Ventilation: Background, Physics, and Sustainable Design Principles

Fundamental Definitions and Terminology of Ventilation

• Ventilation Definition: The provision of fresh air and the subsequent removal of stale air from a building environment.
• Background Ventilation: The controlled provision of the baseline fresh air requirement.
  • It is the designed and intended provision of fresh air at all required times.
  • In practical terms, it involves a specified or designed system that reliably forces air through a building at the required rate.
  • This is considered necessary in cooler climates where maintaining internal thermal comfort is a priority.
• Infiltration: The uncontrolled, accidental, or unintentional provision of fresh air.
  • This is sometimes utilized to provide ventilation in warmer climates or in scenarios where thermal comfort is not a primary concern.
• Purge Ventilation: The provision of a higher level of ventilation for a specific, temporary purpose such as cooking, bathing, or cooling.
• The Paradox of Choice: While it is commonly assumed that people want as much fresh air as possible, Dr. Southall explains that life is rarely that simple. The amount of ventilation required is determined by specific "clues," typically related to temperature and building programming.

Socio-Health and Physical Implications of Under-Ventilation

• Minimal Biological Need: While the most obvious reason for ventilation is to allow occupants to breathe, humans actually require only a small amount of air for basic respiration.
• Primary Drivers for Ventilation:
  • Removal of Carbon Dioxide ($CO_2$).
  • Removal of other human bio-effluents, such as human odors.
  • Removal of moisture: This is critical as moisture leads to condensation, mould growth, and respiratory illness.
• Health Statistics and Case Studies:
  • In the United Kingdom, $31,000$ children under the age of $4$ are hospitalized every year for respiratory infections linked to mould growth.
  • Awaab Ishak: A $2$-year-old child who died in Rochdale in $2020$ due to inadequate ventilation and exposure to mould growth in his home.
  • Awaab's Law: Legislation enacted following the aforementioned tragedy that requires social housing landlords to fix ventilation and mould issues within a rapid timeframe.
  • Blackpool Data: $100\%$ of private rented properties in Blackpool are reported to have a damp or mould problem.
• The General Requirement: To address these issues, the general figure for fresh air requirement is established at $8-10\,\text{litres per second per person}$ ($8-10\,\text{l/s/p}$).

Energetic and Economic Hazards of Over-Ventilation

• Thermal Short-Circuiting: Over-ventilating a building with fresh air short-circuits the thermal insulation. If incoming air is cold, energy must be expended to heat it to a comfortable temperature.
• Ventilation Heat Loss: This refers to heat lost through air exchange, whereas "fabric heat loss" refers to heat lost through the building's physical skin.
• The Magnitude of Energy Loss (Hypothetical Scenario):
  • Dr. Southall provides an example of a single window with an area of $1.2\,\text{m}^2$, half-open for $5\,\text{hours per day}$ in the UK, with air passing through slowly at $0.5\,\text{m/s}$.
  • For a house of $100\,\text{m}^2$, this single window results in an energy requirement of $46\,\text{kWh/m}^2\text{y}$.
  • Contextual Comparison: The RIBA 2030 target for the maximum total energy expenditure in a home is only $35\,\text{kWh/m}^2\text{y}$. Thus, one slightly open window can exceed the entire energy budget for a sustainable modern home.
• Economic and Environmental Impact:
  • Using a gas boiler with $90\%$ efficiency, this ventilation heat loss costs an extra $£354\,\text{per year}$.
  • This causes the release of $920\,\text{kg}$ of $CO_2$, which accounts for approximately $15\%$ of an average UK person's entire annual climate impact.
• Social Implication: Many people cannot afford the cost of heating incoming air, and many heating systems are not powerful enough to maintain thermal comfort against significant over-ventilation. Consequently, opening windows is often not a viable option for the economically vulnerable or for the planet.

Climatic Context and Environmental Control Strategies

• Temperature Difference (\Delta T): Ventilation heat loss is driven by the temperature difference between the interior and exterior.
• Relaxed Ventilation Conditions:
  • In cold climates, if no heating is being provided to the building, designers can be more relaxed about infiltration.
  • When external temperatures (e.g., $+3^\circ\text{C}$) fall within the local comfort band and no artificial heat is needed, over-ventilation is less problematic. This is common in the UK summer when windows are opened for cooling.
• Legal and Safety Warnings: Using operable windows as a year-round strategy for background ventilation in the UK is described as illegal, wasteful, and dangerous.
• High-Temperature Strategies:
  • Short Periods: If external temperatures are briefly above the comfort band, thermal mass can be used to maintain internal comfort.
  • Long Periods: If external temperatures remain high for long durations, incoming air must be controlled and conditioned.
    • Natural Cooling: Water evaporation or ground cooling.
    • Un-natural Cooling: Mechanical air-conditioning.

