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U-Value to R-Value Converter (U-Factor, RSI & R-Value)
Product sheets from North America usually quote imperial R-values and U-factors, while metric sources use RSI and U-values in W/m²·K. Convert one number into all four forms, or add up the R-values of layers you already know.
Quick chart: common R-values in RSI and U-factor
| R-value (IP) | RSI m²·K/W |
U-factor (IP) BTU/h·ft²·°F |
U-value (SI) W/m²·K |
|---|---|---|---|
| R-13 | 2.29 | 0.0769 | 0.437 |
| R-19 | 3.35 | 0.0526 | 0.299 |
| R-30 | 5.28 | 0.0333 | 0.189 |
| R-38 | 6.69 | 0.0263 | 0.149 |
Unit conversions only (U = 1 ÷ R, RSI = R × 0.17611), rounded for reading; not product ratings or code minimums. Select a value to load it into the converter. Fuller charts and worked examples: conversion tables in the R-value, RSI & U-factor guide.
Quick Conversion Table: Imperial R-Value to SI RSI and U-Factor
Reference conversion table between Imperial R-value, SI metric RSI, imperial U-factor and metric U-value, highlighting US building assembly standards and attic insulation requirements. Click any R-value to prefill and compute directly in the converter above:
| R-Value (IP) ft²·°F·h/BTU |
U-Factor (IP) BTU/h·ft²·°F |
Metric RSI m²·K/W |
Metric U-Value W/m²·K |
Typical Building Application |
|---|---|---|---|---|
| R-1 | 1.0000 | 0.18 | 5.678 | Single-pane clear window glass benchmark |
| R-3 | 0.3333 | 0.53 | 1.893 | Standard double-pane window / thin insulation board |
| R-5 | 0.2000 | 0.88 | 1.136 | 1-inch continuous XPS foam board / triple glazing |
| R-8 | 0.1250 | 1.41 | 0.710 | Continuous exterior insulation / insulated sheathing |
| R-10 | 0.1000 | 1.76 | 0.568 | 2-inch continuous XPS / basement wall insulation |
| R-13 | 0.0769 | 2.29 | 0.437 | 2×4 exterior wall fiberglass batt (standard) |
| R-15 | 0.0667 | 2.64 | 0.379 | 2×4 high-density cavity batt insulation |
| R-19 | 0.0526 | 3.35 | 0.299 | 2×6 exterior wall batt / crawlspace floor |
| R-21 | 0.0476 | 3.70 | 0.270 | 2×6 high-density exterior wall insulation |
| R-30 | 0.0333 | 5.28 | 0.189 | Ceiling & attic insulation (warmer climates) |
| R-38 | 0.0263 | 6.69 | 0.149 | Standard residential attic insulation |
| R-49 | 0.0204 | 8.63 | 0.116 | Cold-climate ceiling / attic insulation |
| R-60 | 0.0167 | 10.57 | 0.095 | Very cold climate / superinsulated roof |
Recommended US Attic & Wall Insulation by Climate Zone (IECC / DOE)
Building energy codes and the US Department of Energy (DOE) recommend specific attic insulation levels (R-30 to R-60) and wall assemblies depending on your local climate zone:
| Climate Zone | Representative US Regions | Attic Insulation (R-Value) | Attic Metric RSI m²·K/W |
Attic U-Factor BTU/h·ft²·°F |
Wall Cavity Insulation |
|---|---|---|---|---|---|
| Zone 1 | South Florida, Hawaii, Virgin Islands | R-30 to R-49 | 5.28 – 8.63 | 0.0333 – 0.0204 | R-13 |
| Zone 2 | Southern Texas, Southern California, Gulf Coast | R-38 to R-49 | 6.69 – 8.63 | 0.0263 – 0.0204 | R-13 to R-15 |
| Zone 3 | Carolinas, Georgia, Alabama, Mississippi, Texas, Arizona | R-38 to R-49 | 6.69 – 8.63 | 0.0263 – 0.0204 | R-13 to R-15 / R-20 |
| Zone 4 | Mid-Atlantic, Virginia, Kentucky, Missouri, Kansas, Pacific NW | R-49 to R-60 | 8.63 – 10.57 | 0.0204 – 0.0167 | R-20 or R-13 + R-5 ci |
| Zone 5 | Midwest, Pennsylvania, New York, Ohio, Indiana, Illinois | R-49 to R-60 | 8.63 – 10.57 | 0.0204 – 0.0167 | R-20 or R-13 + R-5 ci |
| Zone 6 | Northern Tier, New England, Wisconsin, Minnesota, Montana | R-49 to R-60 | 8.63 – 10.57 | 0.0204 – 0.0167 | R-20 + R-5 ci / R-13 + R-10 ci |
| Zones 7–8 | Northern Minnesota, North Dakota, Wyoming, Alaska | R-49 to R-60 | 8.63 – 10.57 | 0.0204 – 0.0167 | R-20 + R-5 ci / R-13 + R-10 ci |
Values are theoretical one-dimensional conversions calculated via reciprocal formulas (R = 1/U, RSI = R × 0.17611). More detailed tables and metric-to-imperial comparisons are in the conversion guide reference tables.
