# Natural Ventilation Principles ## Table of Contents - [[#Overview]] - [[#Driving Forces]] - [[#Wind-Driven Ventilation]] - [[#Buoyancy and Stack Effect]] - [[#Fundamental Flow Equations]] - [[#Opening Sizing]] - [[#Single-Sided Ventilation]] - [[#Cross Ventilation]] - [[#Building Depth Limitations]] - [[#Night Purge Ventilation]] - [[#Design Parameters and Targets]] - [[#Wind Data and Site Assessment]] - [[#Practical Design Considerations]] - [[#Simulation and Calculation Methods]] - [[#Key References and Standards]] --- ## Overview Natural ventilation uses wind pressure and buoyancy (stack effect) to drive airflow through buildings without mechanical fans. When effectively designed, natural ventilation provides fresh air, removes heat and pollutants, and contributes to thermal comfort at a fraction of the energy cost of mechanical systems. It is a fundamental component of low-energy building design and is central to the passive strategies identified in [[Bioclimatic Architecture]]. Detailed treatment of specific configurations is provided in [[Cross Ventilation Design]], [[Stack Effect Ventilation]], and [[Mixed Mode Ventilation]]. --- ## Driving Forces Natural ventilation is driven by two physical mechanisms, which may act independently or in combination. ### Wind Pressure Wind striking a building creates positive pressure on the windward face and negative pressure (suction) on the leeward face and roof. This pressure differential drives airflow through openings. **Wind pressure on a surface:** P_w = 0.5 x rho x C_p x V^2 Where: - rho = air density (approximately 1.2 kg/m3 at 20 degC) - C_p = pressure coefficient (dimensionless, depends on building geometry and wind direction) - V = reference wind speed at building height (m/s) ### Buoyancy (Stack Effect) Warm air is less dense than cool air. When warm air rises and exits through high-level openings, cooler air is drawn in through low-level openings. **Stack pressure difference:** delta_P_s = rho x g x H x (Ti - To) / Ti Where: - g = gravitational acceleration (9.81 m/s2) - H = vertical distance between inlet and outlet centres (m) - Ti = indoor air temperature (K) - To = outdoor air temperature (K) --- ## Wind-Driven Ventilation ### Pressure Coefficients (Cp) Cp values depend on building geometry, facade orientation relative to wind direction, and surrounding context. Typical values for a rectangular building: | Surface | Wind Normal to Face | Wind at 45 deg | |---------|-------------------|----------------| | Windward wall | +0.5 to +0.7 | +0.3 to +0.4 | | Leeward wall | -0.3 to -0.5 | -0.3 to -0.5 | | Side walls | -0.5 to -0.7 | Variable | | Flat roof | -0.5 to -0.8 | -0.5 to -0.8 | The total driving pressure for wind-driven cross-ventilation: **delta_P_w = 0.5 x rho x V^2 x (Cp_inlet - Cp_outlet)** ### Wind Speed Correction Meteorological wind data is typically measured at 10 m height in open terrain. Correction is needed for: - **Height:** V(z) = V_met x (z/z_met)^alpha, where alpha depends on terrain roughness (0.14 open, 0.22 suburban, 0.33 urban) - **Terrain:** Shielding from surrounding buildings significantly reduces effective wind speed - **Local effects:** Funnelling between buildings, corner acceleration, turbulence --- ## Buoyancy and Stack Effect Stack ventilation is particularly useful in: - Calm wind conditions - Deep-plan buildings where cross-ventilation is limited - Multi-storey atriums and stairwells - Night cooling strategies ### Key Design Factors - **Stack height (H):** Greater height produces greater pressure difference. Minimum useful stack: 3-5 m - **Temperature difference:** Larger delta_T produces stronger flow. At least 2-4 degC difference needed for significant flow - **Inlet/outlet sizing:** Both must be adequate; flow is limited by the smallest restriction ### Limitations - Stack-driven flow rates are generally lower than wind-driven rates for the same opening areas - Performance is weather-dependent: calm, hot nights produce minimal stack effect - Reverse stack effect can occur when outdoor temperature exceeds indoor temperature --- ## Fundamental Flow Equations ### Orifice Flow Equation The basic equation for airflow through an opening under a pressure difference: **Q = Cd x A x sqrt(2 x delta_P / rho)** Where: - Q = volumetric flow rate (m3/s) - Cd = discharge coefficient (typically 0.60-0.65 for sharp-edged openings) - A = free area of opening (m2) - delta_P = pressure difference across opening (Pa) - rho = air density (kg/m3) ### Combined Wind and Stack When both wind and stack forces act, the combined pressure is: **delta_P_total = delta_P_wind + delta_P_stack** (when acting in the same direction) In practice, the forces may assist or oppose each other depending on wind direction and thermal conditions. For a conservative design, it is common to calculate each separately and take the larger value, or to use vector addition where directions are known. --- ## Opening Sizing ### Ventilation Rate Requirements | Purpose | Ventilation Rate | |---------|-----------------| | IAQ (fresh air) | 10 L/s per person (ASHRAE 62.1) or 0.5-1.0 ACH | | Thermal comfort (air movement) | 5-10 ACH | | Night purge cooling | 6-10 ACH | | Smoke ventilation | Per fire engineering analysis | ### Sizing Procedure 1. Determine required flow rate Q (m3/s) from ventilation rate x room volume 2. Estimate available pressure difference delta_P (from wind, stack, or both) 3. Calculate required opening area: **A = Q / (Cd x sqrt(2 x delta_P / rho))** 4. Check opening area as percentage of floor area ### Rules of Thumb for Opening Area | Strategy | Minimum Free Opening Area (% of floor area) | |----------|---------------------------------------------| | Cross ventilation (comfort) | 5-8% of floor area on each side | | Single-sided ventilation (IAQ) | 3-5% of floor area | | Night purge ventilation | 5-10% of floor area | | Stack ventilation (outlet) | 2-4% of floor area at high level | --- ## Single-Sided Ventilation A room ventilated from one external wall only. ### Mechanisms - **Wind turbulence** -- fluctuating pressures drive air exchange through the opening - **Temperature difference** -- warm air exits at the top of the opening, cool air enters at the bottom (if a single large opening) or through separated high/low openings ### Effective Depth Single-sided ventilation is effective to a maximum room depth of approximately: **D_max = 2.0 to 2.5 x floor-to-ceiling height** For a 3.0 m ceiling: D_max = 6.0-7.5 m ### Flow Rate Estimates For a single opening of area A and height h, buoyancy-driven flow: **Q = Cd x (A/3) x sqrt(g x h x delta_T / T_avg)** Flow rates are typically 50-70% lower than cross-ventilation for the same opening area. --- ## Cross Ventilation Air enters through openings on one facade and exits through openings on the opposite (or adjacent) facade, driven by wind pressure differential. ### Advantages - Higher flow rates than single-sided ventilation - More predictable airflow patterns - Greater effective room depth ### Design Requirements - Openings on at least two facades with pressure differential - Clear airflow path between inlet and outlet - Inlet and outlet on facades with different pressure coefficients - Minimal internal obstructions (open plan or rooms with transfer grilles) ### Outlet-to-Inlet Ratio For maximum internal air velocity: - **Outlet area > Inlet area** (ratio 1.25:1 to 1.5:1) increases velocity at the inlet - Equal areas maximise flow rate but at lower velocity See [[Cross Ventilation Design]] for detailed configuration guidance. --- ## Building Depth Limitations The maximum building depth for effective natural ventilation is a critical planning constraint: | Strategy | Maximum Depth | |----------|---------------| | Single-sided ventilation | 2.0-2.5 x ceiling height (6-8 m) | | Cross ventilation | 5 x ceiling height (12-15 m) | | Cross ventilation with central atrium | 12-15 m per wing | | Stack ventilation (atrium/chimney) | 12-15 m from inlet to stack | **The 14 m rule:** As a widely cited rule of thumb, naturally ventilated buildings should not exceed approximately **14 m** in plan depth for cross-ventilation. Beyond this, the pressure difference is insufficient to maintain adequate air velocity at the centre of the plan. For deeper buildings, consider: - Central atrium or lightwell for stack ventilation - Mixed-mode design (see [[Mixed Mode Ventilation]]) - Segmented plan with multiple ventilation zones --- ## Night Purge Ventilation Night purge (night cooling) ventilation uses cool night air to flush heat