Building performance
R-value / U-value converter
Convert thermal resistance and transmittance with explicit SI and US units, or sum a layer stack with surface resistances.
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What an R-value to U-value converter does
An R-value to U-value converter turns thermal resistance into thermal transmittance and back, and converts both between US and SI units. U is 1 divided by R, and one US R-value unit (h·ft²·°F/Btu) equals 0.1761 SI units (m²·K/W), so R-20 is about RSI-3.52 and U-0.05.
US product labels, energy codes and window ratings mix these quantities. Insulation is sold by R-value, windows are rated by U-factor, and European and Canadian documents often use SI values. Enter any one of the four and the tool returns all four at once.
In single-value mode it converts numbers only. Whether a value describes insulation alone, the center of a cavity, or a whole assembly with framing and air films is up to you. In layer-stack mode it builds that assembly value for you: pick a material and thickness for each layer, add inside and outside surface resistances from ISO 6946 or ASHRAE presets, and read the total R and U in both unit systems, with an optional pass/fail check against a target you enter.
How to use the R-value / U-value converter
- Choose What to convert: Single value for a straight conversion, or Layer stack to sum an assembly.
- Single value: choose the input quantity (R in SI, R in US, U in SI or U in US) and enter the value. The imperial example starts at R-20 (US); the metric example starts at R-3.5 (SI). Read all four results: R · US, U · US, R · SI and U · SI.
- Layer stack: in each layer row pick a layer material (typical R per inch, or fixed items such as an air cavity or hollow block) and its thickness, or choose Custom R and type the layer's own resistance. Add or remove layers as needed.
- Choose the surface resistances: ISO 6946 wall, roof or floor films, ASHRAE winter films, custom values or none. Optionally set a target maximum U or minimum R to get a PASS or FAIL against your code or specification value.
- Read total R and U in both unit systems, the layers-only and surface-film shares, and the layer table. The Imperial/Metric switch relabels thicknesses and targets; results always show both systems.
- Download a PDF or CSV, or copy the result into a specification, energy model or email.
Worked example
Imperial: R-20 h·ft²·°F/Btu corresponds to U-0.05 Btu/(h·ft²·°F) and approximately R-3.52 m²·K/W.
Metric: R-3.5 m²·K/W corresponds to U-0.2857 W/(m²·K) and approximately R-19.87 in US units.
R-value to U-value conversion chart
The chart converts common US R-values. U-values are rounded; use the tool for exact figures. The product descriptions are nominal insulation ratings, not whole-wall or whole-roof performance, which is lower once framing is included.
| R-value (US) | RSI (m²·K/W) | U (Btu/h·ft²·°F) | U (W/m²·K) | Commonly seen as |
|---|---|---|---|---|
| R-5 | 0.88 | 0.2000 | 1.136 | Thin continuous insulation board |
| R-11 | 1.94 | 0.0909 | 0.516 | Batts for 2×4 framing |
| R-13 | 2.29 | 0.0769 | 0.437 | Batts for 2×4 walls |
| R-15 | 2.64 | 0.0667 | 0.379 | High-density batts for 2×4 walls |
| R-19 | 3.35 | 0.0526 | 0.299 | Batts for 2×6 walls |
| R-21 | 3.70 | 0.0476 | 0.270 | High-density batts for 2×6 walls |
| R-30 | 5.28 | 0.0333 | 0.189 | Attic and floor insulation |
| R-38 | 6.69 | 0.0263 | 0.149 | Attic insulation |
| R-49 | 8.63 | 0.0204 | 0.116 | Attic insulation |
| R-60 | 10.57 | 0.0167 | 0.095 | Attic insulation |
RSI to R-value: converting SI values to US units
Going the other way, multiply RSI by 5.678 to get a US R-value. This comes up with Canadian and European specifications, and with product data sheets that list both. To get U in W/m²·K, divide 1 by the RSI.
| RSI (m²·K/W) | R-value (US) | U (W/m²·K) | U (Btu/h·ft²·°F) |
|---|---|---|---|
| 1.0 | 5.68 | 1.000 | 0.1761 |
| 2.0 | 11.36 | 0.500 | 0.0881 |
| 2.5 | 14.20 | 0.400 | 0.0704 |
| 3.0 | 17.03 | 0.333 | 0.0587 |
| 3.5 | 19.87 | 0.286 | 0.0503 |
| 4.0 | 22.71 | 0.250 | 0.0440 |
| 5.0 | 28.39 | 0.200 | 0.0352 |
| 6.0 | 34.07 | 0.167 | 0.0294 |
| 7.0 | 39.75 | 0.143 | 0.0252 |
Window U-factors in US and SI units
Windows, doors and skylights are rated by U-factor rather than R-value, because heat moves through glass, frame and spacers side by side. In the US, NFRC labels give a whole-unit U-factor in Btu/h·ft²·°F. To compare it with an SI U-value in W/m²·K, multiply by 5.678.
