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Cross-market term

R-value and U-value

Two ways of stating how well a construction resists heat flow. R is RESISTANCE and higher is better; U is TRANSMITTANCE and lower is better. They are reciprocals of each other, so a figure read as the wrong one is not merely off — it is inverted.
  • 4Markets
  • NoSame thing everywhere
  • 5Calculators

What it is called, by market

One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.

  • United Statesyours

    Same word, different thing

    R-value

    also R-13, R-30

    The bare R number is in IMPERIAL units. The same construction stated in metric RSI carries a number roughly five and a half times smaller, with the same letter on it.

  • Canada

    Same word, different thing

    RSI

    also R-value, metric R

    Canada uses metric RSI in codes while products are commonly labelled with the American R, so both numbers appear on the same job meaning the same performance.

  • United Kingdom

    Same word, different thing

    U-value

    also thermal transmittance, R-value (materials)

    British practice specifies elements by U-value, where LOWER is better — the opposite direction from R. R appears for individual materials, not for the element.

  • Australia

    Same word, different thing

    R-value

    also total R-value, U-value

    Australia uses metric R for insulation and elements, so an Australian R number is not comparable with an American one despite the identical letter.

The governing standard, by market

  • United States

    ASHRAE 90.1 / IECC

    Prescriptive R-value and assembly U-factor requirements by climate zone.

  • United Kingdom

    Approved Document L / BS EN ISO 6946

    Element U-value limits, and the method for calculating them.

  • Australia

    NCC Section J / AS/NZS 4859

    Total R-value requirements and the rating of insulation materials.

Calculators for this

Each works in either measurement system, and the terminology on the page follows whichever market you have selected.

R-Value to U-Value Calculator →RSI to R-Value Calculator →Insulation Batt Calculator →R-Value to RSI Calculator →Insulation R-Value by Climate Zone Calculator (IECC 2021 and NBC 2020) →

Frequently asked questions

Is a higher number better or worse?
It depends which letter is in front of it, and getting that backwards inverts every comparison. R is RESISTANCE: it measures how much a construction opposes heat flow, so a higher R is a better-insulated element. U is TRANSMITTANCE: it measures how much heat passes, so a lower U is better. They are reciprocals — one divided by the other — so the two describe the same physics from opposite ends. A reader comparing an American product quoting R against a British specification quoting U is comparing numbers that move in opposite directions, and choosing the larger figure in both cases will pick the best insulation in one market and the worst in the other.
Why is an American R-value so much larger than an Australian one?
Because they are different units wearing the same letter. The American R is in imperial units — hour, square foot, degree Fahrenheit per BTU — and the metric R or RSI used in Canada, Australia and Europe is in square metre kelvin per watt. The conversion factor between them is a little over five and a half, so a product labelled R-19 in the United States is roughly R-3.3 in metric terms, and neither number is wrong. Product literature often states only one. That is why an insulation comparison across markets has to establish the UNIT before comparing the figures, and why a specification calling for R-4 means something very different depending on where it was written.
Why does the assembly's figure differ from the insulation's?
Because the insulation is only one layer, and the studs, joists and fixings running through it are not insulation. A wall's actual performance is a weighted result across the whole element, including the framing that bridges the insulation — which is why an element built with R-19 batts does not achieve R-19, and why the gap widens as the framing gets denser. Codes handle this in different ways: some set a prescriptive value for the insulation, some require a calculated whole-element figure, and some allow either. The practical consequence is that a specification naming an insulation product has not specified the wall's performance, and a reader comparing a nominal insulation figure with a calculated assembly figure is comparing two different quantities.