You assume your building’s fresh air comes from the HVAC system. But even with HVAC off, air continuously exchanges between indoors and outdoors through forces you can’t see—wind pushing against walls, temperature differences creating vertical pressure gradients, and envelope imperfections allowing leakage. Modern buildings exchange their entire air volume 0.3-0.7 times hourly through these natural mechanisms alone before mechanical ventilation adds more.
The physics involve three simultaneous forces: wind pressure creating instantaneous differentials of 5-15 Pa (Pascals) on exposed walls depending on gusts and building shape, stack effect generating sustained 10-30 Pa pressure differences between lower and upper floors in tall buildings during winter (cold outdoor temperatures amplify buoyancy), and mechanical systems (HVAC, bathroom exhaust, kitchen fans) adding -5 to +10 Pa building-wide pressurization or depressurization. Research confirms “infiltration caused by wind, negative pressurization of building, and air buoyancy forces known as stack effect”—and these forces combine algebraically creating actual pressure at any given location determining whether air infiltrates or exfiltrates.
Building airtightness determines how much pressure translates into actual airflow. Leaky buildings (ACH50 > 15) measuring >0.35 CFM per square foot of envelope area at standard test pressure exchange air rapidly—sometimes excessively wasting energy. Modern tight construction (ACH50 < 5) achieves <0.1 CFM/sq ft reducing uncontrolled exchange to minimal levels requiring mechanical ventilation for indoor air quality (previous article on ASHRAE 62.2). The envelope acts as variable-resistance membrane where pressure differential (ΔP) drives airflow through available pathways following Q = C × A × (ΔP)^n relationship—doubling pressure increases flow by 40-100% depending on crack geometry.
This guide explains the three pressure mechanisms determining air exchange rates in specific buildings, reveals why tenth-floor apartments experience dramatically different conditions than ground-floor units in same building (stack effect creates floor-specific pressures), and determines when natural infiltration suffices versus when mechanical intervention becomes mandatory for acceptable indoor air quality and energy efficiency.
The Three Forces Driving Air Exchange
Air moves when pressure differences exist across building envelope—three mechanisms create these differentials.
Force 1: Wind Pressure
Mechanism: Wind striking building surfaces creates positive pressure on windward side (air compressing against walls), negative pressure on leeward side (air flowing past creating suction).
Magnitude: Wind pressure calculated as P_wind = 0.00256 × C_p × V² where C_p is pressure coefficient (building shape dependent, typically 0.4-0.8) and V is wind velocity in mph.
Example: 20 mph wind on typical building generates ~5-10 Pa pressure differential between windward and leeward surfaces.
Temporal nature: Wind is variable and unpredictable—pressures fluctuate second-by-second with gusts. Unlike stack effect (steady during stable temperatures), wind creates transient pressure pulses.
Direction dependency: Pressure distribution changes completely with wind direction—north wind creates different envelope pressure map than east wind.
Force 2: Stack Effect (Buoyancy)
Mechanism: Warm indoor air is less dense than cold outdoor air in winter. Buoyancy causes warm air to rise within building vertical shafts (stairwells, elevator shafts, duct chases), creating negative pressure at bottom (where warm air drawn away) and positive pressure at top (where warm air accumulates wanting to escape).
Magnitude: Stack pressure differential proportional to ΔP_stack = C × h × (1/T_out – 1/T_in) where h is height above neutral pressure plane, T_out and T_in are absolute temperatures.
Example: 10-story building (100 ft tall), indoor 70°F (294K), outdoor 20°F (267K):
- Pressure difference bottom to top: ~25-30 Pa
- Much larger than typical wind pressure
Temporal nature:Steady-state during stable weather—unlike wind, stack effect maintains constant pressure as long as temperature differential persists.
Seasonal reversal: Summer with AC creates reverse stack effect—cool indoor air sinks creating negative pressure at top, positive at bottom (opposite of winter).
Force 3: Mechanical Systems
Mechanism: Fans (HVAC supply, bathroom/kitchen exhaust) intentionally move air creating building pressurization (more supply than exhaust = positive pressure) or depressurization (more exhaust than supply = negative pressure).
Magnitude: Depends on system design—typical residential HVAC creates ±5-10 Pa whole-building pressure when unbalanced.
