-19 = RSI 3.35 = U 0.299
R-21 = RSI 3.70 = U 0.270
R-30 = RSI 5.28 = U 0.189
R-38 = RSI 6.69 = U 0.149
R-49 = RSI 8.63 = U 0.116
U 0.15 = RSI 6.67 = R-37.9
U 0.18 = RSI 5.56 = R-31.5
1 RSI = 5.678 R

Common Mistakes Worth Avoiding

Treating imperial R and RSI as the same figure

They differ by 5.678. Quoting an American R-30 batt as RSI 30 overstates it more than fivefold.

Comparing a product R-value with a regulatory U-value

One is a single material, the other a complete assembly including air films and bridging losses. They are not comparable without building up the full assembly first.

Assuming R-value is per inch

An R-value describes a specific thickness of a specific product. Conductivity, not R, is the per-unit property.

Adding U-values

Resistances add in series; transmittances do not. Sum the R-values, then take the reciprocal once at the end.

Conclusion

U is one over R, imperial R is 5.678 times RSI, and thermal resistance built from conductivity is thickness divided by lambda. Add resistances in series and invert once at the end, never the other way round, and remember that a number on an insulation packet describes a product while a number in a building regulation describes an entire wall. Our guide to BTU and joules covers the energy units these ratings are built from.

Try our BTU per hour to watts converter for the heat-flow side of these calculations.