Figure 6-4x: (C) J Wiley, S Bliss Bathroom Ventilation Design Recommendations
     

  • BATHROOM VENTILATION DESIGN - CONTENTS: Bathroom ventilation design specifications, product recommendations, & installation guidelines. Ventilation Rate Required for Small Bathrooms. Ventilation Rate Required for Large Bathrooms. Bath Fan Noise Issues. Recommendations for Bathroom Fan Location. Bath Fan Ductwork Specifications. How to solve or prevent condensation in bath vent ductwork
  • BATHROOM VENTILATION - home
  • POST a QUESTION or READ FAQs about bathroom exhaust venting for control of moisture & odors
  • REFERENCES

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This article discusses the proper design, location, control, and ducting of bathroom vent fans and bath vent duct work. We also discuss the issue of bath vent fan noise, proper location of bath vent controls, and bath vent duct condensation problems.

This article series discusses current best design practices for kitchens and bathrooms, including layout, clearances, work space, and accessible kitchen and bathroom layout, clearances, turning space, grab bars, controls, etc. We include advice on choosing and installing kitchen countertops, cabinets, and kitchen or bathroom flooring, sinks, and other plumbing fixtures and fixture controls such as faucets. A list of kitchen and bath product manufactures and sources is included.

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Bathroom Ventilation Design Recommendations

Bath vent over shower (C) D FriedmanBathrooms produce moisture, odors, and VOCs from aerosols and various personal hygiene products. Effective spot ventilation in these areas is critical for maintaining healthy levels of indoor humidity levels and an overall healthy indoor environment.

Our photo (above left) illustrates location of a bathroom exhaust vent over the shower stall - placed to maximize the effectiveness of exhaust of moisture during shower use. We used a GFCI circuit to assure safety for anyone changing bulbs in this fixture. Photo courtesy Galow Homes.

As noted in Chapter 6 of Best Practices Guide to Residential Construction:

Kitchens and bathrooms are key sources of indoor moisture and other pollutants. Kitchens produce particulates and atomized grease from cooking, and with a gas range, they also produce combustion by-products including nitrogen dioxide and carbon monoxide.

Bathrooms produce moisture, odors, and VOCs from aerosols and various personal hygiene products. Effective spot ventilation in these areas is critical for maintaining healthy levels of indoor humidity levels and an overall healthy indoor environment (see INDOOR AIR QUALITY IMPROVEMENT GUIDE).

Bathroom ventilation should be powerful enough to remove excess moisture before it has time to condense on cold walls and windows, potentially damaging finishes, or escape into wall or ceiling cavities, where it may lead to decay or peeling paint.

Ventilation Rate Required for Bathrooms

The minimum ventilation rate for bathrooms required by the 2003 International Residential Code (IRC) is 50 cfm intermittent or 20 cfm continuous (if part of a whole-house ventilation system). While this may be adequate for a small bath, the guidelines of the Home Ventilating Institute (HVI) are more suitable for larger rooms:

  • Small baths: For bathrooms up to 100 sq ft, HVI recommends using an exhaust fan that provides 1 cfm per square foot of floor space. This will provide approximately 8 air changes per hour. So, for example, an 8x10-foot bathroom would require 80 cfm of ventilation.
  • Large baths: For bathrooms over 100 sq ft, the HVI recommends a ventilation rate based on the number and type of fixtures as shown in Table 6-12. For example, a bathroom with a toilet, shower, and jetted tub would require 200 cfm (50 + 50 + 100) of ventilation either in a single fan or multiple fans placed over the fixtures being vented.

Figure 6-4x: (C) J Wiley, S Bliss

 

Table 6-12

[Click any image or table to see an enlarged version with additional detail, commentary & source citation.]

 

Bath Fan Noise Issues

The biggest homeowner objection to bath fans, their noise, has been addressed with the introduction of whisper-quiet bath fans from a number of manufacturers. Choose the quietest fan for the job, preferably one rated 1.5 sones or less (one sone roughly equals the sound of a quiet refrigerator motor).