Example: Bathroom exhaust fan running 50 CFM in 800 sq ft apartment without compensating supply creates -3 to -8 Pa negative pressure pulling outdoor air through envelope leaks.
Control: Only force designer can modify—wind and stack are environmental variables; mechanical systems are engineered.
Wind Pressure: Instantaneous Differentials on Building Faces
Wind creates surface-specific pressures varying by exposure and building geometry.
Pressure Coefficient (C_p)
Definition: Dimensionless value describing how wind pressure distributes across building surfaces relative to free-stream wind pressure.
Typical values:
- Windward wall: C_p = +0.6 to +0.8 (positive pressure)
- Leeward wall: C_p = -0.3 to -0.5 (negative/suction)
- Side walls: C_p = -0.6 to -0.7 (strong suction)
- Roof: C_p = -0.5 to -0.9 (suction)
Implication: For given wind speed, side walls and roof experience highest suction while windward wall experiences compression—creating infiltration on windward side, exfiltration on leeward/side/roof.
Wind Speed Impact
Pressure quadruples when velocity doubles: P ∝ V²
10 mph wind: ~2-4 Pa differential 20 mph wind: ~8-16 Pa differential
30 mph wind: ~18-36 Pa differential
Reality: Wind gusts create transient spikes—sustained 20 mph with 35 mph gusts creates fluctuating 8-50 Pa pressures stressing envelope.
Building Height and Exposure
Wind velocity increases with height: Ground friction slows near-surface wind; velocity increases logarithmically with elevation.
First floor vs tenth floor: Top floor exposed to 30-50% higher wind speeds than ground level—experiencing proportionally higher pressure differentials.
Urban shelter: Dense city buildings reduce wind exposure (shelter factor 0.5-0.7); rural exposure sees full wind speed (shelter factor 1.0).
Stack Effect: Temperature-Driven Vertical Pressure Gradients
The dominant force in tall buildings during extreme weather.
Physical Mechanism
Winter scenario:
- Indoor air heated to 70°F becomes less dense (lighter) than outdoor 20°F air
- Warm air rises in continuous vertical shafts (stairwells, elevator shafts)
- Rising creates pressure drop at bottom (negative pressure) drawing outdoor air in
- Accumulation creates pressure increase at top (positive pressure) forcing indoor air out
Result:Air infiltrates at lower floors, exfiltrates at upper floors with neutral pressure plane somewhere mid-height where indoor and outdoor pressure equal.
The Neutral Pressure Plane (NPL)
Definition: Vertical location where indoor and outdoor pressures are equal (no net driving force for air movement across envelope).
Location: Rarely at mid-height—depends on envelope leakage distribution, mechanical systems, external wind.
Typical position:1/3 to 2/3 up building height—research notes NPL “rarely at midpoint”.
Example: 15-story building might have NPL at floor 7—floors 1-6 experience infiltration (negative pressure), floors 8-15 experience exfiltration (positive pressure), floor 7 minimal pressure differential.
Pressure Magnitude
Research finding: Stack effect creates pressure differentials “proportional to indoor-outdoor temperature difference and vertical distance from NPL”.
Formula approximation: ΔP_stack ≈ 0.0035 × h × (T_in – T_out) / T_avg Where h = height in feet, T in °F, result in inches water column
Example calculation:
- Building height 100 ft (10 stories)
- Indoor 70°F, outdoor 20°F (50°F difference)
- Bottom floor (50 ft below NPL): -15 Pa (infiltration)
- Top floor (50 ft above NPL): +15 Pa (exfiltration)
- Total differential bottom to top: 30 Pa
Extreme case (Astana study): 58°C (104°F) temperature difference in very cold climate created stack pressures exceeding 100 Pa in tall buildings—creating “difficulty in door operation, whistling through cracks, malfunctioning elevator doors”.
Seasonal Reversal
Summer with AC: Cool indoor air (75°F) is denser than hot outdoor air (95°F)—creating reverse stack effect.
Result: Cool air sinks → negative pressure at top (infiltration), positive pressure at bottom (exfiltration).
Weaker magnitude: Summer temperature differentials typically 5-15°C vs winter’s 20-40°C (in cold climates)—stack effect “usually less pronounced in cooling season”.
Mechanical Pressurization: HVAC and Exhaust Systems
The controllable force—and source of many problems when imbalanced.
Intentional Building Pressurization
Supply > Exhaust: HVAC delivers more outdoor air than building exhausts—creating positive pressure.
