Chapter 9:                      Ventilation

This chapter discusses ventilation, fans, termination fittings, and ducts. Before installing a ventilation system, read Health and Safety for more information on controlling the sources of moisture and indoor air pollutants.

This chapter covers these types of ventilation.

•       Local or spot ventilation

•       Whole-house ventilation

•       Attic and crawl space ventilation

•       Ventilation for cooling

9.1   Pollutant Control

Controlling pollutants at the source is the highest priority for good indoor air quality. Mechanical ventilation can dilute pol­lutants. However, ventilation is ineffective against prolific sources of moisture and pollutants. Ask these questions to eval­uate pollution sources.

•       Do the occupants have symptoms of building-related ill­nesses? See also Health and Safety

•       Do sources of moisture like ground water, humidifiers, water leaks, or unvented space heaters cause indoor damp­ness, high relative humidity, or moisture damage? See Gas Range and Oven Safety

•       Are there combustion appliances, especially unvented ones, in the living space?

•       Do the occupants smoke?

•       Are there paints, cleaners, pesticides, gas or other chemi­cals stored in the home?

9.1.1   Pollution-Control Checklist

Survey the home for pollutants before air-sealing the home. Per­form the following pollutant control measures if needed.

✓     Repair roof and plumbing leaks.

✓     Install a ground moisture barrier over any bare soil in crawl spaces or dirt-floor basements.

✓     Verify that all dryer ducts and exhaust fans move their air to the outdoors.

✓     Verify that combustion-appliance vent systems operate properly.

✓     Verify that kitchen range hoods vent to the outdoors.

✓     Move paints, cleaning solvents, and other chemicals out of the conditioned space if possible.

The home’s occupants are often the source of many home pol­lutants, like candles and deodorizers. Educate the residents about minimizing pollutants in the homes.

9.2   ASHRAE Standard 62.2–2016 Ventilation

Most dwellings in North America currently rely on air leakage for ventilation. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) publishes ventila­tion standards for dwellings.

Their current standard, ASHRAE 62.2–2016, defines the roles and minimum requirements for mechanical and natural ventila­tion systems and the building envelope. The roles and require­ments intend to provide acceptable indoor air quality in residential buildings.

9.2.1   ASHRAE 62.2–2016 Components

If you air-seal dwellings during weatherization, you may need to install whole-dwelling mechanical ventilation systems under ASHRAE 62.2–2016, which has 3 components.

1.      Whole-dwelling ventilation.

2.      Local ventilation.

3.      Natural infiltration credit.

This ventilation standard allows for natural infiltration (air leak­age) to contribute toward the required whole-dwelling ventila­tion rate for single-family homes and horizontally attached multifamily buildings (but not vertically attached multifamily buildings.

9.2.2   Whole-Dwelling Ventilation Requirement

To comply with ASHRAE 62.2–2016, use either the formula or the table shown here to determine the whole-dwelling ventila­tion airflow requirement.

You can provide this mechanical airflow in 4 ways.

1.      A dedicated exhaust or supply fan running continuously or cycling by automatic control.

2.      A bathroom or kitchen exhaust fan running continu­ously or cycling by automatic control.

3.      A central air handler drawing filtered outdoor air into its return.

4.      A balanced ventilation system such as a heat-recovery ventilator (HRV) or energy-recovery-ventilator (ERV).

Room Pressure Imbalances

If any room in the building exceeds ±4 pascals pressure with ref­erence to the common area when all interior doors are closed and while the ventilation system is operating, take action to reduce the pressure. Install transfer grilles or jumper ducts as needed to reduce the room to common area pressure difference to less than ±4 pascals. SWS Detail: 6.6201.2 Primary Ventilation Air Flow between Rooms

Option 1: Finding the Airflow Requirement Using the Formula

If you want to install the minimum ventilation capacity, use these 3 steps to follow the formula option.

1.      Determine the floor area of the conditioned space of the home in square feet (A_floor).

2.      Determine the number of bedrooms (N_br).

3.      Insert these numbers in the formula shown next.

Option 2: Finding the Airflow Requirement Using the Table

Note: If you know the number of occupants, compare the number of occupants with the number of bedrooms plus 1 and use the higher of those two values.

