Insulation R-Value Converter
Convert insulation R-values between US imperial units (ft²·°F·h/BTU) and SI metric units (m²·K/W), and get the corresponding U-values in both systems. Just pick the unit system and enter the R-value.
Convert Insulation R-Values Between US and SI Units
Insulation R-Value Conversion Formula
Where:
- \(R_{US}\) — R-value in US imperial units (ft²·°F·h/BTU), as printed on North American product labels
- \(R_{SI}\) — R-value in SI metric units (m²·K/W), used in European and international building standards
- \(0.176110\) — conversion factor: 1 ft²·°F·h/BTU = 0.176110 m²·K/W exactly (derived from 1 BTU = 1055.05585262 J)
- \(U\) — U-value (thermal transmittance), the reciprocal of the R-value in the same unit system
The first two formulas convert thermal resistance between the US and SI unit systems. The third shows that the U-value is simply the inverse of the R-value: high R means strong insulation, while low U means little heat passes through.
Insulation R-Value – Calculation Example
Example: a US fiberglass batt labeled R-13 (typical for a 2×4 wall) needs to be expressed in metric units for an international energy report.
\[ R_{SI} = 13 \times 0.176110 = 2.289 \ \text{m}^2\!\cdot\!\text{K/W} \]
\[ U_{SI} = \frac{1}{2.289} = 0.437 \ \text{W/(m}^2\!\cdot\!\text{K)} \qquad U_{US} = \frac{1}{13} = 0.0769 \ \text{BTU/(h}\!\cdot\!\text{ft}^2\!\cdot\!\text{°F)} \]
Result: R-13 (US) = 2.289 m²·K/W (SI). The matching U-values are 0.437 W/(m²·K) in metric units and 0.0769 BTU/(h·ft²·°F) in imperial units.
How to use the calculator
- In the field Unit system of the entered R-value, select US: ft²·°F·h/BTU if your number comes from a North American product label, or SI (metric): m²·K/W if it comes from a metric specification.
- Enter your value in Insulation R-value (R). Decimal values such as 5.6 are accepted.
- Read the results in the Converted Values panel: the R-value in the other unit system, plus the U-value in both systems.
What is an R-value?
The R-value measures thermal resistance: how strongly a layer of material opposes the flow of heat through it. The higher the R-value, the slower the heat transfer and the better the insulation. In the US, R-values are written as plain numbers on product labels — R-13 for 2×4 walls, R-19 for 2×6 walls, R-30 to R-49 for attics — with the units ft²·°F·h/BTU implied. Most of the rest of the world uses SI units, m²·K/W, where typical whole-wall values range from about 2 to 8.
Because the two unit systems differ by a factor of about 5.68, a US R-value and a metric R-value can look deceptively similar while describing very different performance. R-13 in US units is 2.289 m²·K/W — not 13. Whenever you compare products, quotes, or building codes across regions, always convert first, which is exactly what this calculator does.
The calculator also returns the U-value, the reciprocal of the R-value. The U-value (thermal transmittance) expresses how much heat actually passes through one square meter (or square foot) per degree of temperature difference. European building regulations and window specifications typically quote U-values: lower is better. A wall with R-13 has U = 0.0769 BTU/(h·ft²·°F) or 0.437 W/(m²·K).
R-values of layers simply add up: two R-13 batts one on top of the other give R-26. U-values do not add — they combine through their reciprocals, which is why working in R-values is more convenient for assemblies.
Common R-values in both unit systems
| Application (US label) | R (ft²·°F·h/BTU) | R (m²·K/W) | U (W/(m²·K)) |
|---|---|---|---|
| 2×4 wall cavity | 13 | 2.29 | 0.437 |
| 2×6 wall cavity | 19 | 3.35 | 0.299 |
| Attic / ceiling (minimum) | 30 | 5.28 | 0.189 |
| Attic / ceiling (cold climates) | 38 | 6.69 | 0.149 |
| High-performance attic | 49 | 8.63 | 0.116 |
Limitations and practical tips
- Nominal vs. whole-assembly R. Label R-values describe the insulation material alone. Real walls lose performance to studs, fasteners, and gaps (thermal bridging), so a 2×4 wall with R-13 batts may perform closer to R-9 to R-11 overall.
