Quick answer: U = 1 ÷ Rtotal, where Rtotal = Rsi + Σ(thickness ÷ λ) + Rse. For a wall Rsi = 0.13 and Rse = 0.04 m²K/W, so a U-value of 0.18 W/m²K is a total resistance of 5.56 and the layers alone are 5.39. US R-values are 5.678 times the SI figure.
Convert
U-value ⇄ R-value
R-value of a layer
Typical λ: PIR 0.022 · phenolic 0.020 · EPS 0.038 · mineral wool 0.032–0.040 · wood fibre 0.038 · aircrete 0.11–0.19 · dense block 1.1–1.6 · brick 0.7–0.9 · plasterboard 0.19.
How the conversion works
Thermal resistance adds up in series, so the total resistance of an element is the sum of its layers plus the two surface films — the still air against the inside face and the moving air against the outside face. BS EN ISO 6946 fixes those films by direction of heat flow: 0.13 inside and 0.04 outside for a wall, 0.10 and 0.04 for a roof, 0.17 and 0.04 for a floor. The U-value is the reciprocal of that total, which is why a U-value can never be read straight off a product's R-value: the same insulation gives a slightly different U-value in a wall, a roof and a floor.
| Element | Rsi (inside) | Rse (outside) |
|---|---|---|
| Wall (horizontal heat flow) | 0.13 | 0.04 |
| Roof (heat flow upwards) | 0.10 | 0.04 |
| Floor (heat flow downwards) | 0.17 | 0.04 |
Surface resistances from BS EN ISO 6946:2017; the values are also what BR 443 tells UK assessors to use.
Common U-values as R-values
| U-value (W/m²K) | Rtotal (m²K/W) | US R-value | Where you meet it |
|---|---|---|---|
| 0.10 | 10.00 | R-57 | Passivhaus-level wall or roof |
| 0.11 | 9.09 | R-52 | Part L 2021 notional roof |
| 0.13 | 7.69 | R-44 | Part L 2021 notional floor |
| 0.15 | 6.67 | R-38 | Part L 2021 limiting roof, new element in an existing dwelling (new dwelling: 0.16) |
| 0.18 | 5.56 | R-32 | Part L 2021 notional wall; limiting floor (new dwelling) and wall (new element in an existing dwelling) |
| 0.26 | 3.85 | R-22 | Part L 2021 limiting wall (new dwelling) |
| 0.30 | 3.33 | R-19 | Typical 2006–2013 new-build cavity wall |
| 0.45 | 2.22 | R-13 | Part L 1995 wall standard — a typical late-1990s cavity wall |
| 1.20 | 0.83 | R-5 | Part L 2021 notional window |
| 1.40 | 0.71 | R-4 | Part L 2021 limiting window / door, new element in an existing dwelling (new dwelling: 1.6) |
| 1.60 | 0.63 | R-4 | Uninsulated cavity wall as built, pre-1976 (RdSAP age-band figure) |
| 2.10 | 0.48 | R-3 | Uninsulated 225 mm solid brick wall as built (RdSAP age-band figure) |
Part L figures from Approved Document L Volume 1, 2021 edition (Tables 1.1, 4.1 and 4.2); the full set, with the Future Homes Standard values, is on the building regulations reference. The "typical" and RdSAP rows are indicative — an existing wall's U-value depends on its actual construction, which is what the free U-value calculator is for.
When a converter isn't enough
Adding layer R-values and inverting is exact for a uniform build-up and wrong for most real ones. Timber studs, rafters and mortar joints bridge the insulation and have to be combined with it by the ISO 6946 upper-and-lower-limit method; unventilated cavities have their own resistance; a ground floor's U-value depends on the ratio of its perimeter to its area, not just its layers; and a build-up with a metal fixing or a thermal bridge at a junction needs a psi-value calculation on top. U-Monkey handles all of that free, with 300+ materials and a free Building Control-ready PDF — start from a converter figure, finish with the calculated one.
Frequently asked questions
How do I convert a U-value to an R-value?
Divide 1 by the U-value. A U-value of 0.18 W/m²K is a total thermal resistance of 1 ÷ 0.18 = 5.56 m²K/W. That total includes the inside and outside surface resistances (0.13 and 0.04 for a wall), so the layers themselves add up to about 5.39 m²K/W.
How do I convert an R-value to a U-value?
Add the surface resistances to the R-value of the layers, then divide 1 by the total. Layers adding up to 4.5 m²K/W in a wall give R_total = 0.13 + 4.5 + 0.04 = 4.67, so U = 1 ÷ 4.67 = 0.21 W/m²K. Leaving the surface resistances out overstates the U-value slightly; leaving them out of an R-value understates it.
What is the R-value of insulation?
Thickness divided by thermal conductivity, both in SI units: R = d ÷ λ with d in metres and λ in W/m·K. 100 mm of PIR at λ = 0.022 is 0.100 ÷ 0.022 = 4.55 m²K/W; 100 mm of mineral wool at 0.035 is 2.86; 100 mm of aircrete block at 0.15 is 0.67. The converter does this for any thickness and conductivity.
Why do US R-values look so much bigger?
Different units. American R-values are in ft²·°F·h/BTU, and one SI m²K/W is 5.678 of those. An R-20 US wall is R = 3.52 m²K/W, which with surface resistances is a U-value of about 0.27 W/m²K. A US U-factor in BTU/h·ft²·°F is the SI U-value divided by 5.678.
Is a higher R-value or a lower U-value better?
Both mean the same thing: more resistance, less heat loss. Higher R is better; lower U is better. Part L works in U-values — the limiting and notional values are U-values in W/m²K — so a UK specification, a SAP calculation and a Building Control submission all need the U-value, which is why the full calculator works that way round.
Can I just add up R-values to get a wall U-value?
For a simple homogeneous build-up, yes — sum the layers and the surface resistances and invert. Real walls are rarely that simple: timber studs or masonry mortar bridge the insulation, cavities have their own resistances, and ground floors depend on the floor's shape as well as its layers. BS EN ISO 6946 handles the bridging with its combined method and ISO 13370 handles floors, which is what U-Monkey does for free.
Related: what a U-value is · what U-value do I need? · how to use U-Monkey.