You can also determine the required continuous fan airflow under ASHRAE 62.2–2016 using the table shown here.

Additional Ventilation Guidance

If the ventilation airflow requirement is less than 15 CFM, ASHRAE 62.2–2016 exempts the mechanical-ventilation requirement.

Residential Energy Dynamics provides a free online tool to help calculate ASHRAE 62.2–2016 ventilation rates.

Refer to the ASHRAE standard for more details, guidance, and exceptions that are beyond the scope of this field guide.

9.2.3   Local Exhaust Ventilation Requirement

There are two options for complying with the local ventilation requirements for kitchens and bathroom: demand-controlled exhaust or continuous exhaust.

1.      For demand-controlled exhaust specify a minimum of 100 CFM for the kitchen, and 50 CFM for each bath­room. An operable window reduces a bathroom’s venti­lation requirement to 30 CFM.

2.      For continuous exhaust specify a minimum of 20 CFM for each bathroom, and 5 ACH for the kitchen (based on volume).

Local Exhaust Deficit

If the existing kitchen or bathroom ventilation doesn’t meet the requirements stated here, you may adjust the whole-dwelling ventilation rate to compensate for the local airflow deficits. Fol­low these steps to calculate the local-ventilation deficit in CFM that must be added to the whole-dwelling ventilation rate.

1.      Determine the total local exhaust ventilation require­ment for all kitchens and bathrooms.

2.      Measure the delivered airflow of existing kitchen or bathroom exhaust fans using flow hood, flow grid, or other airflow measuring device. Subtract this amount from the total local exhaust ventilation requirement.

3.      If the local jurisdiction allows for operable windows to provide for local ventilation, subtract 20 CFM for each kitchen or bathroom that has an operable window.

The result of these steps is the local exhaust ventilation deficit in CFM. Add ¹/₄ of this deficit to the required whole-dwelling ven­tilation rate.

9.2.4   Infiltration Credit

ASHRAE 62.2–2016 allows for infiltration to contribute to the dwelling’s ventilation airflow. Infiltration can supply the entire whole-dwelling ventilation requirement for very leaky build­ings. For moderately leaky buildings, infiltration may supply some of the building’s ventilation.

Both single-family and multifamily buildings that are hori­zontally attached (low rise) are eligible for the infiltration credit.

You can determine the amount of the infiltration credit with a blower door test and weather data based on the building’s loca­tion. Calculating the infiltration credit without software is com­plicated. To simplify the calculations, use the RED Calc Free online tool and select “yes” for the “Use the infiltration credit” option.

9.3   Fan and Duct Specifications

This section covers fan and duct specifications for both local ventilation and whole-building ventilation. Duct sizing, materi­als, and installation determine whether airflow meets the design amount (CFM). Most existing exhaust fans and ventilation sys­tems don’t achieve their design airflow because of installation flaws.

9.3.1   Fan Specifications

Continuous ventilation is highly recommended because it sim­plifies design and control. Continuous ventilation also mini­mizes depressurization by allowing selection of the minimum-sized fan. Exhaust fans, installed as part of weatherization work, must vent to outdoors and should include the following features.

1.      Rated for continuous operation if the fan operates con­tinuously.

2.      A weatherproof termination fitting.

3.      Unless the fan operates continuously, the fan housing or termination fitting should house a backdraft damper.

4.      Noise rating and ventilation efficacy as specified in the table.

Fan Installation

Observe these procedures when installing ventilation fans.

✓     Install the fan or ventilator as close as possible to its termi­nation. New fan must be 1 sone or less.

✓     Orient the fan or ventilator housing so that the exit fittings face toward their termination fittings.

✓     Remove an integral backdraft damper if the fan operates continuously.

✓     Be careful not to inhibit the back draft damper operation by installing screws that interfere with the damper’s move­ment or by damaging the damper housing.

✓     Repair or replace the backdraft damper on an existing fan, if the damper doesn’t open and close freely.

✓     Install in-line fans and multi-port ventilators in remote areas such as attics and crawl spaces and connect the fans to intake grilles in rooms.

✓     Isolate in-line fans and multi-port ventilators from fram­ing to minimize noise.

✓     Measure the fan airflow to verify compliance with Stan­dard 62.2 - 2016.