Lower is better for U, the opposite of R. A U-0.30 window loses heat six times faster per square foot than a wall with an overall U of 0.05 (R-20).
| U-factor (US) | U-value (W/m²·K) | Equivalent R (US) |
|---|---|---|
| 0.20 | 1.14 | 5.00 |
| 0.25 | 1.42 | 4.00 |
| 0.27 | 1.53 | 3.70 |
| 0.30 | 1.70 | 3.33 |
| 0.35 | 1.99 | 2.86 |
| 0.40 | 2.27 | 2.50 |
| 0.50 | 2.84 | 2.00 |
Why you add R-values but average U-values
In a layered assembly, heat passes through each layer in turn, so R-values add. With example layer values of R-13 cavity insulation, R-5 continuous insulation and R-2 for the remaining layers and air films, the path through the cavity totals R-20, a U of 0.05. Adding the U-values of the layers instead gives a meaningless number.
Where heat takes parallel paths, such as studs beside cavity insulation, you average U-values weighted by area, not R-values. With illustrative values of 25% of the wall area at R-9 through the framing and 75% at R-20 through the cavity: U = 0.25 ÷ 9 + 0.75 ÷ 20 = 0.0653, an effective R-15.3. Averaging the R-values would give R-17.25 and overstate the wall.
This is why a wall's whole-assembly R is lower than the number on the batt. For more on the difference, see understanding R-value and U-factor and typical exterior wall assemblies.
Building an assembly R-value with the layer stack
The layer-stack mode multiplies a typical R per inch by each layer's thickness, adds fixed-resistance items and the inside and outside air films, and inverts the total for U. The material values below are the presets the tool uses. They are typical published figures from ASHRAE Handbook—Fundamentals material tables (as tabulated by ColoradoENERGY.org) and ISO 6946, so use the manufacturer's data sheet for a real product and choose Custom R to enter it.
Surface resistances are small but not negligible. ISO 6946 uses 0.13 m²·K/W inside and 0.04 outside for walls (0.10 inside for upward heat flow, 0.17 for downward). ASHRAE winter values are 0.68 h·ft²·°F/Btu inside (still air, horizontal flow) and 0.17 outside at 15 mph. Together they add about R-0.85 (US) to a wall.
| Material | Typical R per inch (US) | Basis |
|---|---|---|
| Fiberglass or mineral wool batt | 3.14 | typical batt |
| Cellulose, loose fill | 3.5 | typical 3.1–3.7 |
| EPS board | 4.0 | expanded polystyrene |
| XPS board | 5.0 | extruded polystyrene |
| Polyiso board | 6.5 | typical aged (LTTR) value |
| Closed-cell spray foam | 6.0 | typical 6.0–7.0 |
| Open-cell spray foam | 3.6 | typical 3.5–3.7 |
| Plywood, OSB or softwood | 1.25 | softwood panel or lumber |
| Gypsum board | 0.90 | ½ in ≈ R-0.45 |
| Brick or cement stucco | 0.20 | clay brick, dense plaster |
| Concrete, normal weight | 0.08 | ≈ 140 lb/ft³ |
| Unventilated air cavity ≥ 25 mm | fixed 0.18 m²·K/W (R-1.0) | ISO 6946, wall |
| 8 in hollow concrete block | fixed R-1.11 | normal-weight CMU |
Diminishing returns: what each extra R saves
Heat flow is proportional to U, which is 1 ÷ R, so each extra unit of R saves less than the one before. Going from R-10 to R-20 halves conduction; going from R-50 to R-60 cuts it by a sixth. The table shows steady-state conduction through 1,000 ft² with a 70 °F temperature difference. To compare whole envelopes, use the envelope heat-loss comparison.
| R-value (US) | U (Btu/h·ft²·°F) | Heat flow (Btu/h) | Saving vs previous row |
|---|---|---|---|
| R-10 | 0.1000 | 7,000 | – |
| R-20 | 0.0500 | 3,500 | 3,500 Btu/h (50%) |
| R-30 | 0.0333 | 2,333 | 1,167 Btu/h (33%) |
| R-40 | 0.0250 | 1,750 | 583 Btu/h (25%) |
| R-50 | 0.0200 | 1,400 | 350 Btu/h (20%) |
| R-60 | 0.0167 | 1,167 | 233 Btu/h (17%) |
Who it is for and when to use it
Reading international product data
Convert RSI values on European or Canadian data sheets into the US R-values used on drawings and in US codes, and back again.