Recommendations for Bathroom Fan Location

Bath vent over shower (C) D FriedmanFor optimal ventilation, locate the exhaust grilles near the source to be ventilated, typically over the tub or shower. In smaller baths, a single, central exhaust point is usually adequate, while in larger baths, multiple smaller fans (or a remote fan with separate pickups) will be more effective.

Bathrooms with enclosed toilet areas or steam showers should have separate exhaust grilles in those areas. Since all exhaust fans require makeup air, the bathroom door needs to be undercut to provide makeup air when bathroom windows are closed.

Our photo (left) illustrates location of a bathroom exhaust vent over the shower stall - placed to maximize the effectiveness of exhaust of moisture during shower use. We used a GFCI circuit to assure safety for anyone changing bulbs in this fixture. Photo courtesy Galow Homes.

Advice for Bath Fan Controls

The Home Ventilating Institute (HVI) recommends that a fan be left on for 20 minutes after use of a bath or shower to remove all excess moisture from the room and ductwork. A timer switch is the simplest way to accomplish this.

Another option is a dehumidistat switch, which will automatically operate the fan whenever the humidity level rises above a preset level. A manual override allows normal operation of the fan for other bathroom uses.

Bath Fan Ductwork Specifications

Examples of good, bad, and horrible fan duct work installations are found at our bath venting home page: BATHROOM VENTILATION

A ventilating fan must overcome the resistance, called “static pressure,” created by the ductwork, including transitions, elbows, and the wall or roof cap. The advertised airflow of bath fans is typically based on a static pressure of 0.1 (inches of water gauge) although some also publish the airflow rating at 0.25 inches, which gives a better estimate of actual airflow in most installations.

Figure 6-4x: (C) J Wiley, S BlissA static pressure of 0.1 is roughly equivalent to 15 feet of straight, smooth 4-inch duct with a 100 cfm fan. A typical installation with about 20 feet of duct, two elbows, and a wall cap has an equivalent duct length closer to 80 feet (see Table 6-13).

Table 6-13

[Click any image or table to see an enlarged version with additional detail, commentary & source citation.]

How well a particular fan can overcome the ductwork’s resistance to airflow is highly variable and is indicated by its fan curve, available from the manufacturer. Inline fans mounted remotely and exterior fans are generally the most powerful.

A typical installation with two elbows and 20 to 30 feet of duct reduces the rated airflow of a standard fan by anywhere from 10 to 30%. With long runs, multiple elbows, or corrugated flex duct, airflow may be reduced by 50% or more. To ensure good airflow, follow these guidelines:

  • Use smooth ductwork of the same size or slightly larger than the fan outlet. Thin-walled PVC pipe works well and is easy to seal.
  • Keep duct runs as short and straight as possible.
  • Where possible, use broad sweeps rather than 90-degree angles to change direction.
  • Seal all joints in metal duct with foil-backed tape or duct mastic, not cloth duct tape.
  • For a standard installation, choose a slightly larger fan size than required. Where duct runs exceed about 25 feet plus two elbows, choose a larger fan size and check the fan curve to determine actual airflow.

Condensation in ductwork is also a concern in cold climates. To avoid problems, insulate the ductwork to at least R-5 or run it below the ceiling insulation. Also keep any metal duct seams facing upward and slope the duct slightly toward the exterior outlet so that any condensation drains to the outside. Avoid any sags in the ductwork, which are potential pooling areas for condensation.

Kitchen and Bath Product Manufacturers, Sources, Associations

Bath Fan Manufacturers

  • American Aldes www.americanaldes.com Remote location single- and multi-port exhaust ventilators
  • Broan-Nutone LLC www.broan.com Low-sone Broan bath fans, also single- and multiport remote location exhaust ventilators; Nutone ceiling-mount bath fans
  • Fan Tech www.fantech.com Remote location inline-duct fans
  • Kanalflakt www.kanalflakt.com Remote location inline-duct fans
  • Marley Engineered Products www.marleymeh.com Ceiling-mount bath fans and general kitchen and room exhaust fans
  • Panasonic www.panasonic.ca/English/ventilationfans Low-sone, Energy-Star-compliant ceiling-mount, inline, and wall bath fans

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

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