R-value vs U-value vs lambda is where insulation and building-material quotations quietly fall apart. Three numbers describe the same physical property from three directions, two of them share a letter with a completely different unit system, and one of them changes the moment you cut the material thinner. A buyer in Sydney and a buyer in Ohio can both ask for "R-2.5" and mean products that differ by a factor of five and a half.
Start with the table. Everything after it is explanation.
The three numbers at a glance
| Lambda (λ) | R-value | U-value | |
|---|---|---|---|
| What it measures | How readily heat passes through the material itself | How much a given thickness resists heat flow | How much heat an assembled element loses overall |
| Units (metric) | W/(m·K) | m²·K/W | W/(m²·K) |
| Units (imperial, US) | BTU·in/(h·ft²·°F) | h·ft²·°F/BTU | BTU/(h·ft²·°F) — usually called U-factor |
| Depends on thickness? | No | Yes | Yes |
| Property of | The material | The material at a stated thickness | The whole build-up: insulation + substrate + air films |
| Better when | Lower | Higher | Lower |
| Typical use | Datasheet for the raw material | Product label for a board or batt | Building code compliance for a wall, roof or window |
| Relationship | λ = thickness ÷ R | R = thickness (m) ÷ λ | U = 1 ÷ (sum of all R-values in the build-up) |
One sentence to keep: lambda describes the material, R-value describes the product, U-value describes the building. A supplier who sends the wrong one of the three is not sending a wrong number — they are answering a question the buyer did not ask.
Why the same product is "R-2.5" in Australia and "R-14" in the US
This is the trap that costs real money, and it is not a rounding issue.
The United States labels insulation using an imperial R-value in h·ft²·°F/BTU. Australia, New Zealand, the UK and continental Europe use the metric R-value in m²·K/W — the same quantity Canada often prints as RSI. The conversion is fixed:
| Direction | Formula | Worked example |
|---|---|---|
| Metric → imperial | R(imperial) = RSI × 5.678 | RSI 2.5 → R-14.2 |
| Imperial → metric | RSI = R(imperial) ÷ 5.678 | R-19 → RSI 3.35 |
| Common ceiling batt | RSI 5.3 | ≈ R-30 |
| Common US wall batt | R-13 | ≈ RSI 2.29 |
An Australian ceiling batt marked R5.3 and an American ceiling batt marked R-30 are approximately the same product. Written next to each other in a comparison table with no units, they look like a six-fold difference in performance. Buyers have specified the wrong product on exactly this basis, and the shipment is not rejectable — the label was accurate in its own system.
The rule: an R-value with no unit system named is not a specification. Write R 2.5 m²·K/W (RSI) or R-14 h·ft²·°F/BTU (US). This is the identical discipline that keeps AWG vs mm² cable specifications from being ordered wrong — two legacy numbering systems, similar-looking numbers, no safe shortcut between them.
Lambda is the only one that belongs to the material
Lambda (λ), thermal conductivity, is measured in watts per metre-kelvin and describes how readily heat moves through a substance regardless of how much of it you have. It is the number a materials engineer wants, because it survives every thickness change.
Indicative values, useful for sanity-checking a datasheet:
| Material | λ, W/(m·K) |
|---|---|
| PIR / polyisocyanurate board | 0.022–0.028 |
| Extruded polystyrene (XPS) | 0.029–0.037 |
| Expanded polystyrene (EPS) | 0.031–0.038 |
| Mineral wool | 0.033–0.040 |
| Softwood timber | 0.12–0.14 |
| Aerated concrete block | 0.11–0.20 |
| Dense concrete | 1.3–1.7 |
| Steel | ~50 |
Two things suppliers get wrong here. First, quoting a declared lambda measured dry at 10 °C as though it applied in service — European practice under EN ISO 10456 distinguishes declared from design values for exactly that reason, and ageing matters for closed-cell foams whose blowing agent diffuses out over years. Second, quoting lambda alone when the buyer needs R-value, then being surprised that the buyer cannot use it: without a thickness, lambda answers nothing about the product in the carton.
R-value: the number that changes when you change the cut
R-value is thickness divided by lambda. Take a board with λ = 0.022 W/(m·K):
| Board thickness | R-value (m²·K/W) | Imperial equivalent |
|---|---|---|
| 25 mm | 1.14 | R-6.5 |
| 50 mm | 2.27 | R-12.9 |
| 80 mm | 3.64 | R-20.7 |
| 100 mm | 4.55 | R-25.8 |
Because R scales linearly with thickness, "R-value per inch" or "per 25 mm" is a legitimate way to compare materials — and a legitimate way to mislead, if the buyer reads a per-inch figure as the value of the whole board. Publish the R-value of the product as supplied, at its actual thickness, and give the per-25 mm figure separately and labelled.
The other omission that generates disputes: nominal versus actual thickness. A board sold as 50 mm that measures 48 mm delivers R 2.18, not 2.27 — about 4% short. If the buyer's compliance calculation was run at the nominal figure and an inspector measures the real one, the shortfall is yours to explain. State the thickness tolerance in the same table as the R-value.
