R-value is the standard metric for thermal resistance in Canadian and American residential construction. Higher R-values indicate greater resistance to heat flow through an insulated assembly. The National Building Code of Canada (NBC) specifies minimum R-values — expressed as RSI in SI units — for each component of the building envelope, and these requirements vary by climate zone.

Thermal insulation material

What R-Value Measures

R-value (or RSI in SI units) quantifies resistance to heat flow per unit area. It is expressed in imperial units as h·ft²·°F/BTU (abbreviated R) or in SI as m²·K/W (abbreviated RSI). In Canada, building product labels often show both. One RSI equals approximately 5.68 R (imperial).

R-value is a material property, not a system property. A wall assembly containing R-20 batt insulation between framing members performs at less than R-20 at the whole-wall level because wood framing has lower thermal resistance than the insulation it displaces. This effect — thermal bridging — is why the NBC 2020 shifted from prescriptive centre-of-cavity R-values to effective thermal resistance values that account for the full assembly.

Thermal bridging through 38×140 mm (2×6) wood studs at 400 mm on centre reduces whole-wall effective R-value by roughly 20–25% compared to the nominal R-value of the cavity insulation alone.

R-Value by Insulation Type

Different insulation materials deliver different R-values per unit thickness. The following figures are approximate mid-range values; actual performance varies by product density, installation quality, and temperature.

Material R-value per inch (imperial) RSI per 25 mm (SI) Typical Application
Fiberglass batt R-3.1 to R-3.7 RSI 0.54–0.65 Wall cavities, attic floor
Mineral wool batt R-3.7 to R-4.2 RSI 0.65–0.74 Wall cavities, soundproofing
Cellulose (blown) R-3.2 to R-3.8 RSI 0.56–0.67 Attics, dense-pack retrofit
Open-cell spray foam R-3.5 to R-3.7 RSI 0.62–0.65 Wall cavities, rim joists
Closed-cell spray foam R-6.0 to R-7.0 RSI 1.06–1.23 Foundations, exterior walls
EPS (Type II) R-3.8 RSI 0.67 Exterior continuous insulation
XPS R-5.0 RSI 0.88 Below-grade, exterior continuous
Polyisocyanurate R-5.5 to R-6.5 RSI 0.97–1.14 Roof assemblies, exterior walls

NBC 2020 Minimum Requirements by Climate Zone

Canada's NBC 2020 defines climate zones based on heating degree days (HDD) below 18°C. Requirements below represent the prescriptive effective thermal resistance minimums for Part 9 residential buildings (houses and small buildings). Provincial codes may specify higher values.

Climate Zone HDD (example cities) Attic (RSI / R) Above-grade wall (RSI / R) Below-grade wall (RSI / R)
Zone 4 <3000 (Vancouver) RSI 8.6 / R-49 RSI 2.97 / R-17 RSI 1.76 / R-10
Zone 5 3000–3999 (Victoria) RSI 8.6 / R-49 RSI 2.97 / R-17 RSI 1.76 / R-10
Zone 6 4000–4999 (Toronto) RSI 8.6 / R-49 RSI 3.08 / R-17.5 RSI 1.76 / R-10
Zone 7A 5000–5999 (Edmonton) RSI 10.0 / R-57 RSI 3.85 / R-21.9 RSI 2.98 / R-17
Zone 7B 6000–6999 (Whitehorse) RSI 10.0 / R-57 RSI 4.67 / R-26.5 RSI 2.98 / R-17
Zone 8 ≥7000 (Inuvik) RSI 10.0 / R-57 RSI 4.67 / R-26.5 RSI 2.98 / R-17

Note: Values above are illustrative based on NBC 2020 prescriptive tables. Designers using the trade-off path or whole-building energy modelling may achieve compliance with different assembly configurations. Verify current requirements with the applicable authority having jurisdiction (AHJ).

Calculating Required Thickness

To determine the thickness needed to reach a target R-value, divide the target by the material's R-value per inch. For a Zone 7A attic requiring RSI 10 (approximately R-57) using blown cellulose at R-3.5/inch:

57 ÷ 3.5 = 16.3 inches (approximately 415 mm) of blown cellulose required at the attic floor.

This calculation assumes level installation without compression or gaps. In practice, settled depth after installation is typically 10–15% less than initial fill depth for blown products, so installers account for settling when determining required fill depth.

Whole-Assembly vs. Nominal R-Value

When combining cavity insulation with exterior continuous insulation, the total assembly R-value is the sum of all layers minus corrections for thermal bridging. For a 2×6 wall with R-21 mineral wool batts and R-10 exterior EPS, the nominal total is R-31, but the framing correction reduces whole-wall effective R-value to approximately R-26 to R-28 depending on framing fraction.

Exterior continuous insulation (CI) placed outside the structural frame is particularly effective at reducing thermal bridging because it covers framing members without interruption.

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