Natural Ventilation

What is Natural Ventilation?

Natural ventilation uses passive strategies to supply outdoor air to a building’s interior for ventilation and cooling.[1] Natural ventilation systems rely on natural forces, such as wind and temperature differences between a building and its environment, to drive the flow of fresh air through a building. Cross ventilation uses air-pressure differentials caused by wind, and stack ventilation or the chimney effect uses the increased buoyancy of air as it warms up. [2] Both work on the principle of air moving from a high-pressure to a low-pressure zone. For example, when cool air enters a home on the first floor or basement, it absorbs heat in the room, rises, and exits through upstairs windows.[3] Stack ventilation works best in open layout home designs with cathedral ceilings and operable windows and skylights on the top floor.

Many modern homes utilize tightly sealed building envelopes and mechanical HVAC systems to achieve energy efficiency (see Air Infiltration). Natural ventilation strategies can introduce fresh air to airtight homes, replacing all or part of an HVAC system, although a home usually needs to supplement natural ventilation with design strategies that help direct airflows such as room-to-room ceiling fans, or whole house fans for larger homes.[4]

Ceiling fans circulate air, remove heat from the home and create a wind chill that cools the body. The wind chill effect can make a room feel 4 to 6 degrees cooler.[5] A whole house fan can move cooler nighttime air into the house through open windows and exhaust warm air through the attic.

How to Implement Natural Ventilation

Implementing a successful natural ventilation design requires an understanding of the local climate, building geometry, occupant behavior, and the design of internal spaces (see Building Orientation, Tree Protection and Placement, and Building Evaluation). For example, in coastal locations, desirable cool sea breezes can reduce cooling loads. It is important to be aware that outdoor air in these areas may also have high moisture content that needs to be managed (see Moisture Control). Other locations may not be ideal for natural ventilation due to local sources of air pollution, allergens, or outdoor noises. The design of internal spaces and the size and placement of openings in the building impacts ventilation as well.[6] Operable windows, fans, and corridors help direct airflow using natural ventilation principles (see Individual Comfort Controls and Views and Operable Windows).

Early in the design stage, project architects and homebuilders can use CoolVent, a natural ventilation simulation tool developed by the Massachusetts Institute of Technology (MIT) to evaluate the effects of different natural ventilation strategies on the proposed home’s energy use and occupant comfort (see Energy Modeling).

The project team may also consider pursuing the Passive House Institute US (PHIUS+ 2018) building certification which sets standards for airtight construction, and natural and controlled mechanical ventilation for zero-energy, high-performance homes.

Example

Harvard HouseZero, Cambridge, MA.

Harvard HouseZero, headquarters for the Harvard Center for Green Buildings and Cities, serves as a model for residential home construction and remodeling. The renovation of the pre-1940’s house into a zero-energy office building utilizes passive strategies, including thermal mass to replace the need for an HVAC system, and active strategies such as a ground source heat pump for peak (extreme) conditions. The building plans to achieve 100% natural daylighting and ventilation using solar vents to instigate buoyancy-driven ventilation and triple-glazed windows for natural cross ventilation through a manual and automated system that monitors for temperature, humidity, and air quality.[7]

Benefits

Natural ventilation strategies can reduce energy use, and lower energy bills and installation and maintenance costs by allowing for smaller mechanical systems and by extending the life of HVAC equipment. Homeowners can install ceiling fans to support natural ventilation without the need to hire a professional, providing additional cost savings. Other benefits include comfortable, year-round indoor temperatures, improved indoor air quality through increased fresh air circulation and increased connection to the outdoors (see Biophilia).

Costs

While passive building typically costs 5-10% more than building a conventional home, natural ventilation strategies considered early in the design phase and as part of an integrated design process provide the opportunity to realize upfront cost savings from installing downsized mechanical equipment and cost savings associated with increased energy efficiency (see Integrated Design Process).[8]

Installing ENERGY STAR certified ceiling fans saves 40% more energy compared to conventional fans, providing savings up to $60 in energy costs over the fan’s 10-year lifetime.[9] The ENERGY STAR website provides guidance on ceiling fan installation and usage as well as comparative product tools and information on available rebates.

Resiliency

In the event of power outages and the loss of air conditioning, natural ventilation can maintain thermal comfort for building occupants, particularly at night, when colder outside air can mix with warmer air inside the building (e.g., nighttime flush). Mixed-mode systems add redundancy and flexibility to the building’s ventilation and HVAC systems, providing back-up during power-outages, mechanical failures, security threats, or the release of air-borne contaminants.

[1] Whole Building Design Guide (WBDG). Natural Ventilation. https://www.wbdg.org/resources/natural-ventilation (accessed March 14, 2018).

[2] CoolVent. 2018. Basics of Natural Ventilation. Massachusetts Institute of Technology. http://coolvent.mit.edu/intro-to-natural-ventilation/basics-of-natural-ventilation/ (accessed June 28, 2018).

[3] US DOE. Energy Savers. Natural Ventilation

https://www.energy.gov/energysaver/natural-ventilation (accessed March 30, 2018).

[4] WindowMaster. Cost and Performance – A design-led approach to effective natural ventilation. https://www.windowmaster.com/white-papers (accessed March 14, 2018).

[5] U.S. Green Building Council’s Green Home Guide. “10 Ways to Beat the Heat.”  http://greenhomeguide.com/know-how/article/10-ways-to-beat-the-heat  (accessed March 30, 2018).

[6] WBDG. Natural Ventilation.  http://www.wbdg.org/resources/naturalventilation.php (accessed March 12, 2018).

[7] Harvard HouseZero. http://harvardcgbc.org/cgbc-announces-first-of-its-kind-housezero-project-extreme-retrofit-of-its-headquarters-requires-no-hvac-or-electric-light/ (accessed March 30, 2018).

[8] Passive House Alliance. 2018. What is Passive Building? Passive House Institute US. http://www.phius.org/what-is-passive-building (accessed June 29, 2018).

[9] Energy Star. Energy Efficient Products. Ceiling Fans. https://www.energystar.gov/products/lighting_fans/(accessed March 30, 2018).