R-Value Calculator
R adds in series, U does not.
Work out R-Value. R adds in series, U does not. Names the misconception directly.
Total R-value
3.01 m²K/W
U-value 0.3322 W/m²K · imperial R-17.1
R-values add in series and U-values do not — U is the reciprocal of total R, so averaging the U-values of individual layers is arithmetically wrong and always flatters the result. Note also that imperial and metric R-values differ by a factor of 5.678, so an R-30 batt in imperial units is about R-5.3 metric. Quoting one against the other is a common and large error. Surface resistances at the inside and outside faces are real and worth including; they are small but they are not zero.
How the R-Value Calculator works
Total R-value of a layered assembly and the U-value that follows, with each layer's share of the resistance. Averaging U-values across layers is arithmetically wrong and always flatters the result.
Also known as: u value from r value · wall thermal resistance calculator · how to add r values · convert imperial r value to metric
R adds, U does not
Thermal resistances in series add, exactly as electrical resistances do. Plasterboard at 0.06, insulation at 2.7 and brick at 0.12 give a total of 2.88 m²K/W, plus the surface resistances at each face.
U-value is the reciprocal of total R, so 1 ÷ 2.88 = 0.347 W/m²K. What cannot be done is averaging the U-values of individual layers, and it is the single most common error in insulation arithmetic.
The error is always in the same direction. Averaging layer U-values here gives roughly 6.2 against the true 0.347 — the assembly appears nearly twenty times worse, or in the more common version of the mistake, an assembly is claimed as far better than it is.
Metric and imperial R are not the same number
Imperial R is in ft²·°F·h/BTU; metric R is in m²K/W. They differ by a factor of 5.678, so an R-30 batt quoted in North America is about R-5.3 in metric units.
Quoting one against the other is a large and common error, and it appears constantly in product comparisons across markets. A wall claimed at R-30 and one claimed at R-5.3 may be identical.
The same applies to U-values, which differ by the same factor inverted. Any thermal figure sourced across markets is worth checking for units before it goes into a calculation.
Thermal bridges and the whole-wall figure
A stud through insulation is a path of much lower resistance. In a timber-framed wall the studs occupy 10 to 25% of the area and conduct several times better than the insulation, so the whole-wall R is meaningfully below the between-studs figure.
The correct treatment is area-weighting: calculate the assembly through the insulated bays and through the studs separately, then weight by the fraction of wall each occupies. Quoting the bay figure as the wall figure overstates performance.
Air gaps behave the same way. A sealed still air gap adds around 0.18 m²K/W; a ventilated or convecting gap adds much less, because convection carries heat across it. A gap that lets air circulate can effectively add nothing at all.
Where to go next
The R-Value question rarely arrives on its own. These are the ones that usually come with it:
- Insulation Calculator — The first hundred millimetres does most of the work.
- Heat Loss Calculator — Floor area alone cannot tell you the answer.
- HVAC BTU Calculator — Oversized is worse than slightly undersized.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I calculate the R-value of a wall?
Add the R-value of every layer, including the inside and outside surface resistances. They add in series because heat passes through each in turn.
What is the difference between R-value and U-value?
U is the reciprocal of total R. R measures resistance to heat flow, U measures the rate of it. Higher R is better; lower U is better.
Why can't I average U-values?
Because resistances add and conductances do not. Averaging layer U-values gives a number far higher than the truth, which always makes the assembly look better than it is.
Are metric and imperial R-values the same?
No — they differ by a factor of 5.678. An R-30 batt quoted in imperial units is about R-5.3 metric, and comparing the two directly is a large and common error.
Do surface resistances matter?
They are small but real — around 0.13 m²K/W inside and 0.04 outside for a wall. Omitting them understates the assembly slightly, and in a poorly insulated wall the share is not negligible.
What is a thermal bridge?
A path of lower resistance through the assembly, such as a timber stud through insulation. It bypasses the insulation locally, and in a timber-framed wall it can reduce the effective whole-wall R-value substantially.
What R-value does my roof need?
Whatever local building code requires, which varies with climate and is revised periodically. Codes set minimums; going beyond is a payback question.
How do I account for timber studs in a wall?
By area-weighting: calculate the R-value through the insulated bays and through the studs separately, then weight by the fraction of wall each occupies. Timber typically takes 10 to 25% of a framed wall.
Does an air gap add R-value?
A sealed still air gap does, around 0.18 m²K/W for a typical cavity. A ventilated or convecting gap adds much less, and a gap that lets air circulate can effectively add nothing.
What is a reflective barrier worth?
It reduces radiant transfer and only works facing an air gap. Its performance depends entirely on the assembly, and manufacturer claims of very high equivalent R-values are frequently measured under conditions that do not apply in a real wall.
Does insulation lose R-value over time?
Some foam boards do, as blowing agents diffuse out — which is why aged R-values are published for them. Mineral wool and cellulose are stable if kept dry and uncompressed.
How much does moisture reduce R-value?
Substantially. Insulation works by trapping still air, and water conducts heat far better than air does. Wet insulation can lose most of its performance and does not recover until fully dry.
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