Magnitude:+5 to +10 Pa typical for commercial buildings; residential often ±2-5 Pa.
Purpose:
- Prevents outdoor pollutant infiltration (keeps dirty air out)
- Controls humidity infiltration (humid climates)
- Prevents odor infiltration from adjacent spaces (hospitals, restaurants)
Design target: ASHRAE recommends +0.02 to +0.05 inches w.c. (+5 to +12 Pa) positive pressure in commercial buildings.
Unintentional Depressurization
Common scenario: Multiple exhaust fans (bathrooms, kitchen, dryer) without sufficient makeup air—creating negative building pressure.
Research finding:“Mechanical systems can generate sustained pressure differentials with much greater impact than wind or stack alone”—creating “constant year-round driving force for air leakage”.
Example: Apartment with:
- 50 CFM bathroom exhaust (continuous)
- 100 CFM kitchen hood (intermittent)
- 120 CFM dryer (intermittent)
- Total exhaust: 270 CFM peak, 50 CFM baseline
- Without compensating supply: -8 to -15 Pa depressurization
Consequences: Excessive infiltration (wasting heating/cooling energy), potential combustion appliance backdrafting (CO hazard), cross-contamination from adjacent units in multifamily.
The Makeup Air Problem
Thermodynamics reality: Air exhausted must be replaced from somewhere.
Sources of makeup air:
- Intentional supply: HVAC outdoor air intake, dedicated makeup air systems
- Envelope infiltration: Air leaking through building envelope gaps
- Adjacent spaces: Air migrating from connected areas (hallways in apartments, neighbor units through shared walls)
Proper design: Match exhaust with supply—balanced airflows preventing excessive pressure differentials.
Common failure: Installing powerful exhaust without adequate supply—“imbalanced airflows leading to building-wide problems” including “forces over 100N on doors, making interior and exterior doors difficult to operate”.
How Pressure Differences Actually Move Air
Pressure differential is driving force—envelope leakage is resistance—airflow is result.
The Fundamental Equation
Q = C × A × (ΔP)^n
Where:
- Q = airflow rate (CFM)
- C = flow coefficient (depends on crack geometry)
- A = effective leakage area (square feet)
- ΔP = pressure differential (Pa or inches w.c.)
- n = flow exponent (typically 0.5-0.7)
Interpretation: Airflow proportional to leakage area and pressure differential raised to power between 0.5 and 1.0.
Non-Linear Relationship
Because n < 1.0: Doubling pressure does NOT double airflow—increases by factor of 2^0.5 to 2^0.7 (approximately 1.4x to 1.6x).
Example:
- 10 Pa pressure → 50 CFM infiltration
- 20 Pa pressure → ~70 CFM (not 100 CFM)—diminishing returns at higher pressures
Implication:“Infiltration is radical function of pressure differential”—large pressure increases yield progressively smaller airflow increases.
Pressure Combination
Three forces act simultaneously: Wind + Stack + Mechanical = Total Pressure
Algebraic addition: Pressures combine—some reinforcing, others opposing.
Example (winter ground floor):
- Stack effect: -15 Pa (infiltration)
- Wind (windward wall): +10 Pa (exfiltration)
- Mechanical (depressurized): -8 Pa (infiltration)
- Net: -13 Pa (infiltration dominates)
Complexity: Every building location has unique pressure based on elevation, exposure, mechanical system operation.
The Neutral Pressure Plane in Tall Buildings
Critical concept for understanding stack effect distribution.
Definition and Location
Neutral Pressure Plane (NPL): Horizontal plane where indoor pressure equals outdoor pressure—no net driving force for air movement.
Location factors:
- Building height: Taller buildings have lower NPL (more area above for positive pressure)
- Envelope leakage distribution: More leakage at top raises NPL; more at bottom lowers NPL
- Mechanical systems: Positive pressurization lowers NPL; negative pressurization raises NPL
Typical range:30-70% of building height from ground—rarely at midpoint.
Pressure Distribution
Below NPL: Increasingly negative pressure with distance downward—air infiltrates through envelope.
Above NPL: Increasingly positive pressure with distance upward—air exfiltrates.
Linear gradient: Pressure changes approximately linearly with vertical distance from NPL.
Research confirms: Pressure differential “typically estimated as linear function of distance from neutral pressure level”.
