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Question DetailsAsked on 3/24/2017

My bathroom is 110 sqft x 8 ft height. Builder installed 70 cfm vent fan. Can I legally force them to upgrade?

I found a formula from internet and it suggest I need 110 cfm ventilation fan for the room.
I am not sure if there is a code/conduct to enforce this item.

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Voted Best Answer

I am assuming this is a single-family home or duplex - not larger, because different rules apply for multi-family homes.

Looks like your formula was from the Home Ventilation Institute - not an "official" code requirements agency, and was a bit off anyway because your floor square footage exceeds the 1 cfm/sf of floor space cirteria in that recommendation. That criteria is minimum 50 cfm for up to 50 sf room footprint area (plan area square footage), 1cfm/sf for 50-100 sf of room area, and for over 100 sf of room plan area (your case has 110) 50 cfm per toilet plus 50 cfm per shower / tub plus 100 cfm per jetted tub - so assuming this is a full bathroom you would need 100 or 150 cfm depending on whether you have a jetted tub or not, assuming only one toilet and shower/tub in the room.

There is also another method, generally applicable to buildings with MORE than 2 families, based on air changes per hour (usually 8 air changes/hr for bathrooms) which also takes into account the length of duct run which might give another answer and cfm rating. For your 110 sf x 8' high room that would give 110 cfm - so maybe that is where you got that number. That 110 cfm might have to be increased based on the length of duct to the roof vent and the type of duct (metal or plastic, smooth wall or flex) and number of any bends - using ASHRAE Standard 62.2 Table 5.3.

However - those are not building code in many or most areas for single family and duplex (and up to fourplex in some areas) homes, which (assuming the 2012 IRC (International Residential Code) is in effect in your area, requires 50 cfm intermittent or 20 cfm continuous ventilation in bathrooms/toilet rooms. (Table M1507.4 in 2012 IRC titled "Local Exhaust Rates For One and Two Family Dwellings")

And actually, if there is a window which opens at least half way and has at least 3 sf of area (so opens to at leat 1.5 sf), the code does not generally require a fan at all.

Obvioously, check with your local building department about local requirements because different areas use different code year versions and some areas insert their own amendments to code requirements, and but I am betting 50 cfm is the requirement for your area.

So - unless your architect (if you have the plans drawn up for the house he should have specified the cfm rating for the fan) specified more airflow (and 70 or 100 cfm is what most would usually have specified for your case), or specified using a specific sizing standard or critera which yields a larger fan size, your only recourse is probably to do a change order to have it changed out - probably about $300-500 I would guess ASSUMING it is still readily accessible from the attic (or is a straight vertical duct shot to the roof vent if a low/zero headroom attic like a flat roof with low-profile trusses attic), or possibly more if a long run like to a gable to avoid roof penetrations, because if he used 3" duct then going above 70 cfm would commonly require larger duct too, which might mean removing the roof hood to do that - and possibly even changing out the roof hood for a larger airflow capacity one depending on wht the one put in is rated for.

My gut feeling - unless you have a jetted tub or steam shower or such, or take a lot of very hot long showers, as long as you turn the fan on BEFORE starting to shower and leave it on for at least 1/2 hour afterwards (preferably with door open unless that causes house humidity problems), you will probably be OK unless this is a bathroom on an outer wall and your area gets below say 10-15 degrees a lot OR your house has very high humidity (like deep south without A/C), which can cause mold issues on walls (though bimonthly or trimonthly wall washing with chlorine cleaner like a non-abrasive tub-safe Ajax commonly controls that). Actually, in cold or very high interior humidity areas, unless you get into the 200-300 cfm fan size like is commonly used in arctic areas, the ventilation has a lot less to do with the mold issue than wall cleanliness and how cold the wall is getting or how high the incoming air humidity is.

Answered 3 years ago by LCD

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