Analytical Clues: Temperature and Wind Data

• Ambient vs. Comfort Temperature:
  • If ambient temperature is near the ASHRAE adaptive comfort bands ($80\%$ or $90\%$), infiltration can be treated with a more relaxed attitude.
  • If there is a significant disparity between ambient and comfort temperatures and energy is being used to maintain comfort, controlled "background ventilation" is mandatory.
• Wind Analysis:
  • Designers must check wind speeds; even in hot climates, a "gale" blowing through a building is undesirable.
  • Wind direction analysis determines if building apertures are aligned with predominant annual or seasonal wind directions.
• Design Responses Based on Clues:
  • Warm + Low Wind Speed: Open the facade extensively.
  • Warm + High Wind Speed: Open the facade but include controls to limit intake.
  • Close to Comfort Band + Solar Gain: Open the facade with seasonal flow controls.
  • Cold + Low Concern for Internal Temp: Rely on infiltration.
  • Cold + High Concern for Internal Temp: Design a dedicated ventilation system with optional seasonal facade openings.

Design Philosophies: "Build Tight - Ventilate Right"

• The Optimal Outcome: To achieve a "Goldilocks" (optimal) level of ventilation—neither too little to harm health nor too much to waste energy—designers use Building Physics principles.
• Core Tagline: "Build tight - ventilate right."
  • Build Tight: Minimise uncontrolled infiltration.
  • Ventilate Right: Design a system that provides a reliable level of background ventilation.
• Regulatory Reality: Opening windows is not legal as a background ventilation strategy for new-builds or modern retro-fits in the UK. However, they are permitted for purge ventilation to remove excess heat.

Methodologies of Ventilation: Natural (Passive)

• Mechanism: Uses natural forces to move air without mechanical assistance.
• The Two Primary Natural Forces:
  1. Stack Effect: Based on the principle that hot air rises.
  2. Newton's Laws / Bernoulli Principle: Driven by the action of wind on the building structure.
• Advantages:
  • Requires little or no additional energy.
  • Quiet operation.
  • Healthy, as there is no supply-side ducting (provided external air quality is high).
• Disadvantages:
  • Variable flow rates.
  • Limited control over the volume of air movement.
• Common Examples: Double skin facades, atria, chimneys/stacks, solar chimneys, and facade apertures.

Methodologies of Ventilation: Mechanical

• Mechanism: Utilizes fans and ducts to move air. Fans are typically located on the extract side to pull air out of the building.
• Features and Benefits:
  • Very controllable; guarantees specific flow rates.
  • Can utilize heat recovery or provide cooling.
  • Can filter and clean dirty outside air before it reaches occupants.
  • Essential for thermal comfort in cold climates.
• Drawbacks:
  • Requires energy to run.
  • Can generate noise pollution.
  • Potential health issues if fresh air does not reach occupants directly.
  • Requires regular maintenance and cleaning.
• Common Examples:
  • MVHR: Mechanical Ventilation with Heat Recovery.
  • Air-conditioning.
  • Constant/Intermittent Mechanical Extract: Often paired with a heat pump for recovery.
• Universal Requirement: Every system—natural or mechanical—must have a dedicated, identified place for air to both enter and leave.

Fluid Dynamics: Newton and Bernoulli Principles

• Newton's Law: High pressure is created where a building slows down or redirects air movement.
• Bernoulli's Principle: Air speeds up around obstructions; any slow-moving air nearby will tend to join the faster stream. Air flows from areas of high pressure to low pressure.
• Application: These principles allow designers to manipulate airflow through building geometry and obstructions. They are most commonly used to promote cross-ventilation (moving air from one side of a building to the other).
• Stack Effect Control: To prevent the flow rates of the stack effect from varying too much, modern designs often use automatically adjusted baffles, flaps, or windows.

Graphical Representation of Ventilation

• Narrative Starting Point: A graphical story of ventilation begins with contextual data: local wind, ambient temperature, and surrounding obstructions.
• Internal Representation: Arrows are used to indicate air temperature, direction, and flow rate within the building.
• External Representation: Streamlines are drawn to illustrate external pressures and the predicted path of air through the structure.

Precedent Analysis

• Dr. Southall notes that all provided precedent buildings utilize natural ventilation.
• Precedent Breakdown:
  • Two precedents use Newton/Bernoulli principles to generate cross-ventilation via infiltration.
  • One precedent uses Newton/Bernoulli principles for cross-ventilation via window control.
  • One precedent utilizes the stack effect to draw air out of the building.
  • One precedent relies entirely on infiltration via floor vents (likely located under beds).