Buildings are generally designed to use natural ventilation. However, in the quest to make buildings more modern, we have jettisoned natural ventilation media for aesthetic reasons like the integration of mechanical systems and interior partition walls. With these compromises come some challenges, including increased cost and adverse environmental impacts. To resolve these challenges, designers and contractors are now returning to the era of natural ventilation.
Natural ventilation does not only save costs but also reduce energy use significantly. It also offers a safe and high-quality, comfortable, and healthy indoor climate. These are more significant perks than what the contemporary option of mechanical ventilation will ever provide. Natural ventilation will come handy as a better alternative to air conditioning plants in places and situations where the climate and building types are favorable. With this arrangement, up to 30% of total energy consumption can be saved.
By working on the principle of pressure differences, natural ventilation systems supply the entire building with fresh air evenly. The change in pressure may be due to a change in humidity or the buoyancy effect created by temperature differences or wind. Irrespective of the cause, the opening sizes and placements determine the amount of ventilation a building gets.
Let’s compare a natural ventilation system to a circuit, without prioritizing exhaust over supply and vice versa. The airflow circuit in a building is completed either by opening between rooms, including grills, louvers, or transom windows, or open-plan techniques. Some of the challenges a natural ventilation designer must contend with include the code requirements concerning smoke and fire transfer. Code requirements have made it impossible to use the stairway as an exhaust stack – an arrangement obtainable in most historic buildings.
What is natural ventilation?
Natural ventilation is a system that serves buildings with fresh air using natural forces of wind and buoyancy instead of mechanical methods. The importance of fresh air in buildings cannot be overemphasized; it offers better thermal comfort and ensures an abundant supply of oxygen for respiration. Most ventilators run at interior air velocities of 160 feet per minute (fpm) and can reduce the perceived internal temperature by up to 5 degrees Fahrenheit. Perhaps, the only shortcoming of the natural ventilation system relative to the true air conditioning system is that the former cannot reduce the humidity of incoming air. And for this reason, it is not the best option in humid climates.
What are the types of natural ventilation effects?
When the wind blows into a building, air passes through wall openings on the windward side while sucking the air out of openings on the roof and the leeward side. Due to the temperature differences between the cool air outside and the warm air inside, the air in the room may rise and exit through the ridge or ceiling, and enter through the lower openings in the wall. In the same vein, the resulting buoyancy from the differences in humidity can enable a column of air that is already pressurized and cooled evaporatively to supply a space. In contrast, the warmer, humid, and lighter air finds its way out near the top.
Below are insights into these three forms of natural ventilation effects.
Wind exerts negative pressure on the leeward side of buildings and positive pressure on the windward side. This necessitates the need to equalize the pressure. And consequently, the fresh air enters via any windward opening and escapes via any leeward opening available. Wind supplies fresh air in generous amounts in the summer. However, the amount of fresh air available during the winter is only sufficient enough to get rid of excess pollutants and moisture.
Wind flow generally in a direction that is perpendicular to the building. However, in cases where the flow is parallel, wind ventilation can be forced by incorporating certain architectural features or adopting casement window openings. In a situation where the wind blows from westwards from the east and along a wall facing the north, the first window opening out and hinging on the left-hand side serves as a scoop that redirects wind into the room. On the other hand, the second window would hinge on the right-hand side to ensure that the opening is down-wind from the open glass plane, with the negative pressure forcing air out of the room.
There should be no obstructions between the leeward exhaust and windward inlet openings. Likewise, no partition should exist in a room with a perpendicular orientation to the airflow. Conversely, the accepted design ensures that the outlet and inlet windows are not directly across each other. This is to provide improved mixing and more effective ventilation.
There are two forms of buoyancy ventilation – humidity-induced, also known as the cool tower, and temperature-induced, also known as the stack ventilation. It is possible to integrate both – create a cool tower that can supply evaporatively cooled air low in space while relying on the increased buoyancy of the humid air to warm to exhaust air from the space via a stack. The weight of the column of cool air above the space pressurizes the cool air supply to the same space. While it is possible to use both stacks and cool towers individually, the integration of both only works in a space where the aim is to stabilize airflow.
Buoyancy is caused by a difference in air density, which, in turn, depends on humidity and temperature. This is plausible, considering that at the same temperature, humid air weighs more than dry air, while at the same humidity, warm air weighs more than the cool air. The effects of humidity and temperature in the cool tower are contradictory in actions; the former pulls upward while the latter pulls downward. The air in the room will rise ordinarily, thanks to the heat and humidity from internal sources and the room’s occupants. The presence of the existing heated air finding its way through the ceiling or roof openings allows fresh air to enter the lower openings to replace it. The effectiveness of the stack effect ventilation is maximum during the winter, considering that the temperature difference of the outdoor and indoor environment is at the peak. Conversely, the effectiveness is reduced significantly in the summer because the requirement of the indoor being warmer than the outdoors cannot be met.
