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ConverterU-value & R-value Converter
Convert between U-value and R-value for building insulation. Calculate total thermal resistance for multi-layer assemblies. Free HVAC tool.
The U-value & R-value Converter helps you convert between thermal transmittance (U-value) and thermal resistance (R-value). Calculate total R-value for multi-layer building assemblies including walls, roofs, and floors for accurate energy modeling and code compliance.
U-value and R-value are reciprocals: U = 1/R and R = 1/U Higher R-value = better insulation. Lower U-value = better insulation.
Have feedback? Report bugs, suggest features, or share your thoughts — we read them allWhat are U-value and R-value?
U-value (thermal transmittance) measures how easily heat flows through a material or assembly. Lower U-values indicate better insulating performance. R-value (thermal resistance) measures a material's resistance to heat flow. Higher R-values indicate better insulation. They are mathematical reciprocals: U = 1/R. In the US, R-value is more commonly used, while international building codes often use U-value. Understanding both is essential for energy modeling, building envelope design, and code compliance.
U-value and R-value Relationship
U = 1 / R and R = 1 / U
Example: R-20 insulation has U-value = 1/20 = 0.05 Btu/(h·ft²·°F)
Thermal Resistance Formulas
1. Reciprocal Relationship
U-value = 1 / R-value OR R-value = 1 / U-value
2. Total R-value (Series Layers)
Rtotal = R1 + R2 + R3 + ... + Rn
Typical R-values (Imperial)
- Exterior air film: R-0.17 (15 mph wind)
- Interior air film: R-0.68 (still air)
- Fiberglass batt insulation: R-3.1 to R-3.4 per inch
- Spray foam (closed cell): R-6 to R-7 per inch
- XPS foam board: R-5 per inch
- Polyisocyanurate foam: R-5.6 to R-6.5 per inch
- Wood studs (2x4): R-4.4, (2x6): R-6.9
- Drywall 1/2": R-0.45, 5/8": R-0.56
Design Tips
- Don't forget air films - they contribute R-0.85 total (exterior + interior)
- Thermal bridging through studs reduces effective R-value by 10-25%
- Continuous insulation (exterior) is more effective than cavity insulation alone
- In cold climates, focus on ceiling insulation first (R-50+)
- Air sealing is as important as insulation - don't ignore infiltration
- Condensation control: May need vapor barriers depending on climate
- Window/door U-values dominate heat loss in well-insulated homes
- Use REScheck or COMcheck software to verify energy code compliance
Frequently Asked Questions
U-value measures how easily heat passes through a building element — the thermal transmittance — expressed in W/m²·K (SI) or BTU/h·ft²·°F (imperial). Lower U-value means better insulation. R-value measures the opposite property: thermal resistance, expressed in m²·K/W (SI) or h·ft²·°F/BTU (imperial). Higher R-value means better insulation. The two are reciprocals: U = 1/R and R = 1/U. So a wall with R = 5 m²·K/W has U = 0.20 W/m²·K. Builders and product manufacturers in the US typically advertise R-values for insulation materials (R-13, R-19, R-30), while European, UK, and Australian codes specify U-values for whole assemblies (walls, windows, roofs).
The calculator handles both SI and imperial units. SI R-value uses m²·K/W; imperial R-value uses h·ft²·°F/BTU. The conversion factor is 1 m²·K/W = 5.678 h·ft²·°F/BTU, so a value of R-30 imperial equals about 5.28 m²·K/W in SI. For U-value, 1 W/m²·K = 0.1761 BTU/h·ft²·°F. Be careful with US insulation labels: an R-19 batt in US imperial is only R-3.35 in SI metric units — a 5.7× factor difference. The calculator converts in both directions and shows both unit systems side-by-side to avoid the common mistake of comparing two regions' insulation specs without unit conversion.
