What is natural ventilation?
Natural forces (e.g. winds and thermal buoyancy force due to indoor and outdoor air density differences) drive outdoor air through purpose-built, building envelope openings. Purpose-built openings include windows, doors, solar chimneys, wind towers and trickle ventilators. This natural ventilation of buildings depends on climate, building design and human behavior.
If well installed and maintained, there are several advantages of a natural ventilation system, compared with mechanical ventilation systems.
- Natural ventilation can generally provide a high ventilation rate more economically, due to the use of natural forces and large openings.
- Natural ventilation can be more energy efficient, particularly if heating is not required.
- Well-designed natural ventilation could be used to access higher levels of daylight.
Design Recommendations
The specific approach and design of natural ventilation systems will vary based on building type and local climate. However, the amount of ventilation depends critically on the careful design of internal spaces, and the size and placement of openings in the building.
- Maximize wind-induced ventilation by siting the ridge of a building perpendicular to the summer winds.
- Approximate wind directions are summarized in seasonal "wind rose" diagrams available from the National Oceanographic and Atmospheric Administration (NOAA). However, these roses are usually based on data taken at airports; actual values at a remote building site can differ dramatically.
- Buildings should be sited where summer wind obstructions are minimal. A windbreak of evergreen trees may also be useful to mitigate cold winter winds that tend to come predominantly from the north.
- Naturally ventilated buildings should be narrow.
- It is difficult to distribute fresh air to all portions of a very wide building using natural ventilation. The maximum width that one could expect to ventilate naturally is estimated at 45 ft. Consequently, buildings that rely on natural ventilation often have an articulated floor plan.
- Each room should have two separate supply and exhaust openings. Locate exhaust high above inlet to maximize stack effect. Orient windows across the room and offset from each other to maximize mixing within the room while minimizing the obstructions to airflow within the room.
- Window openings should be operable by the occupants.
- Provide ridge vents.
- A ridge vent is an opening at the highest point in the roof that offers a good outlet for both buoyancy and wind-induced ventilation. The ridge opening should be free of obstructions to allow air to freely flow out of the building.
- Allow for adequate internal airflow.
- In addition to the primary consideration of airflow in and out of the building, airflow between the rooms of the building is important. When possible, interior doors should be designed to be open to encourage whole-building ventilation. If privacy is required, ventilation can be provided through high louvers or transoms.
- Consider the use of clerestories or vented skylights.
- A clerestory or a vented skylight will provide an opening for stale air to escape in a buoyancy ventilation strategy. The light well of the skylight could also act as a solar chimney to augment the flow. Openings lower in the structure, such as basement windows, must be provided to complete the ventilation system.
- Provide attic ventilation.
- In buildings with attics, ventilating the attic space greatly reduces heat transfer to conditioned rooms below. Ventilated attics are about 30°F cooler than unventilated attics.
- Consider the use of fan-assisted cooling strategies.
- Ceiling and whole-building fans can provide up to 9°F effective temperature drop at one tenth the electrical energy consumption of mechanical air-conditioning systems.
- Determine if the building will benefit from an open- or closed-building ventilation approach.
- A closed-building approach works well in hot, dry climates where there is a large variation in temperature from day to night. A massive building is ventilated at night, then, closed in the morning to keep out the hot daytime air. Occupants are then cooled by radiant exchange with the massive walls and floor.
- An open-building approach works well in warm and humid areas, where the temperature does not change much from day to night. In this case, daytime cross-ventilation is encouraged to maintain indoor temperatures close to outdoor temperatures.
- Use mechanical cooling in hot, humid climates.
- Try to allow natural ventilation to cool the mass of the building at night in hot climates.
- Open staircases provide stack effect ventilation, but observe all fire and smoke precautions for enclosed stairways.
Sunlight Analysis Can Improve Design
Direct sunlight is of critical importance in architecture — for reasons of aesthetics, experience, and comfort, as well as energy performance. Direct sunlight can reduce winter heating, but can dramatically increase summer cooling. Direct sunlight can provide warmth and dynamism to a space, but can also mean visual or thermal discomfort for occupants as diverse as office workers and athletes. And access to direct sunlight is increasingly regulated on both the building and urban scale.
However, until now there hasn’t been a good way for designers to evaluate direct sunlight in a quick, quantifiable way. The architect’s traditional tools — sun angle charts and shadow studies — show only points in time, making it time-consuming to aggregate results across hours, days, or seasons. Direct Sunlight simulation makes these types of studies straightforward — and, in doing so, makes it easier for architects to consider the elemental impact of the sun on their buildings.
Passive Solar Design
You know the orientation of your site and where the sun will be at any given time. Now, you can design a home that will let the sun in when you want it to come in (in the winter) and keep it out when you want to keep it out (in the summer). What you’re trying to do is regulate the temperature inside and out of your home naturally so that you can become less reliant on mechanical and electrical systems.
This is what passive solar design is all about: Designing your house to do as much as it can to heat and cool your house on its own.
Passive solar design is about collecting, storing, distributing, and/or controlling solar energy (both heat and light) so that you can reduce your demand on fossil fuels. This passive approach means that through the basic elements of the house–its walls, windows, floors, and roof–and through its relationship with the surrounding site, the house is able to inherently respond and optimize solar energy, whereby increasing the energy efficiency of your home, making it more comfortable to live in, and being cheaper to run.
Passive solar design works in three ways:
1. Direct gain systems: heat spaces via direct solar gain through glass
2. Indirect gain systems: heat a part of a building like a thick wall or concrete floor then rely on conduction to slowly transfer heat to space over time, aka thermal mass
3. Isolated indirect gain systems: heat an adjacent space then rely on convection to transfer heat to other spaces, aka sunrooms
