Calculators › HVAC and building envelope Opaque-wall resistance within a shared heat limit calculator
Opaque-wall resistance within a shared heat limit
Minimum opaque-wall heat resistance is calculated as A_wall × ΔT / (q_limit − U_glass × A_glass × ΔT). Use the stated system boundary and matching input units.
Units IP (US) SI (metric)
R wall = A wall × Δ T q limit − U glass × A glass × Δ T R_{\text{wall}} = \frac{A_{\text{wall}} \times \Delta T}{q_{\text{limit}} - U_{\text{glass}} \times A_{\text{glass}} \times \Delta T} R wall = q limit − U glass × A glass × Δ T A wall × Δ T R wall = A wall × Δ T 1000 × q limit − U glass × A glass × Δ T R_{\text{wall}} = \frac{A_{\text{wall}} \times \Delta T}{1000 \times q_{\text{limit}} - U_{\text{glass}} \times A_{\text{glass}} \times \Delta T} R wall = 1000 × q limit − U glass × A glass × Δ T A wall × Δ T
Calculator
Enter the values you know and leave one box empty. Calculate finds the empty one, the result or any input.
Worked example
Given
A wall A_{\text{wall}} A wall 8,000 ft² Δ T \Delta T Δ T 40 °F q limit q_{\text{limit}} q limit 100,000 Btu/h U glass U_{\text{glass}} U glass 0.5 Btu/(h·ft²·°F) A glass A_{\text{glass}} A glass 2,000 ft² R wall = 8000 × 40 100 , 000 − 0.5 × 2000 × 40 ≈ 5.333 R_{\text{wall}} = \frac{8000 \times 40}{100{,}000 - 0.5 \times 2000 \times 40} \approx 5.333 R wall = 100 , 000 − 0.5 × 2000 × 40 8000 × 40 ≈ 5.333
Given
A wall A_{\text{wall}} A wall 800 m² Δ T \Delta T Δ T 20 K q limit q_{\text{limit}} q limit 35 kW U glass U_{\text{glass}} U glass 2 W/(m²·K) A glass A_{\text{glass}} A glass 200 m² R wall = 800 × 20 1000 × 35 − 2 × 200 × 20 ≈ 0.5926 R_{\text{wall}} = \frac{800 \times 20}{1000 \times 35 - 2 \times 200 \times 20} \approx 0.5926 R wall = 1000 × 35 − 2 × 200 × 20 800 × 20 ≈ 0.5926
Symbols and units
Symbol Meaning Unit
R wall R_{\text{wall}} R wall minimum opaque-wall heat resistance h·ft²·°F/Btu A wall A_{\text{wall}} A wall opaque wall area ft² Δ T \Delta T Δ T inside-outside temperature difference °F q limit q_{\text{limit}} q limit maximum shared conductive heat loss Btu/h U glass U_{\text{glass}} U glass glazing conductance Btu/(h·ft²·°F) A glass A_{\text{glass}} A glass glazing area ft²
Symbol Meaning Unit
R wall R_{\text{wall}} R wall minimum opaque-wall heat resistance m²·K/W A wall A_{\text{wall}} A wall opaque wall area m² Δ T \Delta T Δ T inside-outside temperature difference K q limit q_{\text{limit}} q limit maximum shared conductive heat loss kW U glass U_{\text{glass}} U glass glazing conductance W/(m²·K) A glass A_{\text{glass}} A glass glazing area m²
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