Depending on the local climate and building type, the approach and design of natural ventilation systems may differ. The best designs or approaches are those that take care of the internal spaces and the placement and size of the openings in the buildings. These two factors majorly determine the amount of ventilation a building gets. You can check the seasonal “wind rose” diagrams on the National Oceanographic and Atmospheric Administration (NOAA) website for an insight into the estimated wind directions. These roses rely on data recorded at airports. Hence, it will not be strange to have slight or dramatic different values at a remote building site.
It is best to build on sites where there are no or little summer wind obstructions. If there are evergreen trees on site, the resulting windbreak will help significantly in mitigating cold winter winds originating from the north.
Narrow buildings work best with natural ventilation.
If a building is very wide, natural ventilation will do a poor job distributing fresh air evenly and effectively to every part. A building that will adopt a natural ventilation system must not exceed a width of 45 ft. This is why most buildings with natural ventilation come with an articulated floor plan.
There should be two individual exhaust and supply openings per room. The stack effect can be maximized through the exhaust high above the inlet. Windows should be across the room and offset from each other; this guarantees minimum airflow obstructions and maximum mixing within the room. Occupants should find the window openings easy to use.
Ridge Vents should be present.
The opening found at the highest point in the roof is called a ridge vent. It serves as a good outlet for wind- and buoyancy-induced ventilation. The ridge opening will only allow maximum air outflow from the building if there are no obstructions.
Ensure adequate internal airflow.
It is essential to ensure proper airflow between the rooms present in a building. It is best to use interior doors that can be opened; they promote whole-building ventilation. Transoms and high louvers may be excellent sources of ventilation if there is a need for privacy.
Vented Skylights or Clerestories are better.
The advantage of a vented skylight or clerestory is that they offer an opening for outward flow of air in a buoyancy ventilation strategy. The skylight comes with a light well that may work as a solar chimney for improving the flow. The ventilation system is incomplete without lower structural openings, for instance, the first-floor windows.
Integrate Attic Ventilation.
If there is an attic in the building, the best way to reduce heat transfer to conditioned rooms below will be to ventilate the attic space. The temperature difference between ventilated and unventilated attics is about 30 degrees Fahrenheit.
Are fan-assisted cooling strategies advisable?
With ceiling and whole-building fans, the effective temperature can be reduced by up to 9 degrees Fahrenheit, while expending one-tenth the electrical energy consumption of what mechanical-air conditioning systems require. It is essential to determine the better approach between the closed-building or open-building ventilation system for a building.
If there is a significant difference in temperature from day to night, the ideal ventilation system is a closed-building approach. In this case, the building is ventilated at night and closed in the morning so that the hot daytime air does not find its way in. The radiant exchange within the giant walls and floor is responsible for cooling the occupants of the building.
If the area is warm and humid, with little or no temperature difference from day to night, the ideal ventilation system is an open-building approach. Here, cross-ventilation is the best way to keep indoor temperatures close to outdoor temperatures.
Natural ventilation is not always enough to make interior conditions optimally comfortable. In fact, 3-5% of the time, building occupants will not enjoy thermal comfort. Therefore, natural ventilation is not ideal for buildings without space conditioning. Designers are faced with the problem of coming up with a solution that integrates both natural ventilation and mechanical cooling systems. While an artificially conditioned building will be fine with a compact plan with sealed windows, a naturally ventilated structure is a blend of large doors and window openings, and articulated floor plan. Designers must also factor in factors like the correct interpretation of wind data, surrounding buildings, vegetation, and local topography when estimating the speed of the wind and its effects on hitting a building.
Construction Methods and Materials Used.
Diverse methods and materials go into the design of adequate natural systems for buildings. These include summer ventilation control methods, wind towers, and solar chimneys, among others. A solar chimney is used in situations where maintaining an indoor temperature that is above outdoor temperature to drive buoyant flow will result in an unacceptably warm environment. It is also more preferable when wind-induced ventilation will fail due to inadequate prevailing breezes. Considering that the chimney is separated from the occupied space, it can be heated by the sun or similar alternatives. The exhaustion of air is usually through the top of the chimney while creating suction at the base for the extraction of the stale air.
The ancient Arabic architectures are characterized by wind towers, also known as “malqafs,” with fabric sails sitting on top. The role of the fabric sails is to direct wind into the building. Once the incoming air is routed past the fountain, it provides evaporative cooling and ventilation. The reverse happens in the night, with the wind tower functioning as a chimney to vent room air. The contemporary form of this design is the “Cool Tower” – a system that puts evaporative cooling elements at the top of the tower, thus pressurizing the supply air with dense and cool air.
The summer is known for an external temperature that is lower than the desired internal temperature. When this happens, opening the windows will help to maximize fresh air intake. The inside temperature can only be maintained at 3-5 degrees Fahrenheit above the outside temperature if there is abundant airflow. When the day becomes hot and calm, there is a very low air exchange rate, and this prompts the internal temperature to increase, way over the external temperature. In such cases of maximum temperatures, the best method of cooling is to use thermal mass or fan-forced ventilation for radiant cooling.