The advantage of natural ventilation for the occupants of buildings is evident. Fresh air from the outside is needed to ventilate an indoor space to meet respiratory, air quality and pollutant removal requirements.

Recent concerns around COVID-19 and other viruses have placed a greater demand for understanding air movement inside buildings. The evolving advice on ventilating indoor spaces converges on one inescapable fact: keeping windows open and allowing as much fresh air as possible. This brings its challenges in maintaining thermal comfort and reveals that most buildings are not designed for natural ventilation.

Many offices, restaurants, high-rise buildings and other habitable structures are entirely dependent on mechanical air supply systems with or without cooling. An elaborate network of ducts, grills, louvers and dampers control the airflow and direct it where needed. There is some allowance for fresh air mixing that reduces with the increasing complexity of building type.

Figure one: Simulating the spread of a virus inside a room using computational fluid dynamics. The darker regions show a higher concentration of simulated virus particles.

Even before recent guidance on COVID-19 mitigation, many building standards and rating systems explicitly encouraged natural ventilation and high air quality levels. Well-known building rating systems, including LEED, BREEAM and the WELL standard, all have sections extolling passive environmental design virtues to maximize fresh air usage. More technical guidelines, such as ANSI/ASHRAE Standard 62.1-2019 Ventilation for Acceptable Indoor Air Quality and CIBSE Application Manual 10 (AM10: Natural Ventilation in Non-Domestic Buildings), give specific details on how natural ventilation requirements and features should be calculated and sized appropriately. Publications such as CIBSE AM11: Building Performance Modelling educates architects and engineers on correctly modeling and simulating natural ventilation using building simulation tools and computational fluid dynamics. The unsteady nature of wind and air movement means designing realistic ventilation strategies in new and existing buildings is a complex topic.

Design Stage Considerations

When designing for ventilation in general, thought must be given to the following considerations.

• What level of ventilation is considered adequate, and how much fresh air is needed? The building and occupancy type will determine this.
• What will the impact of air filters and HVAC equipment be? Each additional component will increase the pressure drop from the outside to the indoor space, adding to the overall project cost.
• How many windows or other opening areas are needed to meet this requirement? The building location, along with other limiting factors, will determine this.
• What is the local climate and, specifically, the wind speed and direction profile over a typical year? This will determine the placement and sizing of openings.
• Will natural ventilation cause drafts and thermal comfort issues for the occupants? Factors such as the building location and facade will determine this.
• How will air quality and CO₂ levels be impacted in a room?
• What control will occupants have over the ventilation system? The type of building and its purpose, such as a hospital versus an office space, will determine this.

An environmental certification system essentially enforces and guarantees the best practice design, construction and operational methodologies for the construction industry. A rating system goes beyond national and local building regulations and guidelines and uses a credit/points system by category. Examples include BREEAM, LEED, WELL, GreenMark, GreenStar, PEARL, QSAS, Estidama and many more. They are Independently reviewed and certified, and practitioners must have professional certifications, such as the LEED AP qualification. Most rating systems will cover several categories of a building’s performance, including:

• Energy (loads, demand and renewables);
• Environmental quality (ventilation and CO₂);
• Water use and efficiency;
• Site sustainability;
• Travel/locality;
• Health and wellbeing; and
• Materials.

For example, let’s look at the LEED Minimum Indoor Air Quality (IAQ) Performance credit. It has specifics on enhanced IAQ strategies and thermal comfort, encouraging the proper design and usage of natural ventilation. BREEAM similarly has the health and wellbeing category with credits for thermal comfort and indoor air quality. The same is true for the other rating systems. Compliance with these standards is demonstrated using mathematical models of a building’s behavior.

Building simulation tools are used to model a 3-D representation of the building and calculate its heating and cooling loads, energy consumption, indoor thermal comfort, and airflow. They use environmental inputs such as climate, location, building material properties and internal gains (including occupancy patterns, incidental heat gains from equipment and lighting). These zonal models give space average results for a unit of time for indoor temperature, CO₂, ventilation and many other variables, allowing engineers to simulate how a building’s design might behave in the real world. Most thermal modeling and building simulation tools use simple airflow network models to calculate ventilation. They do not, however, capture:

• Wind turbulence;
• External wind conditions;
• Local building context; or
• Impacts of terrain.

This means how architects and engineers are sizing natural ventilation openings lacks a correct treatment of the complex fluid dynamics present in the wind and urban environment. Essentially, not enough of the “physics” is captured, leading to a lack of fresh air and causing many buildings to overheat in warmer conditions. The situation is particularly acute for inner-city buildings that lack a means of mechanical ventilation or air conditioning being retrofitted or where the costs might be prohibitive such as schools and hospitals.

Designing buildings to allow for natural ventilation is a complex task. A more advanced set of tools such as computational fluid dynamics (CFD) can be used to evaluate complex building facade pressures, turbulent wind profiles and buoyancy-driven flow, all of which significantly affect the design of a building. Using CFD, we can investigate the following behavior.

• What is the airflow at the occupant level?
• Is natural ventilation simulated correctly, considering it is impacted by wind turbulence and changing building pressures?
• How does opening a window change the airflow path inside a room?
• How is ventilation through various diffuser types affected?

Figure two: A CFD simulation of a ceiling swirl diffuser showing how the Coanda effect, where air entering the room is attached to the ceiling and spreads outwards through the room, instead of coming in as a jet.

COVID Mitigation for Existing Buildings

Figure three: A 3-D model of a room used in CFD simulation to model ventilation and indoor temperatures.

Using the same simulation tools, existing buildings can also be modeled. Figure three shows a CFD model of an open plan learning/office space. The model is used to evaluate various refit and mitigation strategies for ventilation. The top-hung windows can be set at incremental opening angles, and the ceiling air diffusers can have their mechanical air supply turned on/off or changed to any flow rate. They can also be reversed to represent extract fans or as a combination of supply and extract. The model is located in a real-world setting. The local climate data is used to replicate its natural environment, including the average wind speed and direction for the year.

Figure four: A CFD simulation showing the airflow and temperature distribution inside a space, explicitly visualizing how the air interacts with the occupants.

Figure four illustrates the beneficial results an analysis of this type can yield. An architect or engineer can inspect the air movement in a space and what impact making small design changes might have. Ventilation rates, the mean age of air, CO₂ level and the distribution of indoor temperatures are necessary to mitigate COVID-19. A restaurant, for example, might use this to optimally layout seating to stop high concentrations of virus particles from forming in one place. A hospital might use this method to place air diffusers and fans to prevent cross-contamination of stale air. An office building might benefit from identifying how public spaces should be used. Having the ability to simulate people’s movement, airflow and virus particles in one tool is critically important to make our buildings and even homes safer.

Summary

Designing natural ventilation strategies requires the right set of analysis tools from the early stages of design. Traditional approaches have become obsolete. As more computing speed and resources have become available on the cloud, the need for simplified modeling tools has been eclipsed by the need for highly accurate, fast, and accessible software tools that can capture more of the building physics and offer iterative design workflows to allow architects and engineers to converge on an optimal design.