9.3.2   Termination Fittings

Termination fittings for intake and exhaust ducts must exclude pests and water. Termination fittings must comply with these specifications.

✓     Termination fitting must direct water away from its open­ing.

✓     Flash or weather-seal termination fittings.

✓     The termination-fitting collar must be the same diameter as the exhaust or intake fitting on the fan.

✓     If the fan has no backdraft damper and the fan operates intermittently, install a termination fitting with a backdraft damper, to operate in the direction of airflow.

✓     Fasteners must not interfere with backdraft-damper oper­ation.

Locating Termination Fittings

Locate termination fittings using these specifications.

✓     At least 6 inches above grade

✓     At least 10 feet from another mechanical intake

✓     Above local snow or flood line

✓     At least 18 inches above a sloped asphalt based roof

✓     Never on a flat roof

✓     As required by local building authority

✓     Exhaust terminations must be at least 3 feet away from an operable window, an exterior door, or the property line

9.3.3   Duct Sizing

Fans often fail to deliver their rated airflow capacity. Bends, un-straight flex duct, dirty grilles, and backdraft dampers can reduce design airflow by 50% or more.

If you follow the sizing in this table, you may achieve the fan’s rated airflow for short duct runs with a maximum of two elbows.

For more detailed duct-sizing recommendations, seeASHRAE 62.2 Duct Sizing.

9.3.4   Ventilation Duct Materials and Installation

This sections covers SWS requirements and best practices for installing ventilation ducts connected to exhaust fans, ventila­tors, and air handlers.

See also Sealing Duct Leaks

Rigid Duct Installation

Observe these best practices for installing rigid ventilation ducts.

✓     Prefer rigid smooth metal pipe (30 gauge or thicker) or plastic pipe (Schedule 30 or thicker) for ventilation duct.

✓     Limit elbows to a maximum of two per duct run.

✓     Use three equally spaced sheet-metal screws to fasten sec­tions of round metal duct together.

✓     Join rigid duct sections so the edge of male end of a duct section isn’t opposing airflow.

✓     Follow manufacturer’s instructions to join other types of rigid ducts together.

✓     Seal all rigid-duct joints and seams with mastic, mastic and webbing, or metal tape labeled UL181B or UL181B-M. See Sealing Duct Leaks

✓     Support metal ducts with at least ¹/₂-inch, 18 gage strap­ping or at least 12-gage galvanized wire, not less than 10 feet apart.

✓     Insulate metal ducts to R-8 to prevent condensation if they travel through unconditioned spaces. See Duct Insulation

✓     Fasten PVC exhaust ducts together with approved PVC cement.

Flexible Duct Installation

Observe these best practices for installing flexible ducts.

✓     Stretch flex duct and support it every 4 feet with a 1.5-inch duct support.

✓     Use tool-tensioned plastic tie bands to join both the inner liner and the outer liner of the flex duct to the rigid duct or a fitting on the fan or termination fitting.

✓     Install a screw to secure the flex duct and tie band to the metal duct between the tie band and the end of the metal duct.

✓     Flexible air duct material must meet UL 181, NFPA 90A/90B, International Mechanical Code, or the Uniform Mechanical Code.

✓     Seal all joints, connections, seams with mastic, mastic and webbing, labeled UL181B or UL181B-M,

9.4   Commissioning Ventilation Systems

Commission new, retrofitted and serviced whole-dwelling ven­tilation systems to verify that the systems function according to design and the ASHRAE 62.2-2016 standard.

✓     Verify that all the required ventilation-system components are present and connected correctly.

✓     Use airflow and pressure manometers that are appropriate for the operating range and that render accuracy of ± 10%.

✓     Measure total airflow, room airflows, and total static pres­sure to verify that these measurements are within design specifications.

✓     Adjust fan speed, dampers, and registers as necessary to bring airflow into conformance with design specifications.

✓     Verify that all sensors and controls function correctly.

9.5   Whole-Building Ventilation Systems

This section discusses three options for design of whole-build­ing ventilation systems.

✓     Exhaust ventilation

✓     Supply ventilation

✓     Balanced ventilation

See Fan and Duct Specifications

9.5.1   Exhaust Ventilation

Exhaust ventilation systems employ an exhaust fan to remove indoor air, which is replaced by infiltrating outdoor air.