Multi-Zone Reality
Single NPL assumption: Simplified model treating building as single vertical column.
Reality: Complex buildings have multiple neutral planes in different zones depending on internal partitioning, mechanical system zoning.
Example: Apartment building with pressurized corridors vs depressurized units may have different NPL for corridor shaft vs residential spaces.
Infiltration vs Exfiltration: Directional Airflow
Pressure direction determines air movement direction.
Infiltration
Definition:“Unintentional introduction of outside air into building” through envelope cracks.
Driving forces:
- Negative indoor pressure (exhaust > supply)
- Positive outdoor pressure (wind on windward wall)
- Stack effect below NPL (winter)
Impact: Brings unconditioned outdoor air requiring heating/cooling—increasing energy load.
Indoor air quality: Outdoor air provides dilution of indoor pollutants—but uncontrolled infiltration brings moisture (humid climates), outdoor pollution (urban areas), allergens (pollen season).
Exfiltration
Definition:“Leakage of room air out of building”—indoor air escaping through envelope.
Driving forces:
- Positive indoor pressure (supply > exhaust)
- Negative outdoor pressure (wind on leeward/side walls)
- Stack effect above NPL (winter)
Impact: Loses conditioned indoor air—wasting heating/cooling energy.
Moisture risk: Warm humid indoor air exfiltrating through walls in winter can condense within wall cavities when it contacts cold surfaces—potentially causing moisture damage, mold growth if vapor barrier inadequate.
Bidirectional Flow
Simultaneous occurrence: Building experiences both infiltration and exfiltration simultaneously at different locations.
Example (winter 10-story building):
- Floors 1-5: Infiltration (stack effect negative pressure)
- Floors 6-10: Exfiltration (stack effect positive pressure)
- Windward walls: Infiltration (wind positive pressure)
- Leeward walls: Exfiltration (wind negative pressure)
Net effect:Total infiltration = Total exfiltration at steady state (mass balance)—but location-specific impacts vary.
Building Envelope Leakage: Where Air Actually Enters/Exits
Pressure drives flow, but leakage pathways enable it.
Major Leakage Locations
Window perimeters: Gaps between window frame and rough opening—often largest single leakage source in older buildings.
Door frames: Similar to windows—weatherstripping degrades allowing leakage.
Wall-to-roof junctions: Transition areas where different assemblies meet—difficult to seal perfectly.
Wall-to-foundation: Another transition zone prone to gaps.
Penetrations: Pipes, electrical conduits, HVAC ducts, vents penetrating envelope—each creating potential leakage path if not properly sealed.
Material joints: Where different wall materials meet (brick to wood, siding to trim)—caulking degrades over time.
Research notes:“Air leakage mostly occurs at building joints, window frames, and where grout has worn away”—concentrated at transitions, not through solid materials.
Leakage Area Measurement
Blower door test: Standard procedure pressurizing or depressurizing building to 50 Pa measuring resulting airflow.
Effective Leakage Area (ELA): Total equivalent area of all envelope holes/cracks—calculated from blower door CFM50 result.
Typical residential ELA:
- Tight construction: 50-100 square inches
- Average: 150-300 square inches
- Leaky: 400-800 square inches
Visualization: 150 sq in ELA ≈ one 12-inch × 12-inch hole in envelope—distributed across thousands of small cracks.
Air Changes Per Hour (ACH): Measuring Total Exchange Rates
Standard metric quantifying how rapidly building air exchanges.
Definition
ACH = (Infiltration CFM × 60) / Building Volume (cubic feet)
Interpretation: How many times per hour entire building air volume is replaced by outdoor air.
Example:
- 1,500 sq ft house, 8-ft ceilings = 12,000 cubic feet
- Infiltration rate: 60 CFM
- ACH = (60 × 60) / 12,000 = 0.3 ACH
- Full air volume exchanged every 3.3 hours
Typical Natural ACH Ranges
Modern tight construction (ACH50 < 3):0.1-0.3 ACH natural infiltration
Average construction (ACH50 5-10):0.3-0.7 ACH natural
Leaky old buildings (ACH50 > 15):1.0-3.0 ACH natural—sometimes excessive
ASHRAE recommendation:0.35 ACH minimum for acceptable indoor air quality—modern tight buildings fall below this without mechanical ventilation.
