Bath fan size


Good question yesterday–what size bath fan do you need.  For bathrooms up to 100 square feet (e.g., a 10′ x 10′ room, with 8′ ceilings), the Home Ventilating Institute recommends a fan be sized to deliver 1 CFM (cubic foot per minute) of exhaust for every square foot of floor area.

For example, an 8′ x 6′ bathroom, at 48 square feet, should have a 48 CFM fan.  In this case you’d probably round up to a common 50 CFM fan.

Note, this is for bathroom spot ventilation only.  If you’re using you bathroom exhaust as part of a whole-house ventilation strategy, sizing changes based on the size of the home, other fans being used, and how you’ll be using this.  A good home performance contractor can help you determine the right size for you.



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4 Responses to “Bath fan size”

  1. neil Says:

    We recommend a lot more air than HVI, and it’s not just because we sell bath fans.
    If you do not remove water vapor before it condenses on the wall, then you may have moisture problems… water on sheetrock – not good.
    Take a look at our bath fan sizer:

    • Mike Rogers Says:

      We don’t agree. A quality bath fan sized according to HVI recommendation, and properly installed to ensure airflow very close to nominal levels should work very well. Overventilating is not good–and can be as bad as underventilating. Not only is there an energy penalty, in humid climates with too much exhaust ventilation you may actually bring in more moisture than you expelled. And the larger the exhaust, the greater the risk of despressurizing the home, with the potential for combustion safety issues, introducing moisture as noted above, among other potential issues.

      A well functioning fan at HVI reccomended levels and placed near the source should do an excellent job. Making sure you have good insulation and air-sealing details–in cold climates in particular–will help ensure that condensation is not a big issue. In my own home, my bath fan sized to HVI specs, and placed in the shower, means I don’t get even get significant fogging on the mirror during a shower.


  2. neil Says:


    First of all, it’s wonderful to have a discussion about bath ventilation. I don’t disagree with your points regarding over ventilation, however here are a few points you may want to consider.

    1. What is the “source”? Is it the shower or the toilet? Your 50 CFM may be just enough to remove vapour when placed above the shower. However, it probably won’t result in a low odour bathroom (jokes, notwithstanding..). In addition, the low CFM approach does not take human nature into account. Some people like to shower and steam up the bathroom. In this case, the 50 CFM will run for hours, with potential f or condensate on the walls and unpleasant humidity.

    2. Yes, you could de-pressurize the house with a higher CFM fan, and the hypothetical situation you mention regarding combustion safety issues could occur, but this would be a house with very old gas appliances (no combustion fan and accompanying safeties), water heater in the living space,etc, combined with a very well sealed house. Certainly, could happen, not likely, and in such a situation, you would of course highly recommend a balanced ventilation system such as an HRV.

    3. Expanding further upon your concerns about combustion, a far greater concern should be gas stoves. These appliances burn natural gas directly in the living space.
    I’m sure you have some opinions on this …

    4. You can change the speed of (good) bath fans if necessary with a simple triac control.

    5. As all experts say. “It depends” . In this case, seasons and dry climates versus humid climates certainly change our approach to ventilation.

    • Mike Rogers Says:

      Neil–I can’t disagree with your longer response. And you sum it up nicely with #5. It depends. I won’t argue against well thought out ventilation approaches considering all the variables.

      I do encourage a careful examination of combustion equipment with exhaust fans. We still see a lot of atmospherically vented equipment in the living space, and not infrequently, units that fail worst-case depressurization with the exahust fans running. We don’t limit our combustion safety testing to water heaters, and we do look at gas ovens and ranges. And we strongly discourage unvented gas appliances–watch for a post on that soon. In fact, we’ve desginated next week as combustion safety week, and we’ll have a daily post on related topic. Agaiin, there are a variety of approaches to good ventilation, but they must look at the right variables, and they must be safe.

      Thanks for reading and for your thoughtful comments.

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