Thermal Resistance to U-value Converter

To calculate the overall U-value, sum all individual R-values—including insulation layers, structural elements, and air film resistances—then take the reciprocal: $U = 1 / \sum R_{\text{total}}$. Per ISO 6946 and ASHRAE Fundamentals Chapter 26, $R_{\text{total}} = R_{\text{inside air film}} + \sum R_{\text{material layers}} + R_{\text{outside air film}}$. Our tool automates this by accepting an array of layer R-values and adding the default or user-specified air film resistance (0.17 m²·K/W per side, per ISO 6946 Table 1). Note: This method assumes one-dimensional, steady-state conduction and excludes thermal bridging effects—address those separately using ISO 13370 or numerical modeling.

Discrepancies commonly arise from unaccounted thermal bridging (e.g., studs, fasteners), non-standard air film resistances, or differences in test conditions. Manufacturer U-values are typically determined per EN ISO 10211 or ASTM C1363 under controlled lab conditions, while field assemblies include junctions, gaps, and compression that degrade performance. ASHRAE Standard 90.1 mandates accounting for framing factors via parallel-path or isothermal-planes methods—not simple R-summing. Always verify R-values against certified test reports (e.g., ASTM C518) and adjust for real-world installation quality, moisture exposure, and aging per ISO 10456 Annex A.

Per ISO 6946 Table 1 and ASHRAE Fundamentals Chapter 26, use $R_{\text{si}} = 0.13$ m²·K/W (inside surface) and $R_{\text{se}} = 0.04$ m²·K/W (outside surface) for vertical walls in still-air conditions. However, for cold, windy climates, ASHRAE recommends reducing $R_{\text{se}}$ to 0.03–0.04 m²·K/W to reflect increased convective heat transfer. Our tool defaults to 0.17 m²·K/W total air film resistance—equivalent to 0.13 + 0.04—aligning with standard indoor/outdoor assumptions. For precise energy modeling (e.g., EnergyPlus), use climate-specific convection coefficients derived from correlations like McAdams or ISO 15927-2.

No—this tool accepts only SI units: R-values in m²·K/W and outputs U-value in W/m²·K. Imperial R-values must first be converted: $R_{\text{SI}} = R_{\text{IP}} \times 0.1761$. For example, R-19 ≈ 3.34 m²·K/W. Direct conversion without unit correction yields erroneous results. ASHRAE Handbook—Fundamentals (Ch. 26) and ISO 8990 emphasize consistent unit systems to avoid order-of-magnitude errors. Always validate conversions using NIST SP 811 guidelines. The tool’s input validation enforces SI units to prevent accidental misuse—a critical safeguard per ANSI/ASHRAE Standard 103 verification protocols.

Thermal bridging—caused by conductive elements like steel studs, concrete slabs, or window frames—bypasses insulation, lowering effective R-value and raising U-value beyond what simple R-summing predicts. ISO 10211 requires 2D/3D thermal modeling or simplified correction factors (e.g., psi-values) to quantify bridging impact. ASHRAE 90.1 Appendix C permits adjustment via framing factor multipliers (e.g., 0.80 for wood stud walls). Ignoring bridging may underestimate U-value by 20–50%. Our tool calculates the *idealized* U-value; always apply bridging corrections separately using ISO 13370 or manufacturer-provided assembly U-values verified per ASTM C1363.

Vacuum insulated panels (VIPs) achieve R-25–R-40 per inch (≈14–22 m²·K/W·m), followed by polyisocyanurate (R-6–R-7/in ≈ 10.6–12.4 m²·K/W·m) and spray polyurethane foam (R-5.5–R-6.5/in ≈ 9.7–11.5 m²·K/W·m). However, long-term thermal resistance (LTTR) per ASTM C1363 accounts for aging—polyiso can lose 15–20% R-value over time. Selecting high-R/inch materials reduces thickness but increases cost and fire-rating complexity. For compliance with IECC or Passivhaus (U ≤ 0.15 W/m²·K), prioritize stable, tested LTTR values—not initial R-values—and verify conformity with UL 723 and CAN/ULC-S102 flame spread requirements.

Yes—R-values are additive across series layers regardless of material type, provided heat flow is perpendicular to layers and conditions are steady-state (ISO 6946 §4.2). However, interfacial air gaps, compression, or moisture accumulation can invalidate additivity. ASTM C1158 cautions that compressing fiberglass batts against rigid foam reduces effective R-value by up to 30%. Always model layers in actual installed thickness and density, referencing manufacturer data sheets validated per ASTM C518 (hot plate) or C177 (guarded hot plate). Field verification via infrared thermography per ASTM C1060 helps confirm continuity and detect voids compromising summed R-value integrity.