Enter RSI for each layer. RSI values add directly in series. For parallel-path bridging (framing), use the weighted average method or consult ASHRAE Fundamentals.
Conversion Formulas
R-value = RSI x 5.678
U-value (W/m²·K) = 1 / RSI
U-factor (BTU/hr·ft²·°F) = 1 / R-value
°C·m²/W = m²·K/W (same unit, same value)
Common Thermal Resistance Reference Values
| Component / Layer | RSI (m²·K/W) | R-Value (Imperial) | Notes |
|---|---|---|---|
| Interior air film (horizontal) | 0.11 | 0.61 | Upward heat flow (ceiling/floor) |
| Interior air film (vertical) | 0.12 | 0.68 | Wall surfaces; standard ASHRAE value |
| Exterior air film (15 mph wind) | 0.03 | 0.17 | Exterior surface at design wind |
| 12.7 mm (1/2 in) drywall | 0.08 | 0.45 | Interior finish layer |
| 15.9 mm (5/8 in) drywall | 0.10 | 0.56 | Type X fire-rated drywall |
| 12.7 mm (1/2 in) plywood/OSB | 0.11 | 0.62 | Structural sheathing |
| 2x4 stud cavity R-12 batt | 2.11 | 12 | Cavity insulation only; not assembly |
| 2x6 stud cavity R-19 batt | 3.35 | 19 | Cavity insulation only; not assembly |
| 25 mm (1 in) EPS rigid foam | 0.70 | 4.0 | Exterior or under-slab insulation |
| 25 mm (1 in) XPS rigid foam | 0.88 | 5.0 | Below-grade or exterior continuous |
| 25 mm (1 in) polyiso board | 1.06 - 1.14 | 6.0 - 6.5 | Highest R/in; temperature-sensitive |
| Effective 2x6 wall assembly (with bridging) | 2.8 - 3.2 | 16 - 18 | Clear-field after framing correction |
| 2x6 wall + 25mm XPS (effective) | 3.5 - 4.0 | 20 - 23 | Meets NBC for most climate zones |
| R-40 blown attic insulation | 7.04 | 40 | NBC min. zones 5-6 attic |
| R-49 blown attic insulation | 8.67 | 49 | NBC min. zones 7-8 attic (cold climate) |
| R-60 blown attic insulation | 10.57 | 60 | High-performance / EnerGuide 80+ |
NBC 2020 Minimum Effective Thermal Resistance (Part 9)
| Assembly | Zone 4 (mild) | Zone 5-6 | Zone 7-8 (cold) |
|---|---|---|---|
| Attic / ceiling (effective RSI) | 6.97 (R-40) | 7.04 (R-40) | 8.67 (R-49) |
| Above-grade walls (effective RSI) | 2.97 (R-17) | 3.52 (R-20) | 4.23 (R-24) |
| Below-grade walls (effective RSI) | 1.76 (R-10) | 2.11 (R-12) | 2.99 (R-17) |
| Slab-on-grade perimeter (RSI) | 1.76 (R-10) | 1.76 (R-10) | 2.11 (R-12) |
| Exposed floor (effective RSI) | 3.52 (R-20) | 3.52 (R-20) | 5.02 (R-28.5) |
Effective RSI accounts for thermal bridging through framing members. Cavity insulation RSI alone does not equal the effective assembly RSI. Provincial codes and local AHJs may require stricter values. Use the R-Value / RSI Converter for dedicated insulation compliance checks.
How to Use the Thermal Resistance Converter
Enter RSI, R-value, metric U-value, or Imperial U-factor in any field. All four update instantly. Use the quick presets to load common Canadian values: NBC minimum wall RSI 3.52, NBC minimum attic RSI 7.04, or specific R-values like R-49 for cold-climate attics.
Use the Assembly RSI Builder to add multiple layers and get the total assembly RSI, R-value, and U-value. Enter a name and RSI for each layer -- air films, sheathing, cavity insulation, exterior foam, cladding. RSI values add directly in series. The running total updates as you type. This gives you the clear-field RSI before applying any bridging correction.
The NBC minimum effective RSI table shows code requirements for attics, walls, below-grade walls, slabs, and floors across all three Canadian climate zone groupings. Compare your assembly total against the minimum for your project zone to confirm compliance. Note that "effective" RSI is lower than the clear-field RSI due to thermal bridging through framing.
The thermal resistance reference table gives RSI and R-values for 16 common assembly layers from air films to attic insulation depths. Use these as a starting point for assembly calculations, then verify individual layer values from manufacturer datasheets. Feed your converted U-value into the Wall Heat Loss Calculator for full heat loss analysis.
Thermal Resistance in Canadian Building Envelopes
Thermal resistance quantifies how effectively a building assembly resists heat flow. In Canada, building codes and engineering use RSI (m²·K/W) while insulation products and trade conversations use Imperial R-value. Converting fluently between these units, and between thermal resistance and U-value, is essential for envelope design, code compliance, and energy analysis, with reference methods published in the ASHRAE Handbook -- Fundamentals.