Seasonal Variation
Winter: Higher ACH due to strong stack effect amplifying infiltration.
Summer: Lower ACH—weaker stack effect, possibly higher outdoor pressure (heat) reducing infiltration.
Shoulder seasons: Lowest ACH—minimal temperature differential eliminates stack effect, milder weather reduces HVAC operation.
Natural Infiltration Rates by Building Era
Construction practices determine envelope tightness.
Pre-1970 Construction
Typical ACH50: 20-30 (very leaky)
Natural ACH: 1.5-3.0—excessive air exchange wasting energy but ensuring fresh air.
Leakage sources: Single-pane windows, no air sealing, uninsulated walls with numerous penetrations, leaky duct systems.
Ventilation implication: Usually meets ASHRAE 62.2 via infiltration alone—no mechanical ventilation needed.
1970s-1990s Construction
Typical ACH50: 10-15 (moderately leaky)
Natural ACH: 0.5-1.2
Improvements: Double-pane windows, some air sealing, better insulation.
Ventilation implication: Marginal—may meet requirements in mild climates; likely needs mechanical in cold/hot climates.
Post-2000 Construction
Typical ACH50: 3-7 (tight)
Natural ACH: 0.2-0.5—insufficient without mechanical ventilation
Modern practices: Air barriers, sealed windows, spray foam insulation, sealed ductwork.
Ventilation implication:Mechanical ventilation mandatory per ASHRAE 62.2.
High-Performance (Passive House)
Target ACH50: <0.6 (extremely tight)
Natural ACH: <0.1—near-zero uncontrolled infiltration
Technology: Advanced air sealing, triple-pane windows, continuous air barriers.
Ventilation implication:Continuous mechanical ventilation with heat recovery (ERV/HRV) essential.
The Blower Door Test: ACH50 Measurement
Standard procedure quantifying envelope airtightness.
Test Procedure
Equipment: Calibrated fan mounted in door frame creating controlled pressure differential.
Methodology:
- Close all windows, doors (except door with blower)
- Seal fireplace, dryer vents, other intentional openings
- Run fan exhausting air until indoor pressure reaches -50 Pa
- Measure CFM required to maintain -50 Pa—this is CFM50
Alternative: Test at +50 Pa (pressurization) yields similar results.
Converting CFM50 to ACH50
ACH50 = (CFM50 × 60) / Building Volume
Example:
- 1,500 sq ft house, 8-ft ceilings = 12,000 cubic feet
- CFM50 measured: 1,500 CFM
- ACH50 = (1,500 × 60) / 12,000 = 7.5 ACH50
Interpretation: At 50 Pa pressure, house exchanges entire air volume 7.5 times per hour—indicating moderately tight envelope.
Building Code Requirements
IRC 2021: New residential construction must achieve ≤5 ACH50 or ≤3 ACH50 depending on climate zone.
Passive House: ≤0.6 ACH50 required for certification.
Enforcement: Blower door testing required at completion in many jurisdictions to verify compliance.
Converting ACH50 to Natural ACH
ACH50 is artificial—natural conditions create much lower pressure.
The Divider Method
Rule of thumb: Natural ACH ≈ ACH50 / 20 for typical single-family homes.
Example:
- Measured ACH50 = 10
- Natural ACH ≈ 10/20 = 0.5 ACH
Rationale: Natural pressures (4-8 Pa typical) are ~1/10th of test pressure (50 Pa), and airflow is non-linear function—dividing by 20 approximates combined effect.
The Lawrence Berkeley Lab (LBL) Model
More accurate: Accounts for building height, local weather, terrain.
Formula: Natural ACH = f(ACH50, climate, height, exposure)
Typical results: Divider ranges 15-25 depending on specifics—20 is reasonable average.
Limitations
High variability: Natural ACH fluctuates with weather—divider gives long-term average.
Stack effect dominance: Tall buildings in cold climates may have higher natural ACH than divider predicts due to strong stack effect.
Why Modern Buildings Need Mechanical Ventilation
Tight envelopes eliminate natural air exchange requiring engineered replacement.
The Tightness Trend
Energy codes drive airtightness: Each code cycle tightens requirements—reducing allowable ACH50.
Modern achievement: New construction achieves 3-5 ACH50 routinely—translating to 0.15-0.25 natural ACH.
