HVI's Fresh Ideas Home Ventilation & Indoor Air Quality Guide Articles

How Much Ventilation Do I Need?

HVI's ventilation guidelines.

Ventilation products have differing capacities for moving air, making it important to ensure the product being selected has ample capacity for the application. HVI's Certified airflow rating appears on the product or on the HVI label displayed on each unit, in the manufacturer's literature describing the fan and in the HVI-Certified Products Directory.

The following guidelines will help you to determine the fan capacity required for your application.

Bathrooms – intermittent ventilation

HVI recommends the following intermittent ventilation rates for bathrooms:

• An enclosed toilet should have its own exhaust fan.
• Fans approved for installation in wet areas should be located over the shower or tub whenever possible.
• Bathroom doors need to have at least 3/4" clearance to the finished floor to allow proper entry of makeup air.
• A timer or other control that ensures ventilation continues for a minimum of 20 minutes after each use of the bathroom should be installed in each bathroom.
• For steam rooms, HVI recommends a separate fan located in the steam room that can be turned on after use to clear the heat and humidity.

Bathrooms – continuous ventilation

Continuous ventilation at a minimum rate of 20 CFM may be used in lieu of an intermittent 50 CFM exhaust fan.

Kitchen range hoods

Recommended kitchen range hood ventilation rates vary greatly depending on the type of cooking performed and the location of the range. Kitchen range hoods mounted above the range capture contaminants with their canopy shapes and exhaust them effectively with relatively low air volume. Downdraft kitchen exhausters require a higher volume and velocity of air to adequately capture contaminants. They are an alternative when canopy style hoods are not desired due to location of the cooking surface and kitchen aesthetics; however, their performance cannot equal that of hoods that capture the rising column of air above the cooking surface. When considering a downdraft kitchen exhauster, consult the range manufacturer's recommendations.

Kitchen range hoods equipped with multiple speed settings allow low-level, quiet ventilation for light cooking with the ability for higher rates when needed.

• For hoods located over islands multiply the rate by 1.5.
• For "professional-style" cooktops, HVI recommends following the cooktop manufacturer's advice to determine ventilation requirements.
• Inflated performance ratings are common for range hoods that are not HVI-Certified. Selecting range hoods with HVI-Certified Performance Ratings will ensure that ventilation expectations and building code requirements are met.

Note: Nonducted, recirculation kitchen range hoods provide no actual ventilation. For optimum kitchen air quality, always use kitchen range hoods, kitchen fans or downdraft kitchen exhausters that vent directly outside the home.

Heat and energy recovery ventilators

For continuous indoor air quality ventilation, a heat or energy recovery ventilator (HRV or ERV) should provide 0.35 air changes per hour. This calculation must consider the complete occupied volume of the house.

This rate can be more easily calculated by allowing 5 CFM per 100 square feet of floor area.

In addition to this minimum, continuous ventilation rate, HRVs and ERVs often have additional capacity to provide higher ventilation rates to meet the occupants' needs. Such needs could occur as a result of larger gatherings of people; smoking; a hobby or activity using paint, glues or other air-quality contaminants; or for any other reason that requires additional ventilation to improve the quality of indoor air.

Local codes may require different continuous ventilation rates – always check with your building officials to determine the specific requirements for your area.

Whole-house comfort ventilator

HVI recommends that a whole-house comfort ventilator have a minimum capacity to provide approximately one complete air change every two minutes within the occupied area. This flow rate will be great enough to create a perceptible "breeze" through the house. The required flow rate can be calculated by multiplying the gross square footage of the entire house (including unoccupied areas like closets) by 3. Be sure to include the "upstairs" area of multilevel homes. This formula assumes an eight-foot ceiling and takes into account typical unoccupied areas.

A smaller fan can effectively cool the mass of the house, relying on other fans such as "paddle-fans" to create the breeze needed to cool the people. This lower flow rate can be determined by multiplying the square footage by 0.4.

For proper cooling and efficient operation, any whole-house comfort ventilator requires adequate, unobstructed outlets in the attic through soffit vents, grilles or louvers.

To calculate the amount of attic exhaust area you will need, divide the fan's capacity in CFM by 750.

NOTE: Large fans can put the house under significant negative pressure. At least one window should be open before the fan is operated.

Powered attic ventilators – PAVs

Powered attic ventilators should provide at least 10 air changes per hour. Multiplying the total square footage of the attic by 0.7 will provide the rate required. For particularly dark or steep roofs, we recommend a slightly higher rating.

The air being exhausted must be replaced by outside air drawn through vents under the eaves in the soffit. To calculate the total minimum soffit vent intake area in square inches, divide the CFM of the PAV by 300 and multiply the result by 144.

A minimum of one square foot of inlet area for every 300 CFM of HVI-Certified fan capacity is required for proper fan operation.

• Use only soffit vents as fresh-air intakes for powered attic ventilators.
• Do not use gable vents because it's possible to draw rain and snow into the attic.

Static attic ventilation

In all seasons, the attic space is warmer than the outside air. This results in a constant upward air movement due to the buoyancy of warmer air. This characteristic of air may be used to help create a flow of air ventilating the attic. Placing exhaust vents in the roof, the gables or at the ridge of the roof and providing adequate air intake vents in the soffits best accomplishes this. HVI recommends vent selection and placement such that 60 percent of the vent net free area is provided by intake vents placed at the under-eave area and 40 percent of the vent net free area is provided by exhaust vents on the roof, at the ridge or high in the gable area.

To determine the static ventilator net free area (NFA) needed for your attic space, determine the area of the attic in square feet. Divide that area by 150 to determine the square feet of attic ventilation needed. Since manufacturers of static ventilation products rate their products in square inches of NFA, it will be necessary to multiply that value by 144 to determine the square inches required.

Static ventilation needs may be reduced if you have an uninterrupted ceiling vapor barrier installed with a rating of 0.1 perm or less. To calculate the ventilation required with such a vapor barrier, divide the square footage of the attic by 300 instead of 150.

Use these numbers to select, proportion, and place static ventilation products.