R-Value vs U-Value: Thermal Resistance vs Thermal Transmittance Sizing
R-Value measures an insulation material's thermal resistance to conductive heat flow, where higher values indicate superior insulation performance. Conversely, U-Value (overall heat transfer coefficient) quantifies the rate of heat loss or gain through a complete building assembly (glass, frames, wall assemblies), where lower numbers denote superior energy conservation.
Technical Comparison & Conversion Matrix
| Building Element | Typical R-Value (hr-sq ft-°F/BTU) | Equivalent U-Value (BTU/hr-sq ft-°F) | IECC 2024 Climate Zone 4 Code |
|---|---|---|---|
| Single-Pane Clear Glass | R-0.90 | U-1.11 | Non-compliant for new construction |
| Double-Pane Low-E Window | R-3.33 | U-0.30 | Mandatory fenestration baseline (U <= 0.30) |
| Triple-Pane Krypton Glazing | R-6.67 | U-0.15 | Passivhaus / Net-Zero energy standard |
| Standard 2x4 Wall (Fiberglass) | R-13.00 | U-0.077 | R-13 cavity (Zone 1-2 standard) |
| Advanced 2x6 Wall (Mineral Wool) | R-20.00 | U-0.050 | R-20 cavity or R-13 + R-5 continuous |
| Attic Blown Cellulose (14-inch) | R-49.00 | U-0.020 | Standard ceiling baseline (R-49 to R-60) |
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Frequently Asked Questions
Why is U-value used for windows while R-value is used for wall insulation?
Windows and doors are composite assemblies with multiple thermal bridges (glass panes, spacer bars, gas fills, sash framing). U-value accounts for whole-assembly conductive and radiative transfer, whereas R-value typically rates homogeneous insulation batt materials.
How do you calculate heat loss (BTU/hr) through a building envelope assembly?
Heat Loss = U-Value * Area (sq ft) * Delta_T (°F) = (Area * Delta_T) / R-Value. A 200 sq ft wall rated R-20 with a 50°F indoor-to-outdoor temperature differential loses (200 * 50) / 20 = 500 BTU/hr.