For a simple conversion you enter the source value (U or R), select its unit system (SI or imperial), and the calculator returns the reciprocal value in both unit systems plus a clear interpretation (a good code-compliant wall, a window, a poorly insulated roof, etc.). For a layered-assembly calculation, enter each material layer's thickness in millimeters or inches and its thermal conductivity (k-value) in W/m·K or BTU·in/h·ft²·°F. The tool sums each layer's R-value (R = thickness / k) plus the inside and outside film coefficients (typically R = 0.12 m²·K/W inside, R = 0.04 outside for vertical walls) to get the total assembly R, then computes the U-value.
Heat flows through a wall by encountering successive resistances in series: outside air film (typically R = 0.04 m²·K/W for wind-exposed surfaces), exterior cladding, sheathing, insulation, drywall, and the inside air film (R = 0.12 m²·K/W for still indoor air). For series resistances you simply add them up: R_total = R_outside_film + R_cladding + R_insulation + ... + R_inside_film. Then U = 1/R_total. Air films matter most for low-insulation assemblies — for an R-30 wall they add only 1.5 percent to total R; for an R-3 single-pane window they can add 5 percent. For parallel paths (such as studs interrupting insulation), use a weighted area average — the studs create thermal bridges that significantly raise the effective U-value.
Required values depend on climate zone. In the US, IECC 2021 recommends ceiling R-49 to R-60, wall R-20+ continuous, and basement wall R-15 in zones 4-8. UK Building Regulations Part L (2022) calls for wall U ≤ 0.18 W/m²·K and roof U ≤ 0.11 W/m²·K for new builds. EU EPBD targets nearly zero-energy buildings, often U ≤ 0.15 for walls and U ≤ 0.10 for roofs. Passive House standard demands U ≤ 0.15 for opaque envelope and U ≤ 0.80 for triple-glazed windows. For windows, the US 'Energy Star' threshold is roughly U-0.30 imperial (1.7 W/m²·K), while triple-glazed argon-filled units reach U-0.20 (1.1 W/m²·K). Always check the local code first because climate-zone derating is region-specific.
R-value is a steady-state, one-dimensional metric: it assumes parallel heat flow through a uniform material with no air movement, no moisture, and no temperature swings. Real walls have thermal bridges (studs, ties, fasteners) that bypass the insulation, air leakage that bypasses insulation entirely, moisture that degrades insulation performance (wet fiberglass loses up to 50 percent of its R), settling of loose-fill that creates voids, and dynamic effects (thermal mass, radiant gains) that the lab-measured R-value cannot capture. Whole-building energy modeling tools like PHPP, EnergyPlus, or BEopt account for these, while a simple R-value chart does not. Always specify continuous exterior insulation to minimize thermal bridging, and combine R-value with airtightness (ACH50 testing) for honest performance.
ASTM C518 and ASTM C177 cover laboratory measurement of thermal conductivity (k-value) using guarded hot-plate and heat-flow-meter apparatus. ISO 8990 is the international equivalent for whole-component testing (windows, walls in hot-box). For windows, NFRC 100 (US) and EN 673 (Europe) standardize U-value rating. For glazing systems, EN 410 and ISO 10292 cover whole-window thermal performance. ISO 6946 governs the calculation method for plane building elements including air-film coefficients. IECC and ASHRAE 90.1 publish minimum prescriptive U and R values in the US. CIBSE TM23 (UK) and EN ISO 13790 give the corresponding European framework. Manufacturer-published values should always cite the test standard and the testing temperature — k-values drift with temperature.
Closed-cell spray foam, polyisocyanurate, and XPS rigid board contain a fluorinated or hydrocarbon blowing agent that conducts heat much more poorly than air. As the foam ages, atmospheric air slowly diffuses into the cells while the blowing agent diffuses out, raising the conductivity and lowering the R-value. ASTM C1303 prescribes the aged R-value test (a 180-day accelerated weathering) which most modern manufacturers now report. A polyiso panel might be labeled R-6.5 per inch initial but R-5.5 per inch aged. For long-life building design, always use the aged value, and check for ASTM C1303 LTTR (Long-Term Thermal Resistance) certification. EPS (white bead foam) uses pentane that escapes quickly and stabilizes at R-4.0 per inch, so initial-versus-aged difference is small; XPS and polyiso need the aged value for honest comparison.
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