Below minimum: ASHRAE 62.2 requires ≥0.35 ACH equivalent—modern buildings fall short by 30-50% without mechanical systems.
The Indoor Air Quality Risk
Pollutant accumulation: Without adequate air exchange, VOCs, CO2, moisture, combustion byproducts accumulate to unhealthy levels.
Previous articles documented: Tight apartments reach 1,500-2,500 ppm CO2 overnight; VOCs accumulate 2-5x outdoor levels; moisture enables mold growth.
The Solution: Controlled Ventilation
“Build Tight, Ventilate Right”: Achieve envelope airtightness for energy efficiency, then install mechanical ventilation for indoor air quality.
Advantages over leaky envelopes:
- Energy efficient: Controlled ventilation with heat recovery vs random infiltration wasting energy
- Reliable: Ventilation occurs regardless of weather vs infiltration varying with wind/temperature
- Filtered: Mechanical ventilation can filter incoming air vs infiltration bringing unfiltered outdoor air
- Humidity controlled: ERV manages moisture vs infiltration importing humidity or drying excessively
HVAC Integration: Makeup Air and Building Pressure
HVAC systems interact with natural air exchange—proper design critical.
Dedicated Outdoor Air Systems (DOAS)
Design: HVAC delivers measured outdoor air (meeting ASHRAE 62.2) independently of heating/cooling load.
Advantage:Decouples ventilation from thermal conditioning—provides fresh air consistently regardless of heating/cooling calls.
Building pressure: DOAS typically slightly positive (+2-5 Pa) preventing infiltration of unconditioned air.
Return Air vs Relief Air
Conventional HVAC: Returns indoor air to air handler for reconditioning.
Problem: If HVAC supplies outdoor air but only returns (no exhaust), building pressurizes—forcing exfiltration through envelope.
Solution:Relief damper or dedicated exhaust matching outdoor air supply—maintaining pressure balance.
Economizer Operation
Free cooling: When outdoor air cooler than indoor, HVAC uses 100% outdoor air instead of recirculating.
Pressure impact: Massive outdoor air volume (3,000-10,000 CFM) can create excessive positive pressure if relief pathway inadequate.
Design requirement: Relief air capacity must match maximum outdoor air intake during economizer mode.
Unintended Consequences: Cross-Contamination in Multifamily
Pressure imbalances enable air migration between units.
The Apartment Stack Effect Problem
Tall buildings: Stack effect creates pressure differences between floors—lower units negative, upper units positive.
Weak separation: Apartment-to-apartment walls have penetrations (electrical, plumbing) and gaps around doors allowing airflow.
Result: Air migrates vertically and horizontally—lower-floor cooking odors, smoking, pollutants transfer to upper floors via pressure-driven flow.
Research finding:“Cross-floor transport of airborne pollutants” is documented consequence of stack effect in multifamily buildings.
Mechanical System Amplification
Imbalanced exhausts: Unit-by-unit bathroom fans exhausting without supply creates sustained negative pressure pulling air from adjacent units and hallways.
Case study: Research documented “high negative pressures creating forces over 100N on doors, whistling caused by high-speed airflows around entry doors” from imbalanced mechanical systems.
Contamination pathway: Negative-pressure units draw air from neighbors through shared wall penetrations—importing secondhand smoke, cooking odors, VOCs.
Mitigation Strategies
Compartmentalization: Air seal between units—close penetrations, install door sweeps, seal electrical/plumbing gaps.
Balanced ventilation: Match unit exhaust with unit supply—preventing pressure-driven cross-contamination.
Corridor pressurization: Maintain slight positive pressure in hallways preventing unit-to-hallway-to-unit contamination paths.
Seasonal Variation: Winter vs Summer Air Exchange
Temperature differentials create dramatically different conditions.
Winter (Cold Climate)
Strong stack effect: 20-30°C (36-54°F) temperature differentials create 20-40 Pa pressure differences in tall buildings.
Dominant force: Stack often exceeds wind and mechanical combined—becoming primary driver of air exchange.
Infiltration location: Ground floors, north exposures (wind + stack align).
Energy impact:Highest infiltration rates—cold air enters requiring maximum heating energy.
Indoor air quality: Dry outdoor air imported causes low indoor RH (10-25% without humidification)—previous ERV article discussed moisture recovery necessity.
Summer (Hot Climate)
Weak reverse stack: 5-15°C (9-27°F) differentials create 5-10 Pa pressures—much weaker than winter.
