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Convert thermal resistance (R-value) to thermal transmittance (U-value) for insulation calculations
Formula: U-Value = 1 / R-Value
SI Equivalent: R-Value (SI) = 1.7610 m²·K/W, U-Value (SI) = 0.567859 W/(m²·K)
| Material | R (Imperial) | U (Imperial) | R (SI) | U (SI) |
|---|---|---|---|---|
| Single pane glass | 0.91 | 1.100 | 0.16 | 6.25 |
| Double pane glass | 2 | 0.500 | 0.35 | 2.84 |
| Triple pane glass | 3 | 0.330 | 0.53 | 1.89 |
| Fiberglass batt (3.5") | 11 | 0.091 | 1.94 | 0.52 |
| Spray foam (3.5") | 21 | 0.048 | 3.70 | 0.27 |
| Brick wall (4") | 0.8 | 1.250 | 0.14 | 7.10 |
R-Value (Thermal Resistance) measures how well a material resists heat flow. Higher R-values indicate better insulation. It's the inverse of U-value and is commonly used in North America.
U-Value (Thermal Transmittance) measures how much heat passes through a material. Lower U-values indicate better insulation. It's the standard metric used in Europe and international building codes.
Simply divide 1 by the R-value. For example, R-10 becomes U = 1/10 = 0.1 BTU/(hr·ft²·°F). The relationship is reciprocal.
Both indicate the same thing from different perspectives. High R-value = good insulation (more resistance to heat flow). Low U-value = good insulation (less heat transmission). They're inverses of each other.
No! U-values are NOT additive. To combine layers, convert each U-value to R-value (R = 1/U), add the R-values, then convert back (U = 1/R_total). R-values ARE additive.
It depends on climate zone and building component. Generally, cold climates need R-49 to R-60 in attics, R-13 to R-21 in walls. Check local building codes for specific requirements.
Multiply Imperial R-value by 0.1761 to get SI R-value (m²·K/W). Divide SI R-value by 0.1761 to get Imperial R-value. For example, R-20 Imperial = 3.52 m²·K/W.
Yes, R-values can vary with temperature, especially for foam insulations. Manufacturers typically rate products at 75°F (24°C). Performance may differ in extreme conditions.