How to convert R-value to U-value and U-factor to R-value by hand
U-factor and R-value are reciprocals in matching units. Follow these steps to convert manually:
- Identify the unit system. Check whether you have an imperial U-factor (BTU/h·ft²·°F), imperial R-value (ft²·°F·h/BTU), metric U-value (W/m²·K) or metric RSI (m²·K/W).
- Take the reciprocal (1 ÷ value). For imperial U-factor to R-value: R = 1 ÷ U (e.g. U-0.30 → R-3.33). For imperial R-value to U-factor: U = 1 ÷ R (e.g. R-13 → U-0.0769). For metric U-value to RSI: RSI = 1 ÷ U (e.g. U 0.25 → RSI 4.00).
- Convert between metric and imperial if needed. Convert RSI to imperial R-value: R = RSI × 5.678 (or R = 5.678 ÷ U-value). Convert imperial R-value to metric RSI: RSI = R ÷ 5.678 (or U-value = 5.678 ÷ R).
| You have | Formula | Example |
|---|---|---|
| U-factor, imperial (BTU/h·ft²·°F) | R (IP) = 1 ÷ U | U-0.30 → R-3.33 |
| U-value, metric (W/m²·K) | RSI = 1 ÷ U | U 0.25 → RSI 4.00 m²·K/W |
| U-value, metric (W/m²·K) | R (IP) = 5.678 ÷ U | U 0.25 → R-22.7 |
| R-value, imperial | U (SI) = 5.678 ÷ R | R-20 → U 0.284 W/m²·K |
| RSI (m²·K/W) | R (IP) = RSI × 5.678 | RSI 3.0 → R-17.0 |
Longer charts (R-1 to R-60, window U-factors, metric U-values and RSI) are in the guide's U-value to R-value conversion tables, and the FAQ answers common questions such as whether U-factor and U-value are the same thing.
Frequently asked questions
How do I convert an R-value to a U-value?
To convert an R-value to a U-value, take the reciprocal: U = 1 ÷ R in matching units. For imperial units, an R-13 wall has a U-factor of 1 ÷ 13 ≈ 0.0769 BTU/h·ft²·°F. To convert to metric U-value (W/m²·K), divide 5.678 by the imperial R-value: 5.678 ÷ 13 ≈ 0.437 W/m²·K.
How do I convert a U-factor to an R-value?
To convert a U-factor to an R-value, divide 1 by the U-factor: R = 1 ÷ U. For example, a window with an imperial U-factor of 0.30 has an equivalent thermal resistance of 1 ÷ 0.30 ≈ R-3.33 (RSI 0.59). For metric U-values, R = 5.678 ÷ U.
What is the formula to convert R-value to U-factor?