- R-value is not everything. It only covers conductive heat flow at steady state. Air leakage, moisture, thermal mass, and radiant gain can matter as much as, or more than, the R-value — an airtight R-13 wall often outperforms a leaky R-19 one.
- Conditions matter. Laboratory R-values are measured at a standard mean temperature (about 24 °C / 75 °F). Some materials, particularly loose-fill and foams, lose or gain resistance at temperature extremes or as they age and settle.
- Mind the U-value direction. When reading U-values, remember that lower is better — the opposite of R-values. If a specification quotes a U-value, this calculator's reciprocal outputs let you cross-check it.
- Units are not optional. A bare "R-5" in a European document almost certainly means 5 m²·K/W (≈ R-28 US), not 5 ft²·°F·h/BTU. Always confirm the unit system before comparing.
If you work with thermal and climate calculations, you may also find our Heat Index Calculator, Dew Point Calculator, and Wind Chill Calculator useful for related HVAC and weather analysis.
Frequently Asked Questions
How do I convert an R-value by hand without the calculator?
Multiply a US R-value by 0.176 to get m²·K/W, or divide it by 0.176 (equivalently, multiply by about 5.68) to go from metric to US. For example, R-19 US: \(19 \times 0.176 = 3.35\) m²·K/W. For a quick mental estimate, remember that metric R-values are roughly one-sixth of the US number, so R-30 US ≈ 5.3 m²·K/W.
What is a good R-value for walls, attics, and floors?
For US 2×4 walls, R-13 to R-15 is standard; 2×6 walls typically use R-19 to R-21. Attics need R-30 to R-49 depending on climate (higher in colder regions), and floors over unheated spaces are often R-19 to R-30. In metric units, that translates to roughly 2.3–3.7 m²·K/W for walls and 5.3–8.6 m²·K/W for attics. Always check your local building code, since requirements vary by climate zone.
Why is a US R-value about 5.7 times bigger than the metric value?
The two unit systems measure the same physical property but use different base units. One ft²·°F·h/BTU equals exactly 0.176110 m²·K/W, derived from the definition of the BTU (1 BTU = 1055.05585262 J), square feet versus square meters, and Fahrenheit versus Kelvin temperature scales. This is why R-13 US is only 2.29 m²·K/W — the numbers describe identical insulation performance.
Can I add R-values together, and how do I combine U-values?
R-values of layers in series simply add: R-13 batt plus R-5 foam board gives R-18 total. U-values cannot be added directly — you must convert each to its R-value first, sum them, then take the reciprocal. For example, layers with U = 0.5 and U = 0.2 give R = 2 + 5 = 7, so the combined U is about 0.143 W/(m²·K). This is why assembly calculations are usually done in R-values.
Is R-5 on a European product the same as R-5 on a US product?
No, and mixing them up is one of the most common mistakes. A bare "R-5" in a European or international document almost always means 5 m²·K/W, which equals about R-28 in US units — a dramatically different level of insulation. Before comparing products, codes, or quotes across regions, confirm which unit system the number uses and convert it first.
What's the difference between R-value and U-value, and which is better?
They are reciprocals describing the same thing from opposite directions: R-value measures resistance to heat flow (higher is better), while U-value measures transmittance, the heat that actually passes through (lower is better). Europe and window manufacturers favor U-values; North America favors R-values. If a spec sheet quotes U = 0.25 W/(m²·K), the equivalent R-value is \(1/0.25 = 4\) m²·K/W ≈ R-23 US.
Why is a whole wall's real performance lower than the labeled R-value?
Labeled R-values describe only the insulation material, but real assemblies contain studs, nails, headers, and gaps that conduct heat faster — a phenomenon called thermal bridging. A 2×4 wall filled with R-13 batts often performs closer to R-9 to R-11 overall. For accurate energy modeling, look up whole-assembly R-values or use parallel-path calculation rather than relying on the label value.