Q: What is ΔCp and why does wind direction matter?

Cp is the dimensionless surface-pressure coefficient: positive on the windward facade (typically +0.6 to +0.8), negative on the leeward facade (typically −0.2 to −0.4). ΔCp = Cp_in − Cp_out is the driving pressure difference. For wind perpendicular to the inlet facade, ΔCp ≈ 0.95; at 30° off-axis, ≈ 0.65; at 60°, ≈ 0.25; at 90° (parallel to the opening wall), ΔCp ≈ 0 and wind-driven flow vanishes entirely — relying purely on stack. This is the single largest source of real-world ventilation failure in Indian apartments: the opening is oriented wrongly to the local wind rose.

Q: When does buoyancy (stack) flow dominate over wind?

Three conditions must hold for stack to dominate: (i) vertical distance between inlet and outlet ≥ 1.5 m (e.g., low window + clerestory or ventilator above door); (ii) indoor-outdoor ΔT ≥ 5 K; (iii) wind speed at the facade < 1.5 m/s. Stack is most valuable on hot still nights when wind drops but indoor temperatures remain elevated — this is when cross-ventilation relying purely on wind fails. Traditional Indian courtyard houses exploit this: the courtyard heats during the day, rises at night, and pulls in cool air from shaded rooms at ground level.

Q: What ACH benchmark should I design to?

IS 3362 / ASHRAE 62.1 / CIBSE TM40 residential benchmarks (per habitable room): living/dining 6 ACH design, 3 ACH minimum; bedroom 4 ACH design, 2 ACH minimum; kitchen 10 ACH design with range hood, 6 ACH minimum without; bathroom 8 ACH minimum exhaust. For naturally-ventilated design, target 1.5× the minimum on a typical (not peak) wind day — this gives headroom for seasonal variation. Peak design-wind-day ACH can safely be 2–3× target; above 15–20 ACH, occupants report paper/plant disturbance and thermal drift.

Q: How does wind speed at the facade relate to IMD/city wind data?

Published IMD wind speeds are measured at 10 m above open terrain. At urban sites the facade wind speed is lower due to terrain roughness and upstream obstruction, modelled by the power-law profile: V_z = V_ref · (z/z_ref)^α, where α ≈ 0.22 for suburban, 0.33 for dense urban. For a 3rd-floor apartment (z ≈ 10 m) in dense urban: V_facade ≈ 0.60 × V_IMD. For ground-floor rooms in a dense colony: V_facade ≈ 0.30–0.40 × V_IMD. IS 875 Part 3 terrain-category correction is the proper method for performance-based submissions; this tool uses a simplified urban-suburban multiplier.

Q: Which Indian climate zones benefit most from natural ventilation?

ECBC 2017 + NBC 2016 climate classification: Hot-Dry (Jodhpur, Ahmedabad, Jaipur, Delhi) — stack ventilation at night is essential; large daytime openings lose; courtyard typology works. Warm-Humid (Chennai, Mumbai, Kochi, Kolkata, Bhubaneswar) — cross-ventilation at all hours is critical; stack flow is limited by low ΔT; prioritise opposite-wall openings with Cd > 0.5. Composite (Bengaluru, Pune, Hyderabad, Nagpur) — cross-ventilation works most of the year; night-flush cooling in dry season. Cold/Moderate (Shimla, Srinagar, Shillong) — controlled ventilation only; envelope-tight design for winter, openable in summer.

Question 1

What model does this calculator use?

Accepted Answer

The CIBSE AM10 (2005) simplified model for naturally-ventilated buildings, as adopted in IS 3362-1977 (Code of Practice for Natural Ventilation of Residential Buildings). Wind-driven flow: Q_w = Cd · A_eff · V_wind · √|ΔCp|, where Cd is the discharge coefficient, A_eff = (A_in · A_out) / √(A_in² + A_out²) is the effective opening area for two-opening systems, V_wind is the free-stream wind speed at roof height, and ΔCp is the pressure-coefficient difference between windward and leeward facades (typically 0.8–1.0 for perpendicular wind, 0.3–0.5 at 45°, zero at ±90°). Stack-driven flow: Q_s = Cd · A_eff · √(2g·Δh·ΔT/T_avg), where Δh is the vertical distance between inlet and outlet centres and ΔT is the indoor-outdoor temperature difference. Combined: Q_total = √(Q_w² + Q_s²), the standard envelope superposition when wind and stack are additive but independent.

Question 2

What does NBC 2016 Part 8 require for natural ventilation?

Accepted Answer

NBC 2016 Part 8 Section 1 (Lighting and Ventilation) specifies minimum openable area as a percentage of floor area: 10% for habitable rooms (living, dining, bedroom, kitchen), 5% for bathrooms and WCs, and 7.5% for staircases and corridors. Of the openable area, at least 50% must be operable for ventilation. This is prescriptive only — it guarantees a minimum window size, not minimum ACH. IS 3362-1977 Clause 4 provides performance criteria: 30 m³/h per person minimum air change, rising to 85 m³/h per person in tropical-humid zones.

Question 3

How do I pick the discharge coefficient (Cd)?