Installing a two-speed bathroom fan is a common ventilation strategy. The new fan runs continuously on low speed for whole-building ventilation. A built-in occupancy sensor switches the fan automatically to a high speed to remove mois­ture and odors from the bathroom quickly.

A remote fan that exhausts air from several rooms through ducts (4-to-6 inch diameter) may provide better ventilation for larger more complex homes, compared to a single-point exhaust fan.

✓     Fans must conform to Fan Specifications.

✓     Ducts must conform to Ventilation Duct Materials and Installation.

✓     Termination fittings must conform toTermination Fittings.

Exhaust ventilation systems create a negative house pressure, drawing outdoor air in through leaks in the shell. This keeps moist indoor air from traveling through building cavities, which would occur with a positive house pressure. The negative house pressure reduces the likelihood of moisture accumulation in building cavities during the winter months in cold climates. In hot and humid climates, however, this depressurization can draw moist outdoor air into the home through building cavities. Therefore we recommend supply ventilation for warm humid climates rather than exhaust ventilation.

9.5.2   Supply Ventilation

Supply ventilation, using the home’s air handler, is never oper­ated continuously as with exhaust ventilation because the fur­nace or heat-pump blower is too large and would over-ventilate the home and waste electrical energy. Supply ventilation may not be appropriate for tight homes in very cold climates because supply ventilation can push moist indoor air through exterior walls, where moisture can condense on cold surfaces.

Motorized Outdoor-Air Damper

A motorized damper that opens when the air-handler blower operates must control outdoor-air supply. The furnace/air con­ditioner heats or cools the outdoor air as necessary before deliv­ering it to the living spaces.

The damper control estimates how much ventilation air is needed. The damper closes after the required amount of ventila­tion air has entered during heating or cooling. The control also activates the damper and the blower for additional ventilation air as needed without heating or cooling the air, during mild weather.

Supply-Ventilation System Requirements

Supply ventilation typically uses the furnace or heat pump as a ventilator. A 5-to-10 inch diameter duct connects the furnace’s main return duct to a termination outdoors.

✓     The existing duct system must leak less than 10% of the air handler flow when measured at 25 Pa. WRT outside.

✓     The outdoor air must flow through a MERV 6 or better air filter before flowing through heating and cooling equip­ment.

✓     Ducts must conform to Ventilation Duct Materials and Installation.

✓     Termination fittings must comply withTermination Fittings.

9.5.3   Balanced Ventilation

Balanced ventilation systems exhaust stale air and provide fresh air through a ducted distribution system. Of the three ventila­tion systems discussed here, balanced systems do the best job of controlling pollutants in the home.

Balanced systems move equal amounts of air into and out of the home. Most balanced systems incorporate heat-recovery venti­lators or energy-recovery ventilators that reclaim heat and mois­ture from the exhaust air stream.

Balanced ventilation systems can improve the air quality and comfort of a home, but they require a high standard of care. Testing and commissioning is vital during both the initial instal­lation and periodic service calls.

Heat-Recovery and Energy-Recovery Ventilators

The difference between heat-recovery ventilators (HRVs) and energy-recovery ventilators (ERVs) is that HRVs only transfer heat, while ERVs transfer both sensible heat and latent heat (moisture) between air streams.

HRVs are often installed as balanced whole-building ventilation systems. The HRV core is an air-to-air heat exchanger in which the supply and exhaust air streams pass one another and exchange heat without mixing.

9.5.4   Adaptive Ventilation

The home’s residents can maintain good indoor air quality by using spot ventilation together with opening windows and doors. Depending on climate and season, residents can control natural ventilation to provide clean air, comfort, and energy effi­ciency.

✓     Choose windows and screen doors in strategic locations to ventilate using prevailing winds.

✓     Make sure that windows and screen doors, chosen for ven­tilation, open and close and have effective insect screens.

✓     Open windows to provide make-up air when an exhaust fan or the clothes dryer is operating.

✓     Understand that dust and pollen may enter through win­dows or screen doors and consider the consequences.

9.6   Multifamily Ventilation

Exhaust-only ventilation is the most common ventilation strat­egy for multifamily buildings. Vertical chases surround the ven­tilation ducts and installers cut holes in the chase for the ventilation registers.

