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I am building a half bath in my basement. Is an exhaust fan required by code? Since there is no shower can it be vented into unfinished space?

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I can't say for sure about code - it varies a lot by location. But generally either code requires or it is a good idea to have:

  • An exhaust fan or
  • An operable window on an outer wall

so that you can vent hot/moist air, chemical fumes from cleaning, etc. A half-bath (normally that means no tub or shower) will not have as great a need for ventilation as a full bath, but it still needs a fan or window.

If you install an exhaust fan, you must vent it to the outside. If you vent it into an attic or other unfinished space then you will eventually get some combination of mold, mildew, damage to insulation, damage to wood and other problems. Venting to the outside is easiest if you can basically go "up" and put a hole (properly caulked/sealed) in the roof, but venting off to the side is acceptable. The total length of the ductwork can be an issue - check the specs on the fan for more information.

There is quite a variety of exhaust fans available, but the biggest criteria to consider are noise (typically measured in either sones or decibels) and speed (typically measured in CFM). You generally want the quietest fan you can afford that has at least the minimum speed for your bathroom - which will depend on the size of the bathroom.

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A bathroom that has a window opening less than 1-1/2 square feet is required to have a fan. The fan cannot vent inside the structure.

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Yes, a 50 cfm exhaust fan is required and an 80 cfm is required if there is a tub or shower. (See ICC Table R1507.4)

Note: 2017 Code does not mention the use of a window as a substitute for an exhaust fan...news to me...it’s always been an option.

Exhaust duct to terminate a minimum of 3’ from any window.

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  • 2015 Code does - as Ed Beal & I both said (and Ed with the specific size of 1-1/2 sq ft) - see codes.iccsafe.org/public/document/IRC2015/… R303 Oct 11 '18 at 23:17
  • @manassehkatz Yes, as shown in 2015 Chapter 3, Section R303.3, a 3 s.f. Window is allowed if one-half is openable. Oops, I do see in the 2017 Code, a 3’ window if one-half is openable is permitted if there is no tub or shower...
    – Lee Sam
    Oct 12 '18 at 0:30
  • It has definitely changed over the years. My house originally had a fan in only the interior full bath. The 1/2 bath (with window) did not and also a full bath (with window) did not, but I added heat/fan/light to the full bath. Opening a window facing the street for ventilation right after a shower isn't such a great idea... Oct 12 '18 at 1:49

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