Dominant force:AC operation (depressurization from return air) and wind often exceed stack effect.
Infiltration location: Upper floors (reverse stack), all exposures (wind-dependent).
Energy impact: Moderate—hot humid air infiltrates requiring AC to cool and dehumidify.
Shoulder Seasons
Minimal stack effect: Small temperature differentials (5-10°C) generate <5 Pa stack pressures.
Dominant force:Wind and mechanical systems—stack effect negligible.
Lowest natural ACH: Without temperature-driven buoyancy, tight buildings exchange air <0.2 ACH naturally—highlighting mechanical ventilation necessity.
Frequently Asked Questions
What causes air to exchange in buildings?
Three simultaneous forces create pressure differentials driving air through envelope leaks: (1) Wind pressure generating 5-15 Pa on exposed walls depending on gusts; (2) Stack effect from temperature differences creating 10-30 Pa vertical pressure gradients in tall buildings (warm indoor air rises in winter, cool air sinks in summer); (3) Mechanical systems (HVAC, exhaust fans) adding ±5-10 Pa building pressurization/depressurization. Research confirms “infiltration caused by wind, negative pressurization, and air buoyancy forces known as stack effect”—forces combine algebraically determining actual pressure at any location. Airflow through cracks follows Q = C × A × (ΔP)^n where doubling pressure increases flow ~1.4-1.6x (not 2x).
How many air changes per hour is normal for a house?
Depends on construction era and airtightness. Modern tight buildings (ACH50 < 5): 0.2-0.5 natural ACH—insufficient without mechanical ventilation per ASHRAE 62.2 requiring ≥0.35 ACH. Average construction (ACH50 5-10): 0.3-0.7 ACH—marginal, may need mechanical in extreme climates. Leaky old buildings (ACH50 > 15): 1.0-3.0 ACH—often excessive wasting energy but ensuring fresh air. Conversion: Natural ACH ≈ ACH50/20 for typical homes. Seasonal variation: Winter shows higher ACH from strong stack effect; shoulder seasons show lowest (<0.2 ACH in tight buildings) highlighting mechanical ventilation necessity.
What is stack effect in buildings?
Temperature-driven vertical pressure gradient where warm indoor air rises in cold weather creating negative pressure at bottom (infiltration), positive pressure at top (exfiltration), with neutral pressure plane mid-building where pressures equal. Stack pressure proportional to temperature difference and height: ΔP_stack ≈ 0.0035 × h × (T_in – T_out)/T_avg. Example: 10-story building (100 ft), 70°F indoor, 20°F outdoor creates ~30 Pa differential bottom to top. Research confirms stack effect causes “difficulty in door operation, whistling through cracks, cross-floor transport of airborne pollutants, compromised ventilation performance” in tall buildings. Summer reversal: Cool AC air sinks creating opposite pressures but weaker magnitude (smaller temperature differentials).
Why do tall buildings have more air infiltration?
Stack effect magnitude proportional to building height—taller buildings create larger vertical pressure differences. 3-story building (30 ft): ~10 Pa stack differential. 30-story building (300 ft): ~100 Pa differential—10x greater driving force. Additionally, upper floors experience 30-50% higher wind speeds than ground level due to reduced ground friction, creating higher wind pressures. Research documents “stack effect increases with building height” and becomes “very pronounced in high-rise structures.” Result: Tall buildings show excessive infiltration at lower floors, excessive exfiltration at upper floors requiring specialized mitigation (compartmentalization, mechanical pressurization, improved envelope) versus low-rise buildings where stack effect negligible.
How does HVAC affect building air pressure?
HVAC creates building pressurization (supply > exhaust) or depressurization (exhaust > supply) adding ±5-10 Pa to natural pressures from wind and stack effect. Balanced systems (equal supply/exhaust) maintain neutral pressure; imbalanced systems create sustained pressure differentials. Research warns “mechanical systems can generate pressure differentials with much greater impact than wind or stack alone” creating “constant year-round driving force for air leakage.” Common problem: Bathroom/kitchen exhausts without makeup air depressurizing building -8 to -15 Pa causing excessive infiltration, potential combustion backdrafting. Proper design: Match exhaust with supply, install relief dampers, control building pressure within ±5 Pa target preventing energy waste and IAQ problems.