Chapter 9:                      Ventilation

This chapter discusses ventilation, fans, termination fittings, ducts, and passive vents.

This chapter covers these 4 types of ventilation.

1.      Local ventilation

2.      Whole-dwelling ventilation

3.      Attic and crawl space ventilation

4.      Ventilation for cooling

Before installing a mechanical whole-dwelling ventilation sys­tem, read “Health and Safety” on page 21 for more information on controlling the sources of moisture and indoor air pollutants.

9.1   Pollutant Control

Controlling pollutants at the source is the highest priority for good indoor air quality. Mechanical ventilation dilutes 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 dwelling-related ill­nesses? See also "Health and Safety" on page 21.

•       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” on page 28.

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

•       Do the occupants smoke?

•       Do the occupants use plug-in air freshers, scented candles or other chemical fragrances?

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

•       Deferral must be considered when there are pollutant sources that cannot be controlled such as moisture produc­tion or excessive animals.

9.1.1   Pollution-Control Checklist

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

ü       Repair roof and plumbing leaks.

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

ü       Verify that dryer ducts and exhaust fans move exhaust air to the outdoors.

ü       Verify that combustion-appliance vent systems operate properly.

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

ü       Air-seal between attached garages and a dwelling’s condi­tioned areas.

ü       Weatherization cannot be performed on a dwelling with an un-vented combustion heater as its primary heat source.

The dwelling’s occupants are often the source of many home pollutants, such as candles, deodorizers, and pesticides. Educate the residents about minimizing such pollutants in their dwell­ings.

Note: Ventilation specialists now use the term “dwelling” to describe either a single-family home or multifamily living unit, ASHRAE 62.2–2016 now applies to both single-family and multifamily dwellings.

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 the formula shown here to determine the whole-dwelling ventilation airflow requirement.

Note: If the ventilation airflow requirement is less than 15 CFM, ASHRAE 62.2–2016 exempts the mechanical-ventila­tion 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 ±3 pascals pressure with ref­erence to the outdoors when all interior doors are closed and while the occupant ventilation system is operating, take action to reduce the pressure. Install transfer grilles or jumper ducts as needed to reduce the room to outdoors pressure difference to less than ±3 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 below.

Option 2: Use the ASHRAE 62.2 Ventilation Sizing calculator located at Intelligentweatherization.org (Retro App A) to determine the continuous ventilation CFM requirement.

http://intelligentweatherization.org/weatherization-forms/

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.

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 above, you may adjust the whole-dwelling ventilation rate to compensate for the local airflow deficits. Uti­lize the Intelligent Weatherization ASHRAE Calculator or the RED Calc Free to determine ventilation needs or 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 Intelligent Weatherization ASHRAE Calculator or 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-dwelling ventilation. Duct sizing, materi­als, and installation determine whether airflow meets the design airflow rate (CFM). Most existing exhaust fans and ventilation systems don’t achieve their design airflow because of installation flaws.

9.3.1   Fan Specifications

We highly recommend continuous ventilation. Continuous ven­tilation simplifies design and control. Continuous ventilation also minimizes depressurization by allowing selection of the minimum-sized fan.

General Fan and Ventilator Specifications

Exhaust fans, installed as part of weatherization work, must vent to outdoors and should meet all of the requirements outlined in SWS Section 6.

•       Installed fans must meet SWS compliance directives along with local code and manufacturers installation require­ments.

Fan Installation

Observe these procedures when installing ventilation fans.

ü       Install the fan or ventilator as close as possible to its termi­nation.

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

ü       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.

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

ü       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.

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

ü       Measure the fan airflow to verify compliance with ASHRAE 62.2–2016.

ü       Install a MERV 6 filter in all ducted supply ventilation sys­tems. See “Air Filter Effectiveness” on page 339.

9.3.2   Termination Fittings

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

ü       The hole in the building shell shall be no greater than 1/4” larger than the size of the hole required to accommodate the termination fitting.

ü       Termination fitting must direct water away from its open­ing.

ü       Flash or weather-seal termination fittings.

ü       Termination fittings must have insect screens over the openings. Insect screen openings must be between 1/4” and 1/2” in size.

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

ü       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.

ü       Fasteners must not interfere with backdraft-damper oper­ation.

Locating Termination Fittings

Locate termination fittings using these specifications.

ü       At least 10 feet from another fan termination

ü       Above local snow or flood line

ü       Never on a flat roof

ü       As required by local building authority

ü       Exhaust terminations must be at least 3 feet away from an operable opening to houses and the property line.

9.3.3   Duct Sizing

Fans often fail to deliver their rated airflow capacity. Bends, un-straight flex duct, dirty grills, 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, see “ASHRAE 62.2 Duct Sizing” on page 530.

9.3.4   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" on page 349.

Rigid Duct Installation

Observe these best practices for installing rigid ventilation ducts.