Forced-air HVAC systems also provide ventilation to multifam­ily buildings. The HVAC system delivers a portion of heated and cooled air as outdoor ventilation air.

See the following sections for more information on multifamily ventilation.

•       ASHRAE Standard 62.2–2016 Ventilation

•       Whole-Building Ventilation Systems

•       Air Filtration for Indoor Air Quality

9.7   Air Filtration for Indoor Air Quality

Efficient air filters can reduce particle pollution in homes where particles are a air-quality problem. Ventilation isn’t effective at removing small particles. Suggest that customers run their air handlers during heavy air pollution in cities, proximity to dirt roads, seasonal forest fires, and other particle-generating events.

You can run an air-handler fan using the “fan only” setting on a thermostat. You can even program the fan to run for a period each day using a programmable thermostat.

The best places for filters are in forced-air HVAC systems or in balanced ventilation systems. Room air cleaners can also be effective particle removers if there is natural circulation among rooms.

Air filters affect the airflow and energy consumption of forced air HVAC systems and balanced ventilation systems. Before choosing the type of air filter and deciding to use the filter to remove particles, consider the filter’s MERV rating and a home’s need for particle removal.

9.7.1   Installing Filters for Outdoor Air

Provide filters for outdoor air supplied through ducted ventila­tion systems and observe these specifications.

✓     Select a filter with a MERV rating of 6 or greater.

✓     The filter’s pressure drop must not result in insufficient ventilation airflow.

✓     Install the filter on the inlet side of the fan.

✓     Make the filter accessible for changing or cleaning.

✓     Instruct the occupants or building manager on how and when to service the filter.

9.8   Attic Ventilation

Attic ventilation has the following functions.

•       To keep the attic insulation and the attic’s other building materials dry by circulating dry outdoor air through the attic.

•       To prevent ice dams in cold weather by preventing snow melt by keeping the roof deck cold during the winter.

•       To remove solar heat from the attic during the summer.

9.8.1   Attic Ventilation as a Solution for Moisture Problems

The best way to keep attic insulation dry is to air-seal the attic floor to block moist air from entering the attic. Adding attic vents may help to solve certain attic moisture problems.

•       Seasonal moisture deposition removed by vents.

•       Ice damming in areas that currently lack high and low vents.

Adding attic vents won’t solve these attic moisture problems.

•       Moisture deposited by air leaks between the living space and the attic.

•       Rain driven through attic vents.

•       Roof leaks that dampen building materials beyond the capacity of the vents to dry.

9.8.2   When to Install Attic Ventilation

Install more attic ventilation only if you believe that the home needs one of the attic-ventilation functions listed above. Con­sider the following discussion points.

•       Don’t increase attic ventilation without first sealing attic air leaks and testing the attic air barrier for adequate airtight­ness.

•       Avoid cutting new vents through the roof to avoid the pos­sibility of roof leaks.

•       Attic ventilation may not provide a useful function in some climates, such as persistently damp climates or windy, rainy climates.

Important note: An outright exception to ventilating attics is offered by the IRC if a code official determines that “atmo­spheric or climatic conditions” aren’t favorable to attic ventila­tion.

9.8.3   Attic Ventilation Requirements

Always vent exhaust fans directly to outdoors (through a gable or wall) and never into a ventilated attic.

Net free area is the area of the vent minus the vent’s solid obstructions such as screens and louvers. The net free area is typically 50% to 70% of the gross vent area.

The IRC and SWS state these requirements for attic ventilation.

✓     Provide a minimum ratio of one square foot of net free vent area to 150 square feet of attic area.

✓     The minimum net free ventilation area must be 1/300 of the vented space provided, if between 40% to 50% of the required ventilating area is provided by ventilation in the upper portion of the attic or rafter space.

✓     Vents must have screens, with ¹/₄-to-¹/₁₆ inch or less open­ing, to prevent the entry of pests and to reduce the entry of snow and rain.

✓     Vertical vents must have louvers to deflect rain.

✓     Install vent chutes or baffles to prevent blown insulation from entering the space between soffit vents and the attic. The baffles should allow the maximum amount of insula­tion to be installed over top plates without restricting ven­tilation paths. Vent chutes or baffles also help prevent the wind-washing of insulation caused by cold or hot air entering soffit vents. They should extend upwardly along the rafter to at least 4 inches above the finished insulation level.

