“Ventilation” is defined as “the natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space.” “Infiltration” can be defined as the uncontrolled inward leakage of air through cracks and gaps in the building envelope, especially around windows and doors. Notice that infiltration deals with uncontrolled situations. In our homes we want to be able to control air movement.
In the past, air leakage was relied on to provide ventilation. However, a leaky building does not guarantee good indoor air quality. Why? Air leaks often bring in air quality problems from attics, crawl spaces, and the outside. These can include:
- Radon from crawl spaces and under-slab areas
- Mold (fungi)
- Pollen and other allergens
- Dust and other particles
Furthermore, if the building is leaky it can cost up to 30-40% more to heat or cool due to conditioned air escaping and outside air infiltrating.
Indoor air typically contains more types—and higher concentrations of—pollutants than outdoor air, even in industrialized areas. Common home indoor air pollutants include biological pollutants (mold spores, dust mites, bacteria, viruses, pollen, animal dander); combustion pollutants (including carbon monoxide); lead from old paint or lead-tainted soil; volatile organic compounds (VOCs) emitted from many paints, glues, and other building materials (this is called “outgassing”); and, in some areas of Florida, radon. The other contributor to poor indoor air quality in Florida is moisture. When done correctly, proper ventilation will assist in pollutant and moisture removal and control.
In order to ensure a healthy indoor environment, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends that a home have 0.35 air changes per hour (ACH). An older home may have infiltration rates in the order of 2.5ACH or greater, while a building built to today’s standards and practices will usually have infiltration rates from 0.5 to 1.0 ACH.2 A new energy efficient designed home may have far less than 0.35 air exchanges per hour.
Building infiltration may vary depending on conditions. During cold, windy periods, infiltration into the building may be accelerated, whereas very little air exchange may occur during calm, warm weather.2 To ensure proper indoor air quality, mechanical or natural ventilation should be a consideration in the design and maintenance of any home.
What types of ventilation are available?
Natural ventilation depends on uncontrolled air movement or infiltrations that cannot be depended on to ventilate the house uniformly. It also brings with it unwanted moisture. Natural ventilation relies on the opening of windows and doors for the most part. However, when people don’t open their windows and doors, natural ventilation is more or less infiltration through cracks and small holes in the home. This type of ventilation is not typically recommended for Florida homes.
Spot ventilation in bathrooms and kitchens is another approach used to control moisture and odors. Many of the exhaust fans installed in standard construction are ineffective—a prime contributor to interior moisture problems for buildings in hot, humid climates.
Florida Building Codes require that, where used, all bath and certain kitchen exhaust fans vent to the outside of the building envelope—not just into an attic or crawl space. The ducts used with these exhaust fans should be straight and exhaust to the outside; however, most fans are vented with flexible duct which creates resistance and on extremely long duct runs, reduces the exhaust flow.
Florida Building Code guidelines call for providing a minimum of 50 cubic feet per minute (cfm) intermittent air flow for bathrooms and 100 cfm intermittent for kitchens. Manufacturers should supply a cfm rating for any exhaust fan. Over-ventilation can cause unwanted air to be drawn into the home, leading to humidity and other problems. To allow for proper run times in bathroom exhausts, install a humidistat or timer. Also consider the level of fan noise, rated by sones. Choose a fan with a rating of 2.0 or less; top quality models are often below 0.5 sones. See the fact sheet Spot Ventilation for more information on spot ventilation and Ventilating Fans Key Product Criteria to see the requirements for ventilating fans to be ENERGY STAR® certified.
Improving spot ventilation will help control specific moisture problems but will not provide adequate ventilation for the entire building. Another type of ventilation—whole-house—can exhaust air from the kitchen, baths, living area and bedrooms. Whole-house ventilation systems are usually classified as exhaust ventilation if the mechanical system forces inside air out of the home, supply ventilation if the mechanical system forces outside air into the home, or balanced ventilation if the mechanical system forces equal quantities of air into and out of the home. Each system has advantages and disadvantages, must be correctly sized, and its presence included when determining the cooling/heating requirements of the house along with any filtering or dehumidification needs. See the fact sheet Whole-House Ventilation Systems and the Energy.gov web page Whole-House Ventilation for more information.