U-value: the number the building code actually checks
U-value is the reciprocal of the total thermal resistance of a complete element — insulation, sheathing, cladding, cavities and the internal and external surface air films — calculated for a wall, roof, floor or window and expressed in W/(m²·K). ISO 6946 sets out the calculation method for opaque elements.
No supplier can quote a U-value for a product in isolation, and any datasheet that does is describing an assumed build-up that is probably not the buyer's. Windows and doors are the exception: they are supplied as complete assemblies, so a whole-window U-value is meaningful — and it is not the same as the centre-of-glass U-value, which is always the flattering number.
What to do when a buyer asks for your product's U-value: give the R-value at the supplied thickness, the lambda, and the thickness with tolerance, and say plainly that the U-value depends on their build-up. That answer takes one line and reads as competence. Sending a U-value you cannot substantiate reads as either confusion or a claim you will be held to.
Writing the spec line so it survives translation
The failure is almost never the physics. It is that the number leaves your factory correct and arrives on a listing, a translated datasheet or a marketplace form with the units dropped. A spec line that holds up looks like this:
| Field | Value |
|---|---|
| Product | PIR insulation board, foil-faced |
| Nominal thickness | 50 mm (tolerance −0 / +2 mm) |
| Declared lambda (λD) | 0.022 W/(m·K), EN 12667, 10 °C mean |
| Thermal resistance (R) | 2.27 m²·K/W at 50 mm |
| Imperial equivalent | R-12.9 h·ft²·°F/BTU |
| Per 25 mm | R 1.14 m²·K/W |
| U-value | Depends on build-up; calculate per ISO 6946 |
Seven rows, no ambiguity, and every number traceable to a stated thickness and standard.
Then put the thickness and the R-value on the product image, not only in a table three scrolls down. Buyers scanning a catalogue or a marketplace listing look at the picture first, and a board photographed against a white background is indistinguishable at 25 mm and 100 mm. Tools built for dimension and spec annotation let you mark the measured thickness and the governing spec directly onto the photo and export it at each channel's required size — measured from the actual board, which is the part a generic image editor makes tedious and an AI image generator cannot do at all, because it produces a plausible-looking number it never measured. The same approach applies across the category, as how to label building material dimensions sets out, and you can see what a cleanly dimensioned product image looks like in this dimensioned product image example.
Pre-quotation checklist
- Every R-value carries its unit system —
m²·K/Worh·ft²·°F/BTU, never a bare number - Lambda quoted with the test standard and mean temperature, and marked declared or design
- R-value given at the supplied thickness, with any per-25 mm figure labelled separately
- Thickness stated with tolerance, nominal and actual distinguished
- No U-value quoted for a product that is not a complete assembly
- For windows and doors, whole-unit U-value given, not centre-of-glass
- Imperial equivalents calculated once at source with the 5.678 factor, not re-derived downstream
- Thickness and R-value marked on the product image, not only in the datasheet
- Ageing behaviour disclosed for closed-cell foam products
FAQ
What is the difference between R-value and U-value?
R-value measures resistance to heat flow and is higher for better insulation; U-value measures heat transmittance through a complete building element and is lower for better performance. They are reciprocals: U = 1 ÷ (sum of all R-values in the build-up, including surface air films). R-value belongs to a product; U-value belongs to a wall, roof, floor or window.
What is lambda value in insulation?
Lambda (λ) is thermal conductivity in watts per metre-kelvin, W/(m·K) — how readily heat passes through a material independent of its thickness. Lower is better. Divide the thickness in metres by lambda to get the R-value: a 50 mm board with λ = 0.022 gives R = 0.05 ÷ 0.022 = 2.27 m²·K/W.
How do I convert R-value to RSI?
RSI = imperial R-value ÷ 5.678, and imperial R-value = RSI × 5.678. RSI is the metric thermal resistance in m²·K/W; the imperial R-value is in h·ft²·°F/BTU. US R-19 is RSI 3.35; Australian R2.5 is about US R-14.2. Canada frequently prints both on the same label, which is the practice worth copying.
Why does Australian R-value look so much lower than American R-value?
Because they are different units, not different performance. Australia, New Zealand, the UK and Europe use metric R-values in m²·K/W; the US uses imperial R-values in h·ft²·°F/BTU, which are 5.678 times larger for the same product. An Australian R5.3 ceiling batt is roughly a US R-30 batt. Comparing the bare numbers across markets without converting is the most common specification error in insulation trade.
Can I quote a U-value for my insulation board?
No, not on its own. A U-value describes a complete element including substrate, cavities, cladding and surface air films, so it changes with the buyer's build-up. Quote lambda, thickness with tolerance, and R-value at that thickness, and let the buyer or their engineer calculate the U-value per ISO 6946. Complete assemblies such as windows and doors are the exception — for those, quote the whole-unit U-value rather than the centre-of-glass value.
Sources & References
- Kingspan — What are U-values, R-values and lambda values?
- TheGreenAge — Thermal conductivity, R-values and U-values simplified
- Sager — What are the lambda value, R-value and U-value?
- SOPREMA — R and RSI value converter
- TakeCharge — Difference between R-value and RSI value
- HTRC — Insulation values and comparisons, metric and imperial
- NIST — Unit conversion reference