| Wall Assembly | Area | R-Value | U-Factor | Heat Loss | % of Total |
|---|
Wall heat loss in residential load calculations
Wall conduction is typically the largest single component of residential heating load, accounting for 25-40% of total heat loss depending on the building. The formula is straightforward: Q = U × A × ΔT. But getting the right U-factor for a real wall assembly requires accounting for framing, insulation, air films, and any continuous insulation. The U-value calculator breaks down wall assemblies layer by layer.
R-value vs. U-factor
R-value and U-factor are reciprocals: U = 1/R. The R-value printed on insulation batts is for the insulation only — not the whole wall assembly. A 2×6 wall with R-20 batt insulation has a whole-wall R-value of roughly R-17 to R-19 after accounting for framing (thermal bridging), air films, sheathing, and drywall. Don't use the insulation R-value alone in heat loss calculations.
Thermal bridging in wood-framed walls
Wood framing at 16" or 24" on-centre creates thermal bridges that reduce the effective R-value of the wall assembly. At 16" OC, framing occupies about 25% of the wall area and has an R-value of roughly R-6 (2×6 stud). The cavity insulation is R-20. The area-weighted average is about R-10.5 — far less than R-20. Continuous exterior insulation is the most effective way to address framing thermal bridges. See the U-value calculator for detailed assembly analysis including framing factors.
Frequently Asked Questions
A standard 2x6 stud wall at 16" on-centre with R-20 batt insulation has a whole-wall effective R-value of approximately R-16 to R-18. Framing at 16" OC occupies about 25% of the wall area and has an R-value of only R-6.9 for a 2x6 stud. The area-weighted effective R-value of cavity plus framing is roughly R-15. Adding 5/8" drywall (R-0.5), exterior sheathing (R-1.0), and air films (R-1.0 total) brings the assembly to about R-17. Use the U-value calculator for a precise layer-by-layer analysis.
NBC 2020 Part 9 requires an effective thermal resistance of RSI 3.5 (R-20 effective) for above-grade walls in Climate Zone 6 (most of Ontario). This is the effective whole-wall value, not just the insulation R-value. A 2x6 wall with R-20 batt typically achieves about RSI 3.0 effective, falling slightly short. Adding R-5 continuous exterior insulation brings it to RSI 3.8, exceeding the requirement. Ontario SB-12 provides compliance paths for different wall assemblies.