The Department of Energy recommends the practice of integrating mechanical ventilation into the heating, ventilation and air conditioning (HVAC) system. The Florida Building Code 5th Edition (2014), Energy Conservation requires Mechanical Ventilation in new construction using ASHRAE Standard 62.2 Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. The design air change per hour minimums for residential buildings shall be the maximum rates allowed for residential application with ASHRAE 62.2. The ventilation air, or make-up air, shall NOT be provided to conditioned spaces from attics, crawlspaces, attached enclosed garages or outdoor spaces adjacent to swimming pools or spas. Outdoor air intakes and exhausts shall have automatic or gravity dampers that close when the ventilation system is not working.
Whole-house ventilation systems can provide fresh, filtered, outside air in a controlled amount using the existing HVAC delivery system for even distribution and mixing. The most common systems involve:
- an exterior air intake NOT located in attics, crawlspaces, attached and enclosed garages and is located away from moisture sources like swimming pools or spas
- R-6 ductwork running to the return air side of the HVAC system,
- dampers that allow control of the volume of air intake,
- an electronic control to ensure that the HVAC fans operate often enough to draw in adequate fresh air as well as to treat and distribute it.
In summary, all homes need ventilation to remove stale interior air, pollutants, and excessive moisture. Researchers recommend whole-house mechanical ventilation systems for all buildings. The amount of ventilation required will depend on a number of variables including:
- the infiltration rate of the building envelope
- the size of the building and the ceiling height (building volume)
- the number of occupants and lifestyle,
- The overall design of the building.
More research is needed in the area of mechanical ventilation in buildings in hot-humid climates like Florida. We do know that buildings are being constructed with tighter building envelopes (less unwanted infiltration) and that air changes are needed to ensure safe and healthy indoor environments. The amount needed is provided in the ASHRAE standard and should be calculated by an HVAC professional.
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Certified Ventilating Fans
References and Resources
ASHRAE Standard 62.2 Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings.
Florida Building Code 5th Edition (2014). Energy Conservation. Accessed May 29, 2015.
Russell, M, Sherman, M., and Rudd, A. (2005). Review of Residential Ventilation Technologies. Ernest Orlando Lawrence Berkeley National Laboratory.
Sherman, M. (2014). Residential air. ASHRAE Journal 56.5, pp. 28-29.
Straube, J. (October-November 2012). Air leaks: How they waste energy and rot houses. Fine Home Building. The Taunton Press. pp. 45-49. Retrieved May 29, 2015.
U.S. Department of Energy. (November 18, 2014). Ventilation. Retrieved June 1, 2015.
U.S. Department of Energy. (May 30, 2012). Whole-house ventilation. Retrieved June 1, 2015.
U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE), Building Technologies Program. (December, 2002). Technology Fact Sheet: Spot Ventilation. Retrieved June 1, 2015.
U.S. Environmental Protection Agency. (July 3, 2012). Remodeling your home? Have you considered indoor air quality?. Retrieved June 1, 2015.
U.S. Environmental Protection Agency, ENERGY STAR. (n.d.). Ventilating fans key product criteria. Retrieved June 1, 2015.
Authors: Wendell A. Porter1 and Craig R. Miller2
Reviewer: Kathleen C. Ruppert2
First published May 2009; revised June 2015.
1 Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL
2 Program for Resource Efficient Communities, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL
 Straube, J. (2012). Air Leaks: How They Waste Energy and Rot Houses. Fine Home Building. The Taunton Press.
 Sherman, Max. Residential Air. ASHRAE Journal 56.5 (2014): 28-29.