Assembly Cross-Section Visualization
Layer-by-Layer Breakdown
| # | Layer | Thickness | R-Value | R-Value (SI) | % of Total |
|---|
Compare to Building Code Benchmarks
Common Material R-Values Reference
| Material | R-Value per inch (Imperial) | RSI per 25mm (Metric) | Notes |
|---|---|---|---|
| Fiberglass batt (standard) | R-3.1 to R-3.4/in | RSI 0.54–0.59/25mm | 2×4: R-11–13; 2×6: R-19–21 |
| Fiberglass batt (high-density) | R-3.7 to R-4.3/in | RSI 0.64–0.75/25mm | 2×6: R-21–25; 2×8: R-30 |
| Mineral wool / Rockwool batt | R-3.7 to R-4.2/in | RSI 0.64–0.73/25mm | Better fire & moisture resistance |
| Blown cellulose (attic) | R-3.2 to R-3.8/in | RSI 0.55–0.66/25mm | Settles ~20% over time |
| Blown fiberglass (attic) | R-2.2 to R-2.7/in | RSI 0.38–0.47/25mm | Less settling than cellulose |
| Spray polyurethane foam (closed-cell) | R-6.0 to R-6.5/in | RSI 1.04–1.13/25mm | Air barrier; moisture barrier |
| Spray polyurethane foam (open-cell) | R-3.5 to R-3.8/in | RSI 0.61–0.66/25mm | Air barrier; vapour permeable |
| EPS rigid foam (Type II) | R-3.6 to R-4.0/in | RSI 0.63–0.70/25mm | Common for continuous insulation |
| XPS rigid foam (extruded) | R-5.0/in | RSI 0.87/25mm | Excellent moisture resistance |
| Polyisocyanurate (polyiso) | R-6.0 to R-6.5/in | RSI 1.04–1.13/25mm | Best R/inch; loses value in cold |
| OSB sheathing (½″) | R-0.5 total | RSI 0.09 | Structural; low R-value |
| Plywood sheathing (½″) | R-0.6 total | RSI 0.10 | N/A |
| ½″ Drywall / Gypsum | R-0.45 total | RSI 0.08 | N/A |
| Wood siding (½″) | R-0.80 total | RSI 0.14 | N/A |
| Brick veneer (4″) | R-0.44 total | RSI 0.08 | Thermal mass, not insulation |
| Exterior air film | R-0.17 total | RSI 0.030 | ASHRAE standard condition |
| Interior air film (horizontal) | R-0.68 total | RSI 0.120 | Heat flow upward (winter ceiling) |
| Interior air film (vertical) | R-0.68 total | RSI 0.120 | Walls |
| 2×4 Douglas Fir stud (3.5″) | R-4.4 total | RSI 0.77 | Used in parallel path calc |
| 2×6 Douglas Fir stud (5.5″) | R-6.9 total | RSI 1.22 | N/A |
Understanding R-Value and U-Value in HVAC
R-value and U-value are the two most important thermal performance metrics in HVAC and building science. Understanding the difference between them, and how to calculate assembly values from individual material layers, is essential for accurate heat load calculations, equipment sizing, and energy code compliance.
R-Value vs. U-Value: The Key Relationship
R-value (thermal resistance) measures how well a material resists heat flow. It is expressed in hr·ft²·°F/BTU (Imperial) or m²·K/W (Metric, called RSI). Higher R-value = better insulation. R-values are additive for layers in series: R_total = R₁ + R₂ + R₃ + ... This is why you simply add up all the layers of a wall assembly.
U-value (thermal transmittance, also called U-factor) measures the rate of heat transfer through a material. It is expressed in BTU/hr·ft²·°F or W/m²·K. Lower U-value = better insulation. U-value is the reciprocal of total R-value: U = 1/R_total. The heat loss formula uses U-value: Q = U × A × ΔT, where Q is heat loss in BTU/hr, A is surface area in ft², and ΔT is the indoor-outdoor temperature difference in °F. This is the fundamental formula in the heat load calculator.
Why Framing Correction Matters
A 2×6 wall with R-20 fiberglass batts does NOT have an effective R-value of 20. Wood studs, plates, and headers conduct heat much better than insulation, typically R-1.25/inch vs. R-3.5+/inch for fiberglass. Once plates, headers, jack studs, and corners are counted (not just the studs themselves), framing typically occupies about 20-25% of the wall area in a 2×6 wall at 16″ o.c. Applying the parallel path calculation, the effective R-value of the cavity is reduced by a framing correction factor of approximately 0.72, so R-20 batts in 2×6 framing yield about R-14.4 in the cavity zone, not R-20. Add the other layers (sheathing, drywall, air films) and you might get R-16 to R-17 overall. This is why the heat load calculator asks for effective R-value, not nominal insulation R-value. The official NRCan effective thermal resistance tables publish this parallel-path calculation in full for every common framing size and spacing.
Continuous Insulation (CI) and Its Impact
Continuous insulation (CI) applied to the exterior of a wall (rigid foam, mineral wool batts, or spray foam) bypasses the framing entirely and is not subject to the framing correction factor. This is why adding even R-5 of exterior CI to a 2×6 wall can improve effective R-value more than upgrading from R-20 to R-24 batts inside. When entering CI in the layer list, mark it as non-framed to exclude it from the correction. If you're pricing out a CI upgrade, the insulation cost calculator can help estimate the project cost.
NBC Canada R-Value Requirements
National Building Code (NBC) Canada requires minimum effective RSI (SI R-value) for building envelopes by climate zone. The NBC Canada compliance guide has the full zone map and requirements. Use this calculator to verify your assembly meets or exceeds the required effective RSI before entering it into load calculations.
Worked Examples
For worked examples of split-insulated wall assemblies meeting these minimums across every Canadian climate zone, see NRCan's exterior insulation selection guide.
Frequently Asked Questions
Yes, R-values of layers in series add directly. This is one of the most useful properties of R-value and why it is preferred for assembly calculations. If your wall has exterior air film (R-0.17) + OSB sheathing (R-0.5) + R-20 insulation + drywall (R-0.45) + interior air film (R-0.68), the total is R-21.8 before framing correction. U-values do NOT add, always convert to R, sum, then convert back to U.
Nominal R-value is the rated R-value of the insulation product alone (e.g., "R-20 batt"). Effective R-value is the actual thermal performance of the entire wall assembly including framing correction, all other layers, and surface air films. Effective R-value is always lower than nominal due to thermal bridging through framing. For example, R-20 batts in 2×6 framing yield approximately R-16-17 effective total assembly value. Always use effective R-value in heat load calculations.
Multiply Imperial R-value by 0.176110 to get RSI (m²·K/W). Divide RSI by 0.176110 to get Imperial R-value. For example: R-20 imperial = 20 × 0.176110 = RSI 3.52. This calculator displays both automatically. Note: RSI is also called R-value (SI) or metric R-value in Canadian building codes.
Yes, for calculating heat loss (U × A × ΔT), you should include surface air films in the total R-value because they are a real resistance to heat flow. ASHRAE standard surface film resistances are: exterior (winter, 15 mph wind): R-0.17 imperial (RSI 0.030); interior (still air, vertical surface): R-0.68 (RSI 0.120); interior (still air, horizontal, heat flow upward): R-0.61 (RSI 0.107). This calculator includes air films in its presets and the "Add Air Films" button adds them automatically.