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

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

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

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

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

ü       Seal all rigid-duct joints and seams with mastic, mastic and webbing, or metal tape, labeled UL181B or UL181B-M. See “Sealing Duct Leaks” on page 349.

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

ü       Insulate metal ducts to R-8 to prevent condensation if they travel through unconditioned spaces. See “Duct Insulation” on page 354.

ü       Fasten PVC exhaust ducts together with approved PVC cement.

Flexible Duct Installation

Observe these best practices for installing flexible ducts.

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

ü       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.

ü       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.

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

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 standard.

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

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

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

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

ü       Verify that all sensors and controls function correctly.

ü       Provide required compliance documentation with ASHRAE 62.2 2016. It must be included in client file.

9.5   Whole-Dwelling Ventilation Systems

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

ü       Exhaust ventilation

ü       Supply ventilation

ü       Balanced ventilation

See “Fan and Duct Specifications” on page 399.

We begin by discussing ducts for all types of ventilation systems.

9.5.1   Exhaust Ventilation

Exhaust ventilation systems employ an exhaust fan to remove indoor air, which infiltrating outdoor air then replaces.

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 dwellings, compared to a single-point exhaust fan.

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 building pressure. The negative building 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.

Single-Family Exhaust-Ventilation-System Specifications

ü       Fans must conform to “Fan Specifications” on page 400.

ü       Ducts must conform to “Duct Materials and Installation” on page 405.

ü       Termination fittings must conform to“Termination Fit­tings” on page 402.

ü       Use passive intake vents only if you can air seal the build­ing to 1 ACH₅₀ or less. Otherwise the ventilation fans may draw their makeup air from air leaks rather than the pas­sive vents.

Multi-room Exhaust-Ventilation-System Specifications

ü       Evaluate the seal between the roof-mounted ventilator and the its ducts and the roof.

ü       Evaluate ventilation-duct chases for air leakage.

ü       Install backdraft dampers on intermittently operating sys­tems.

ü       Measure airflow through registers to ensure a correct air­flow rate. Adjust register size if necessary to decrease or increase ventilation airflow.

ü       Adjust ventilator airflow if building ventilation airflow is either excessive or insufficient.

ü       Insulate ducts outside the thermal boundary to R-8.

ü       Fire dampers must be accessible for inspection and testing.

ü       Educate occupants or building manager on maintenance procedures.

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 dwellings 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.

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

ü       The outdoor air must flow through a MERV 6 or better air filter before flowing through heating and cooling equip­ment. See “Air Filter Effectiveness” on page 339.

ü       Ducts must conform to “Duct Materials and Installation” on page 405.

ü       Termination fittings must comply with“Termination Fit­tings” on page 402.

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 are 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 transfer heat only, 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   Rooftop-Unit (RTUs) Economizer Ventilation

Many buildings, particularly those with RTUs use economizers as their ventilation system. Economizers don’t normally have heat recovery or energy recovery function, so ventilating with an economizer can have an energy penalty compared to a heat recovery ventilators. See “Economizers” on page 333.

Mild climates are ideal for ventilating with an economizer. When buildings use an economizer for ventilation, its dampers are open a small amount while the HVAC system is heating, cooling, or operating on the fan-only option. The fan-only option is for when you need ventilation but not heating or cool­ing.

To run optimally, the economizer requires a programmable thermostat that tracks the amount of ventilation air delivered during the free cooling mode and during the ventilation mode. See “RTU Maintenance and Improvement” on page 334.

For information on cooling with an economizer, see “Economiz­ers” on page 333.

9.6   Garage Exhaust Ventilation

Attached garages, particularly garages beneath living areas, may require exhaust ventilation to prevent indoor CO contamina­tion. Only consider garage exhaust ventilation when all attempts to air-seal the garage from the house have been insufficient.

•       Use pressure diagnostics to assist in determining the level of connection from the garage to the house and in making the determination for the need to add garage ventilation.

•       Provide requests for garage ventilation to IHCDA and they will evaluate the request on a case-by-case basis.

•       For single family homes, install 100-CFM-capacity exhaust fans on the exterior wall or surface mounted inside the garage.

•       Make sure the fan doesn’t cause an unacceptable depressur­ization in a nearby CAZ. Provide pressure-relief if neces­sary.

•       For larger multifamily garages, do thorough air sealing and then provide exhaust ventilation necessary to reduce indoor CO to a negligible level.

•       Operate the fan using an occupancy sensor with a 15-min­ute runtime or continuous for larger garages as necessary.

See also "Garages Underneath Living Areas" on page 199.

9.7   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” on page 395

•       “Whole-Dwelling Ventilation Systems” on page 407

•       “Air Filtration for Indoor Air Quality” on page 416

•       “Rooftop-Unit (RTUs) Economizer Ventilation” on page 413

•       “Air Filtration for Air Handlers” on page 338

9.8   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 clients 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. For more information on MERV rat­ings, see “Air Filter Effectiveness” on page 339.