The fundamental formula is U = 1 ÷ R (and R = 1 ÷ U) within the same unit system. In imperial units, U-factor is in BTU/h·ft²·°F and R-value is in ft²·°F·h/BTU. When converting between metric (W/m²·K) and imperial, multiply or divide by 5.678263.
What is the difference between U-value and U-factor?
Both describe thermal transmittance (heat flow rate per unit area per degree of temperature difference). In North America, "U-factor" usually refers to imperial units (BTU/h·ft²·°F) and whole-product ratings like windows. "U-value" is standard in metric contexts (W/m²·K). 1 BTU/h·ft²·°F equals 5.678 W/m²·K.
Can I sum R-values and U-values of wall layers?
You can add R-values of layers in series directly: R(total) = R1 + R2 + ... + Rn. You cannot add U-values directly for sequential layers. Always sum the resistances (R-values) first, then take the reciprocal to find the overall assembly U-factor.
What are the US attic insulation requirements by climate zone?
Under the International Energy Conservation Code (IECC) and Department of Energy (DOE) benchmarks, recommended attic insulation levels range from R-30 to R-49 in warm climates (Zones 1 to 3) and R-49 to R-60 in cold climates (Zones 4 to 8). In metric RSI, this translates to RSI 5.28 to RSI 10.57 m²·K/W.
How do I convert imperial R-value to metric RSI for building plans?
Multiply the imperial R-value by 0.176110 to obtain metric RSI (in m²·K/W), or divide the R-value by 5.678263. For example, standard US wall insulation R-13 equals RSI 2.29 m²·K/W, and attic insulation R-38 equals RSI 6.69 m²·K/W.
What is the difference between prescriptive R-value and U-factor in US building codes?
Prescriptive R-value sets the minimum thermal resistance for individual insulation layers (such as cavity batt or continuous exterior foam). The U-factor alternative evaluates the overall heat transmission of the complete assembled wall or roof, accounting for thermal bridging through studs and joists.
What R-value is required for residential exterior walls in the US?
Current US residential building energy codes (IECC / IRC) commonly prescribe R-13 to R-20 in 2x4 or 2x6 wall cavities for mild regions (Zones 1 to 3), and R-20 cavity or R-13 cavity plus R-5 continuous insulation for colder northern climates (Zones 4 to 8).
Formula summary and conversion standards
RSI = R(imperial) × 0.1761101838
U = 1 ÷ R (in matching units)
U(W/m²·K) = U(BTU/h·ft²·°F) × 5.678263
R(imperial) = 5.678263 ÷ U(metric)
R(total, series) = R₁ + R₂ + … + Rₙ
- Reciprocal law: thermal resistance (R) and transmittance (U) are exact reciprocals:
R = 1/UandU = 1/Rwhen expressed in matching unit systems. - Conversion constant: the factor
1 ft²·°F·h/BTU = 0.1761101838 m²·K/W(and its inverse5.678263) derives from standard physical definitions listed in the SI/IP conversion tables of the ASHRAE Handbook—Fundamentals. - Series layers: adding resistances in series (
R_total = R₁ + R₂ + … + Rₙ) is the standard one-dimensional method for heat crossing continuous layers sequentially (ASHRAE Fundamentals, chapter on heat transfer through building components).
Limits
- The series sum assumes every layer is continuous. Studs, fasteners, slab edges and other thermal bridges create parallel heat paths that lower the real performance of a wall, so a series total of insulation layers overstates the whole-assembly R-value.
- Window U-factors from rating labels already describe the whole product; do not add them to wall layers.
- Results are calculations from the numbers you enter. They are not a measurement of any product and do not show whether an assembly meets an energy code.
Reading the result
- Higher R-value or RSI means more resistance to heat flow; lower U-factor or U-value means less heat passes through. The four rows describe the same number.
- When comparing products, check that both figures use the same unit and the same basis (for example whole-window U-factor vs centre-of-glass). See comparing like with like.
- For a whole wall or roof, the series total is an upper bound when framing interrupts the insulation. The guide shows a parallel-path example.
Background, reference tables and examples: U-value and R-value: how to convert U-factor to R-value (and RSI).