Preparing energy code documentation
Energy codes such as the IECC list both prescriptive R-values and maximum U-factors. Convert between them before entering values in compliance software. See IECC 2018 to 2021 changes.
Comparing windows with walls
Put a window's U-factor and a wall's R-value on the same scale to see how much more heat the glazing loses per square foot.
Setting up a heat-loss calculation
Convert R-values to U-values before multiplying by area and temperature difference in the envelope heat-loss comparison.
Method, formulas & assumptions
Single value: U(SI) = 1/R(SI). R(SI) = R(US) × 0.1761101838. U(SI) = U(US) × 5.67826334. Layer stack: total R = sum of layer R (typical R per inch × thickness, fixed items, or your custom R) + inside and outside surface resistances (ISO 6946 or ASHRAE presets, custom, or none); U = 1 ÷ total R; optional pass/fail against a target U or R you enter.
Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.
Common mistakes to avoid
- Comparing a product R-value with a whole-assembly U-factor. Framing, fasteners and air films make them different quantities.
- Averaging R-values across framed and insulated areas. Average U-values by area instead, then invert.
- Converting in the wrong direction. US R to RSI multiplies by 0.1761; RSI to US R multiplies by 5.678.
- Confusing a total R-value with R per inch. Some data sheets give resistance per inch or per 25 mm, which must be multiplied by the installed thickness first.
- Treating a higher U as better. For U-values and U-factors, lower means less heat flow.
- Forgetting that nominal R assumes correct installation. Compressed batts and gaps reduce real performance, and no conversion accounts for that.
Scope & limits
Use like-for-like values. Layer R values are typical published figures (ASHRAE Handbook—Fundamentals tables, ISO 6946), not product data; parallel paths such as studs are not averaged and thermal bridging is not included. The target check compares with a value you enter, not a code lookup.
Frequently asked questions
How do you convert R-value to U-value?
Divide 1 by the R-value, staying in one unit system. R-20 in US units gives U = 1 ÷ 20 = 0.05 Btu/h·ft²·°F, and RSI-3.5 gives U = 0.286 W/m²·K.
How do you convert a US R-value to RSI?
Multiply by 0.1761. R-19 is about RSI-3.35 and R-30 is about RSI-5.28.
How do you convert RSI to a US R-value?
Multiply by 5.678. RSI-3.5 is about R-19.87 and RSI-5 is about R-28.39.
What is the U-value of R-30 insulation?
1 ÷ 30 = 0.0333 Btu/h·ft²·°F, or about 0.189 W/m²·K. That describes the insulation layer alone, not a whole roof or wall assembly.
Is a lower U-value better?
Yes. U-value measures how much heat passes through, so lower is better. For R-value, higher is better.
What is the difference between U-value and U-factor?
They describe the same thing: heat flow per unit area per degree of temperature difference. U-factor is the usual US term, especially for NFRC-rated windows, while U-value is common internationally.
Why is the conversion factor 5.678?
One Btu/h·ft²·°F equals 5.678 W/m²·K, which follows from the definitions of the Btu, the foot and the Fahrenheit degree. The R-value factor, 0.1761, is its inverse.
Can I add R-values together?
Yes, for layers that heat passes through one after another, which is what the layer-stack mode does, including the inside and outside air films. For side-by-side paths such as studs and cavity insulation, average the U-values by area instead.
Does the tool check my assembly against the energy code?
It checks against a target you enter, not a code lookup. Type the maximum U-factor or minimum R-value from your code table or specification and the tool reports PASS or FAIL in both unit systems.
Do I need an account to download the result?
No. All calculations, local file processing and exports on this page are free, without an account.
Where are my inputs stored?
The tool keeps inputs in this browser tab. It does not upload them or create a cloud copy. Save or download anything you want to keep before leaving the page. Standard site analytics may operate independently of the tool.
Can I work in metric instead of imperial?
Yes. Use the Imperial / Metric switch above the inputs. Imperial (feet, inches, pounds, gallons) is the default; switching converts every dimensional field and result, and your choice is remembered across the free tools in this browser.
Further reading
- Understanding R-value and U-factor
- Typical exterior wall assemblies
- IECC 2018 to 2021 changes
- All building performance tools
- All free architecture & construction tools
- Harth resource library
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