RSI and R-Value -- The Core Pair
The conversion factor is 5.678, the exact relationship set out in the NIST Guide to the SI, Appendix B: R-value = RSI x 5.678, and RSI = R-value / 5.678. Key pairs to memorize: RSI 1.76 = R-10, RSI 2.11 = R-12, RSI 3.52 = R-20, RSI 4.23 = R-24, RSI 7.04 = R-40, RSI 8.67 = R-49. These cover the NBC minimum requirements for below-grade walls, above-grade walls, and attics across all Canadian climate zones. For dedicated R-value to RSI work with insulation material context, use the R-Value / RSI Converter.
Thermal Resistance and U-Value
U-value is the inverse of thermal resistance: U (W/m²·K) = 1/RSI and U-factor (BTU/hr·ft²·°F) = 1/R-value. A wall with RSI 3.52 has U-value 0.284 W/m²·K. A window with RSI 0.71 has U-value 1.4 W/m²·K -- meeting the NBC maximum for zones 7-8. U-value is preferred for windows because their low thermal resistance makes comparison easier with higher numbers. R-value is preferred for insulation because the large numbers are easier to relate to -- RSI 8.67 is harder to visualise than R-49. The U-Value Converter handles window-focused thermal transmittance work.
Adding Thermal Resistance Layers
For assembly layers in series (stacked), RSI values add directly: RSI_total = RSI_1 + RSI_2 + RSI_3 + ... For a wall: interior air film (0.12) + drywall (0.08) + R-19 batt (3.35) + OSB sheathing (0.11) + 25mm XPS (0.88) + exterior air film (0.03) = total clear-field RSI 4.57 (R-25.9). After correction for 15% wood stud framing (k = 0.12 W/m·K vs. cavity insulation k = 0.040 W/m·K), the effective assembly RSI drops to approximately 3.7-3.9 (R-21-22). The Assembly RSI Builder on this page handles the clear-field total; consult ASHRAE Fundamentals Chapter 27 for the parallel-path bridging correction method, or use the Wall Heat Loss Calculator to run the full corrected assembly.
Effective Thermal Resistance and Building Codes
The NBC requires effective assembly RSI -- not cavity insulation RSI alone. Effective RSI accounts for thermal bridging through framing members, ties, and penetrations. A 2x6 stud wall with R-19 cavity insulation has a cavity RSI of 3.35, but the effective assembly RSI after accounting for wood stud bridging (16 inches on centre, 15% framing fraction) is only about 2.8-3.0 -- below the NBC minimum of 3.52 for zone 6. Adding 25mm of exterior continuous insulation (RSI 0.70-0.88) brings the effective RSI to 3.5-3.9, meeting or exceeding code. This is why the NBC minimum effective RSI for walls differs from the simple cavity insulation R-value that fits a 2x6 stud, and why the Heat Load Calculator asks for effective assembly values rather than nominal cavity R-values.
Frequently Asked Questions
RSI and R-value both measure thermal resistance -- how strongly an assembly resists heat flow -- in different unit systems. RSI uses m²·K/W (SI metric); R-value uses ft²·°F·hr/BTU (Imperial). The conversion: RSI = R-value / 5.678, or R-value = RSI x 5.678. Canadian building codes use RSI; insulation product packages and trade conversations use R-value. R-value numbers are always about 5 to 6 times larger than the equivalent RSI: R-20 = RSI 3.52, R-40 = RSI 7.04, R-49 = RSI 8.67.
Multiply RSI by 5.678 to get Imperial R-value. So RSI 3.52 = R-20, RSI 4.23 = R-24, RSI 7.04 = R-40, RSI 8.67 = R-49.2 (approximately R-49). To go the other way, divide R-value by 5.678. This is one of the most frequent conversions in Canadian building envelope work since code documents specify RSI while product labels show R-value. Use the quick presets above or the R-Value / RSI Converter for a dedicated lookup table.
U-value is the inverse of thermal resistance: metric U-value (W/m²·K) = 1/RSI, and Imperial U-factor = 1/R-value. A wall with RSI 3.52 (R-20) has U = 1/3.52 = 0.284 W/m²·K and U-factor = 1/20 = 0.05 BTU/hr·ft²·°F. A window with U = 1.4 W/m²·K has RSI = 1/1.4 = 0.714 m²·K/W (R-4.06). Windows use U-value rather than R-value because their low resistance makes higher U numbers more intuitive for comparison. The U-value fields in the converter above show this inverse automatically.
Typical effective RSI (after thermal bridging correction): 2x4 stud wall with R-12 batt, no exterior insulation: effective RSI 1.8-2.1. 2x6 stud wall with R-19 batt: effective RSI 2.8-3.2 (below NBC minimum for most zones). 2x6 wall plus 25mm XPS exterior: effective RSI 3.5-4.0 (meets NBC zone 6 minimum of RSI 3.52). 2x6 wall plus 50mm mineral wool exterior: effective RSI 4.3-4.7 (exceeds NBC for all zones). The Assembly RSI Builder above lets you total any layer combination. Check the result against the NBC minimum effective RSI table for your climate zone.
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