✓     Don’t use powered ventilators to increase attic ventilation because of their energy consumption and doubtful effec­tiveness.

High and Low Vents

A combination of high and low vents is the best way to move ventilating air through the attic. Soffit vents and ridge vents are an ideal combination for high-low attic ventilation. However, gable vents and roof vents, located high or low, also create acceptable ventilation.

9.8.4   Power Ventilators

Power ventilators have limited value ventilating attics for air-conditioning energy savings or moisture mitigation.

•       Power ventilators typically run longer than necessary.

•       Power ventilators often consume more electricity than they save in reduced air conditioning.

•       Power ventilators can pull conditioned air out through ceil­ing air leaks, counteracting their ventilating or cooling ben­efit.

9.8.5   Unventilated Attics

According to the IRC, new attics may be unventilated if the two conditions listed here are both met.

1.      There is no vapor barrier installed in the ceiling.

2.      The roof assembly is insulated with an air-impermeable insulation, such as high-density sprayed polyurethane, to the bottom of the roof sheathing or foam board on the top side of the roof sheathing.

9.9   Crawl Space Ventilation

Before taking steps to improve crawl-space ventilation, comply with these requirements.

✓     The crawl space should have an access hatch or door that is adequate for a worker or resident to enter or exit.

✓     Correct grading, drainage, and gutter-and-downspout problems related to crawl-space moisture problems.

✓     Install a ground moisture barrier as specified in Crawl Space Moisture and Safety Issues. With this requirement met, foundation ventilation must be at least 1 square foot of vent for every 1500 square feet of crawl-space area.

✓     If necessary, install a sump pump with its discharge drained to daylight.

9.9.1   Naturally Ventilated Crawl Spaces

A ground moisture barrier that covers 100% of the exposed crawl space floor will be installed.

9.9.2   Power-Ventilated Crawl Spaces

The IRC allows you to seal the crawl-space vents completely when you insulate the foundation walls and power-ventilate the crawl space. These three specifications apply to power-venti­lated or conditioned crawl spaces.

1.      Remove moisture sources like standing water and install a seam-sealed and edge-sealed ground-moisture bar­rier, before sealing the foundation vents.

2.      The IRC requires 1 CFM per 50 square feet of crawl space floor area in continuous powered exhaust ventila­tion. The IRC requires openings from the crawl space into the home so that make-up air comes from the liv­ing space. Some installers depend on floor air leakage to provide make-up air instead of intentional openings between the home and crawl space.

3.      An acceptable alternative to option 2 is controlling the exhaust fan with a dehumidistat (moisture sensitive control). Such an exhaust fan typically operates contin­uously until the crawl space is dry and then intermit­tently after that. This option isn’t IRC-approved.

9.9.3   Conditioned Crawl Spaces

The IRC requires 1 CFM per 50 square feet of crawl space floor area in conditioned supply air from a forced-air system. The IRC requires openings from the crawl space into the home for this option. The conditioned option requires code-compliant level of foundation insulation appropriate for the home’s cli­mate.

The conditioned crawl space, although allowed by the IRC, may be an ineffective moisture-and-energy solution for existing crawl spaces, especially in dry locations. In humid climates with damp crawl spaces, the conditioned crawl space has succeeded in reducing moisture problems and even energy costs, when combined with an airtight ground moisture barriers and foun­dation insulation.

9.10   Ventilation for Cooling

Ventilation cooled homes for centuries before air conditioning was invented. Ventilation is still an effective method for clients who can’t afford air conditioning.Ventilate with fans during the coolest parts of the day and night, and close the windows during the hottest periods.

Some fans come with a tight-sealing winter cover. If the fan doesn’t have such a cover, or if the attic access doesn’t allow you to cover the fan easily, then you can fabricate a cover for the grille on the ceiling. A seasonal cover, held in place with rotating clips or spring clips and sealed with foam tape, works well. If the clients switch between air conditioning and cooling with a whole-house fan as the summer weather changes, build a tightly-sealed, hinged door for the fan opening that is easy to open and close when they switch cooling methods.

9.11   SWS Alignment