Ventalation for Homes

If too little outdoor air enters a home, pollutants can sometimes accumulate to levels that can pose health and comfort problems. Likewise, one approach to lowering the concentrations of indoor air pollutants in your home is to increase the amount of outdoor air coming in.

Outdoor air enters and leaves a house by: infiltration, natural ventilation, and mechanical ventilation. In a process known as infiltration, outdoor air flows into the house through openings, joints, and cracks in walls, floors, and ceilings, and around windows and doors (air may also move out of the house in this manner -- this is called exfiltration). In natural ventilation, air moves through opened windows and doors. Air movement associated with infiltration and natural ventilation is caused by air temperature differences between indoors and outdoors and by wind. Finally, there are a number of mechanical ventilation devices, from exhaust (vented outdoors) fans that intermittently remove air from a single room, such as bathrooms and the kitchen, to air handling systems that use fans and duct work to continuously remove indoor air and distribute filtered and conditioned outdoor air to strategic points throughout the house. The rate at which outdoor air replaces indoor air is described as the air exchange rate. When there is little infiltration, natural ventilation, or mechanical ventilation, the air exchange rate is low and pollutant levels can increase.

Unless they are built with means of mechanical ventilation, homes that are designed and constructed to minimize the amount of outdoor air that can "leak" into and out of the home may have higher pollutant levels than other homes. However, because some weather conditions can drastically reduce the amount of outdoor air that enters a home, pollutants can build up even in homes that are normally considered "leaky."

Most home heating and cooling systems, including forced air heating systems, do not mechanically bring fresh air into the house. Opening windows and doors, operating window or attic fans, when the weather permits, or running a window air-conditioner with the vent control open increases the ventilation rate. Local bathroom or kitchen fans that exhaust outdoors remove contaminants, including moisture, directly from the room where the fan is located and also increase the outdoor air ventilation rate.

Ideally, new homes will be built to minimize leakage to control energy loss, improve comfort, and minimize the transport of moisture and pollutants through the building shell. These homes should then also have mechanical ventilation to remove pollutants generated in the home and provide outdoor air in a controlled manner. Whether a mechanical ventilation system makes sense in your existing homes depends on the house, your existing heating, ventilation, and air-conditioning (HVAC) system, and the changes you have planned. You should discuss this with your HVAC contractor. A local Weatherization office, or building performance contractor, might also be able to help you with this decision or point you to local experts.

How much ventilation do I need?

The American Society of Heating, Refrigeration and Air-Conditioning Engineering (ASHRAE recommends (in its Standard 62-1999, "Ventilation for Acceptable Indoor Air Quality") that homes receive .35 air changes per hour, but not less than 15 cubic feet per minute (cfm) per person. A common rule of thumb is the 15 cfm multiplied by number of bedrooms in the house plus one. For example, a 3 bedroom house would require at least 60 cfm of outdoor air. [(3 bedrooms + 1) x 15 cfm = 60 cfm]. Kitchens should have an intermittent exhaust capacity of 100 cfm; bathrooms an intermittent capacity of 50 cfm. (ASHRAE also notes that "dwellings with tight enclosures may require supplemental ventilation supply for fuel-burning appliances, including fireplaces and mechanically exhausted appliances. See information on combustion appliances.)

For a basic overview of ventilation, including different types ventilation systems, try the following resources:

Homeowner's Guide to Ventilation. by the New York State Energy Research and Development Authority's (NYSERDA). In addition to a basic general discussion of ventilation, this document includes recommendations which are especially useful for homes in cold-climates.

ENERGY STAR® has finalized the ventilation fan specification. www.energystar.gov/index.cfm?c=vent_fans.pr_vent_fans

Home Ventilation Guide. by the Home Ventilating Institute.

Guide to Controlled Ventilation by the Oregon State University Extension Service at (503) 731-4104.

"Home Ventilation Options for Home Builders from Energy Source Builder.

For a detailed analysis of ventilation system options for new homes, see the Lawrence Berkeley National Laboratory report Recommended Ventilation Strategies for Energy-Efficient Production Homes. Copies of ASHRAE Standard 62 are available from ASHRAE.

Recommended Ventilation Strategies for Energy-Efficient Production Homes - Project Description:

Because low-infiltration homes often need mechanical ventilation to maintain indoor air quality, the US Environmental Protection Agency's (EPA) Energy Star® Homes program asked our research team in the Energy Analysis Department of Lawrence Berkeley National Laboratory (LBNL) to recommend the most appropriate mechanical ventilation strategies for new, single-family Energy Star® production (site-built tract) homes in four climates.

Mechanical ventilation is uncommon in US single-family homes because until recently it was thought that residential buildings were leaky enough to provide adequate air exchange. However, as homebuilding materials and practices have changed, building air leakage (infiltration) levels have decreased, and it has become obvious that mechanical ventilation is a residential design issue. This report does not address whether mechanical ventilation is necessary or what level of air exchange is appropriate. It does evaluate and compare the cost and effectiveness of nine residential ventilation systems and recommends the most suitable systems for new single-family production homes in cold, moderate, hot humid, and hot arid climates.

Our analysis is based on computer simulations of ventilation systems in prototypical homes and requires assumptions about climate, home characteristics, indoor pollutants, and occupant behavior that do not necessarily apply to every situation. Besides making climate-specific recommendations for mechanical ventilation of new homes, we also provide information that can enable contractors to determine the most suitable ventilation strategy for each set of circumstances.

We compared residential ventilation strategies in four climates according to three criteria: total annualized cost (the sum of annualized capital cost and annual operating cost), predominant indoor pressure induced by the ventilation system, and distribution of ventilation air within the home. The mechanical ventilation systems modeled deliver 0.35 air changes per hour continuously, regardless of actual infiltration or occupant window-opening behavior.

Based on the assumptions and analysis described in this report, we recommend independently ducted multi-port supply ventilation in all climates except cold because this strategy provides the safety and health benefits of positive indoor pressure as well as the ability to filter and dehumidify ventilation air. In cold climates, we recommend that multi-port supply ventilation be balanced by a single-port exhaust ventilation fan, and that builders offer balanced heat-recovery ventilation to buyers as an optional upgrade.

For builders who still install forced-air integrated supply ventilation, we emphasize that (1) forced-air ductwork should be airtight or within conditioned space, (2) systems should include a control that automatically operates the forced-air fan at least 20 minutes during each hour that the fan does not operate for heating or cooling, and (3) because of high operating costs and other disadvantages of operating a standard forced-air fan for ventilation, home buyers should be given the option of upgrading to an efficient, variable-speed forced-air fan with an integrated-control motor (ICM).

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