from the building's thermal mass, reducing the following day's cooling requirement. ### Requirements - Thermal mass exposed to room air (no suspended ceilings over concrete soffits) - Secure, weather-protected openings that can remain open overnight - Night-time temperatures falling below daytime comfort threshold - Adequate airflow rate: **6-10 ACH** for effective mass cooling ### Effectiveness Night purge ventilation can reduce next-day peak temperatures by **2-5 degC** in buildings with adequate exposed mass. It is most effective in climates with diurnal temperature swings exceeding 10 degC. ### Practical Concerns - Security of open windows (high-level louvres, screened openings) - Rain ingress protection - Insect prevention - Acoustic privacy (urban noise) - Automated control via BMS for reliable operation --- ## Design Parameters and Targets ### Air Speed at Occupant Level | Condition | Target Air Speed (m/s) | |-----------|----------------------| | Sedentary work (comfort) | 0.3-0.8 | | Light activity | 0.5-1.0 | | Hot humid climate comfort | 1.0-2.0 | | Draught discomfort threshold (cool conditions) | > 0.2 at < 20 degC | ### Noise Considerations Open windows expose occupants to external noise. Natural ventilation is most viable where: - External noise level < 55 dB LAeq at the facade - Acoustic louvres can attenuate 10-15 dB while maintaining airflow (with pressure drop penalty) - Building set back from major roads --- ## Wind Data and Site Assessment ### Sources - Meteorological stations (airport data, national weather services) - Wind roses showing speed and direction frequency - Computational wind analysis (CFD) for complex urban sites - On-site measurement for critical projects ### Site Effects - **Urban canyon:** Wind speed reduced 30-50% compared to open terrain - **Funnelling:** Gaps between buildings accelerate wind - **Corner acceleration:** Wind speed at building corners can increase by 50-100% - **Downwash:** Tall buildings create downdraft on leeward side - **Stack between buildings:** Temperature differences between sunlit and shaded spaces drive airflow --- ## Practical Design Considerations ### Window Types and Airflow | Window Type | Effective Open Area (% of window area) | Airflow Characteristics | |------------|---------------------------------------|------------------------| | Casement (90 deg) | 90% | Excellent, can redirect airflow | | Sliding | 45-50% | Good, no projection | | Hopper (bottom-hung, in) | 50-60% | Good, redirects air upward | | Awning (top-hung, out) | 50-60% | Good, rain-protected when partially open | | Pivot (horizontal/vertical) | 50-70% | Good, depends on opening angle | | Louvred | 70-85% | Excellent, maintains security and weather protection | | Double-hung sash | 45-50% | Fair, limited effective area | ### Transfer Openings For cross-ventilation through partitioned spaces: - Transfer grilles or louvred panels in walls or doors - Undercut doors (minimum 25 mm, preferably 50 mm gap) - High-level transom openings between rooms and corridor - Acoustic transfer devices where noise control is required --- ## Simulation and Calculation Methods | Method | Complexity | Application | |--------|-----------|-------------| | Hand calculation (orifice equation) | Low | Single-zone preliminary sizing | | BS EN 16798-7 simplified method | Low-Medium | Regulatory compliance | | Network airflow models (CONTAM, Comis) | Medium | Multi-zone buildings, stack + wind | | CFD (Computational Fluid Dynamics) | High | Complex geometries, furniture-level resolution | | CoolVent (MIT) | Medium | Natural ventilation preliminary design | | EnergyPlus Airflow Network | Medium-High | Integrated thermal-airflow simulation | --- ## Key References and Standards - CIBSE AM10 (2005). *Natural Ventilation in Non-Domestic Buildings* - CIBSE Guide A -- Environmental Design (ventilation requirements) - BS EN 16798-1:2019 -- Indoor environmental input parameters for design - ASHRAE Standard 62.1 -- Ventilation for Acceptable Indoor Air Quality - ASHRAE Fundamentals Handbook -- Ventilation and infiltration chapter - Allard, F. (1998). *Natural Ventilation in Buildings* - Etheridge, D. (2012). *Natural Ventilation of Buildings* - Awbi, H. (2003). *Ventilation of Buildings* --- #environment #ventilation