9.8.1   Installing Filters for Outdoor Air

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

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

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

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

ü       Make the filter accessible for changing or cleaning.

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

9.8.2   Adaptive Ventilation

The dwelling’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.

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

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

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

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

9.9   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 and keeping the roof deck cold during the winter.

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

9.9.1   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 indoor air from entering the attic. Adding attic vents may help to solve certain attic moisture problems.

•       Seasonal moisture deposition.

•       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.

9.9.2   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: The IRC offers an outright exception to venti­lating attics if a code official determines that “atmospheric or climatic conditions” aren’t favorable to attic ventilation.

9.9.3   Attic Ventilation Requirements

Always vent exhaust fans directly to outdoors (through a soffit, 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.

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

ü       The IRC requires only one square foot of net free area per 300 square feet of attic area for cool-climate ceilings, that have an interior vapor barrier, or well distributed ventila­tion (high and low), or with thorough air-sealing of the ceiling.

ü       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.

ü       Vertical vents must have louvers to deflect rain.

ü       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.

ü       Don’t install power 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.

9.9.4   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.

•       Existing power attic ventilators will no longer be needed if the attic floor is well sealed and insulation is sufficient. Try to convince the client to allow Wx to disable the fan to save energy.

9.9.5   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.

9.10   Crawl Space Ventilation

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

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

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

ü       Install a ground moisture barrier as specified in “Crawl Space Moisture and Safety Issues” on page 38.

ü       Install a sump pump with its discharge drained to daylight or a French drain to drain persistent standing water.

9.10.1   Naturally Ventilated Crawl Spaces

When insulating the floor, the crawl space is usually ventilated naturally through passive vent openings in the foundation wall. A ground moisture barrier protects the floor insulation and other building materials from moisture. The vent openings can remove small amounts of moisture from the crawl space. Two specifications apply to ventilated crawl spaces.

1.      A crawl-space with a ground-moisture barrier may have vent openings equal to 1 square foot of vent area to 300 square feet of crawl-space floor area. A minimum of two vents should be installed on opposite corners of the crawl space.

2.      In a dry crawl space with a ground-moisture barrier, ventilation openings may be minimized to one square foot of net free ventilation area for every 1500 square feet of crawl-space floor area, according to the 2012 IRC.

9.10.2   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 barrier and founda­tion insulation.

9.11   Ventilation for Cooling

Ventilation cooled dwellings for centuries before the invention of air conditioning. 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.

9.11.1   Whole-House Fans

Whole-house fans range in diameter from 24 inches to 42 inches, with capacities ranging from 3,000 to 10,000 cubic feet per minute (cfm). The capacity of the fan in cfm is rated for two different conditions: 1) free air; and 2) air constricted by 1 inch of static pressure. The second condition is closer to the actual operating conditions of the fan in a home, and the cfm rating at 1 inch of static pressure may still be 10 to 30 percent higher than the actual volume of air moved by the installed whole-house fan. This means you should probably install a fan with a greater capacity than the sizing recommendations that follow.

Whole-house fans require 2 to 4 times the normal area of attic vent openings. Install a minimum of 1 square foot of net free area for every 750 cfm of fan capacity. However, more vent area is better for optimal whole-house-fan performance because the extra vent area increases airflow.

To estimate the suitable size of a whole-house fan in cubic feet per minute,

1.      First determine the volume of the building in cubic feet. Multiply the square footage of the floor area in your building that you want to cool by the height from floor to ceiling.

2.      Take that volume and multiply by 15 to 40 air changes per hour, depending on how much ventilation you want.

3.      Then, divide by 60 minutes to get cubic feet per minute of capacity for the whole-house fan.

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.

9.11.2   Window Fans

Window fans are best used in windows facing the prevailing wind or away from it to provide cross ventilation. Window fans can augment any breeze or create a breeze when the air is still. If the wind direction changes in your area, use reversible type win­dow fans so you can either pull air into the home or push air out, depending on which way the wind is blowing. Experiment with positioning the fans in different windows to see which arrange­ment works best.

9.11.3   Air Circulation

Air circulating fans are very effective cooling energy savers. Air circulating fans may allow a 4 degree rise in the thermostat set­ting with no decrease in comfort.

Use circulating fans with air conditioners, evaporative coolers, whole-house fans, or by themselves. Circulating fans save cool­ing energy by increasing air movement over the skin to help occupants feel cooler.

Ceiling fans and various types of portable fans provide more comfort at less cost than any other electrically powered cooling strategy. Options include: small personal fans that sit on table­tops, or heavier units that sit on the floor or on metal stands with wheels.

Ceiling fans produce high air speeds with less noise than oscil­lating fans or box fans. High quality ceiling fans are generally more effective and quieter than cheaper ones. Ceiling fans are a key element to providing low-cost comfort to a home.

9.12   SWS Alignment