Accepted Answer

Cd is controlled by the opening geometry. Plain rectangular window opening (no mullions, no screens): Cd = 0.60–0.65. Louvered shutter (45° vanes): Cd = 0.35–0.45. Jali / perforated screen: Cd = 0.25–0.35 depending on open-area ratio (typical stone jali 40–60% open). Clerestory (short-throat opening in the slab): Cd = 0.55. Door opening: Cd = 0.70. Fly-screen / mosquito-net reduces Cd by 15–20%. Security grille with < 50% open area effectively halves Cd. Pick the Cd of the lower-flow opening in the pair — it controls flow.

Question 4

What is ΔCp and why does wind direction matter?

Accepted Answer

Cp is the dimensionless surface-pressure coefficient: positive on the windward facade (typically +0.6 to +0.8), negative on the leeward facade (typically −0.2 to −0.4). ΔCp = Cp_in − Cp_out is the driving pressure difference. For wind perpendicular to the inlet facade, ΔCp ≈ 0.95; at 30° off-axis, ≈ 0.65; at 60°, ≈ 0.25; at 90° (parallel to the opening wall), ΔCp ≈ 0 and wind-driven flow vanishes entirely — relying purely on stack. This is the single largest source of real-world ventilation failure in Indian apartments: the opening is oriented wrongly to the local wind rose.

Question 5

When does buoyancy (stack) flow dominate over wind?

Accepted Answer

Three conditions must hold for stack to dominate: (i) vertical distance between inlet and outlet ≥ 1.5 m (e.g., low window + clerestory or ventilator above door); (ii) indoor-outdoor ΔT ≥ 5 K; (iii) wind speed at the facade < 1.5 m/s. Stack is most valuable on hot still nights when wind drops but indoor temperatures remain elevated — this is when cross-ventilation relying purely on wind fails. Traditional Indian courtyard houses exploit this: the courtyard heats during the day, rises at night, and pulls in cool air from shaded rooms at ground level.

Question 6

What ACH benchmark should I design to?

Accepted Answer

IS 3362 / ASHRAE 62.1 / CIBSE TM40 residential benchmarks (per habitable room): living/dining 6 ACH design, 3 ACH minimum; bedroom 4 ACH design, 2 ACH minimum; kitchen 10 ACH design with range hood, 6 ACH minimum without; bathroom 8 ACH minimum exhaust. For naturally-ventilated design, target 1.5× the minimum on a typical (not peak) wind day — this gives headroom for seasonal variation. Peak design-wind-day ACH can safely be 2–3× target; above 15–20 ACH, occupants report paper/plant disturbance and thermal drift.

Question 7

How does wind speed at the facade relate to IMD/city wind data?

Accepted Answer

Published IMD wind speeds are measured at 10 m above open terrain. At urban sites the facade wind speed is lower due to terrain roughness and upstream obstruction, modelled by the power-law profile: V_z = V_ref · (z/z_ref)^α, where α ≈ 0.22 for suburban, 0.33 for dense urban. For a 3rd-floor apartment (z ≈ 10 m) in dense urban: V_facade ≈ 0.60 × V_IMD. For ground-floor rooms in a dense colony: V_facade ≈ 0.30–0.40 × V_IMD. IS 875 Part 3 terrain-category correction is the proper method for performance-based submissions; this tool uses a simplified urban-suburban multiplier.

Question 8

Which Indian climate zones benefit most from natural ventilation?

Accepted Answer

ECBC 2017 + NBC 2016 climate classification: Hot-Dry (Jodhpur, Ahmedabad, Jaipur, Delhi) — stack ventilation at night is essential; large daytime openings lose; courtyard typology works. Warm-Humid (Chennai, Mumbai, Kochi, Kolkata, Bhubaneswar) — cross-ventilation at all hours is critical; stack flow is limited by low ΔT; prioritise opposite-wall openings with Cd > 0.5. Composite (Bengaluru, Pune, Hyderabad, Nagpur) — cross-ventilation works most of the year; night-flush cooling in dry season. Cold/Moderate (Shimla, Srinagar, Shillong) — controlled ventilation only; envelope-tight design for winter, openable in summer.

Opening type	Cd	Open-area ratio	Note
Plain window (sliding / casement)	0.62	1.00	Baseline — openable sash with no screen or grille
Window with mosquito screen	0.50	1.00	Deduct ~18% for fly screen
Window with security grille	0.35	1.00	Most grilles reduce effective flow by 40-50%
Louvered shutter (45° vanes)	0.40	0.75	Typical jhilmil / Venetian louver
Jali / perforated screen (dense, 35% open)	0.30	0.35	Fine stone jali, privacy-first
Jali / perforated screen (open, 55% open)	0.34	0.55	Rajasthani / Indo-Saracenic proportions
Clerestory / ventilator	0.55	1.00	Excellent for stack; good pair with low opening
Door opening	0.70	1.00	Use for internal transfer only; Cd high but flow often obstructed

Cross-Ventilation Analyzer

Inputs

Ventilation Analysis

Opening discharge coefficients (Cd)

The CIBSE AM10 simplified model

Cross-ventilation design questions