Convert C Factor to U Factor: Formula & Guide
- Total R-value
- m²·K/W
- Result
- C factor
- 0.5 W/m²·K
- You entered
- Surface film resistance (Rsi + Rse)
- 0.17 m²·K/W
- You entered
To convert a C factor (thermal conductance) to a U factor (thermal transmittance), you use the equation U = C × 1 when the C factor already includes surface air films; otherwise, you must add the film resistances. In practice, the C factor is the steady-state heat transfer rate through a unit area of a building assembly for a one-degree temperature difference, and the U factor is the same quantity but typically includes the effects of interior and exterior surface air films. This article explains the difference, the exact conversion steps, and common pitfalls.
Formula
The conversion depends on whether the C factor already includes surface air films. The general relationship is:
- If C includes surface films: U = C (no conversion needed, but verify the test conditions).
- If C does not include surface films (common for material data): U = 1 / (1/C + Rsi + Rse)
Where:
- C = thermal conductance (W/m²·K or Btu/h·ft²·°F)
- Rsi = interior surface air film resistance (m²·K/W or h·ft²·°F/Btu)
- Rse = exterior surface air film resistance (m²·K/W or h·ft²·°F/Btu)
- U = thermal transmittance (W/m²·K or Btu/h·ft²·°F)
The sum Rsi + Rse is often taken as 0.17 m²·K/W for vertical surfaces in standard conditions, but it varies with wind, emissivity, and position.
Worked Example
Using the calculator defaults: C = 0.5 W/m²·K, and Rsi + Rse = 0.17 m²·K/W.
- Compute the material R-value: Rmaterial = 1 / C = 1 / 0.5 = 2.0 m²·K/W
- Add surface film resistances: Rtotal = 2.0 + 0.17 = 2.17 m²·K/W
- Take the reciprocal to get U: U = 1 / 2.17 ≈ 0.461 W/m²·K
The calculator shows U = 0.461 and Rtotal = 2.170. This matches the manual calculation.
When to Use This Conversion
You need this conversion when working with building material data sheets, energy modeling software, or code compliance. Manufacturers often list a C factor for a specific product (e.g., insulated panels, concrete masonry units, or window assemblies) without the surface air film resistances. To get the whole-assembly U factor for a wall or roof, you must add those films. Conversely, if you have a U factor from a whole-assembly test (like a window rating), you can back-calculate the C factor by subtracting the film resistances.
How to Read the Result
Lower U factors mean better insulation. A U factor of 0.5 W/m²·K allows 0.5 watts per square meter per degree Kelvin of temperature difference. In US units, a U factor of 0.1 Btu/h·ft²·°F is roughly equivalent to 0.568 W/m²·K. Always check the units: the calculator outputs in metric, but you can convert using 1 W/m²·K = 0.1761 Btu/h·ft²·°F.
Common Mistakes
- Forgetting surface films: The most common error is using U = C directly when the C factor does not include films, which overestimates heat loss by 5–10%.
- Unit mix-ups: Mixing metric and imperial R-values in the same equation. Convert everything to one system first.
- Assuming constant film resistance: The 0.17 m²·K/W value is a standard approximation for vertical surfaces with moderate wind. For high winds or low-emissivity surfaces, the actual value differs.
- Using C for non-homogeneous assemblies: If the assembly has thermal bridges (e.g., steel studs), the simple series calculation fails. Use a two-dimensional or three-dimensional model instead.
Limits of the Formula
This conversion assumes steady-state heat flow, one-dimensional heat transfer, and no moisture or air movement. Real walls have framing, insulation, air gaps, and varying surface conditions. The formula is accurate for homogeneous layers in series, but for complex assemblies, you should use a thermal modeling tool or a hot-box test. Also, the C factor is temperature-dependent for some materials (e.g., insulation with temperature-dependent conductivity), so the conversion is only valid at the test temperature.
Frequently Asked Questions
What is the difference between C factor and U factor?
The C factor (thermal conductance) measures heat flow through a material or assembly without surface air films. The U factor (thermal transmittance) includes the surface air films, so it represents the total heat flow from the indoor air to the outdoor air. For a wall, the U factor is always lower than the C factor because the air films add resistance.
How do I convert U factor to C factor?
Use the same equation rearranged: C = 1 / (1/U – Rsi – Rse). For example, if U = 0.3 W/m²·K and the total film resistance is 0.17, then C = 1 / (1/0.3 – 0.17) = 1 / (3.333 – 0.17) = 1 / 3.163 ≈ 0.316 W/m²·K.
What is the typical Rsi + Rse value?
For vertical surfaces, the combined surface film resistance is often taken as 0.17 m²·K/W (0.97 h·ft²·°F/Btu) in standard conditions. For horizontal surfaces (e.g., roofs), it may be higher or lower depending on heat flow direction. Always check the standard you are following (e.g., ASHRAE, ISO 6946).
Can I use this for windows?
Yes, but windows are typically rated with a U factor that already includes surface films. If you have a C factor for the glass center-of-glass, you need to add the film resistances and also account for the frame and edge effects. For simplicity, use the whole-window U factor directly.
Why does my calculator show a different U factor than a product datasheet?
Product datasheets may use different test conditions (e.g., winter vs. summer, different wind speeds, or different surface film resistances). Always verify the assumptions. The calculator uses a standard 0.17 m²·K/W film resistance, but your datasheet might use 0.04 for exterior and 0.13 for interior, totaling 0.17, or another combination.