GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Code question regarding HRVs and bathroom fans

Troy Stevenson | Posted in Mechanicals on

Everything was going well in our green home planning process until a short conference call with our mechanical contractor today.

His assertions:

1. our home will be too dry (we are omitting a humidifier)
2. our bathrooms still need exhaust fans (we are installing an HRV) in order to pass inspection and meet code
3. an HRV should never pull exhaust air from the laundry room as it will create negative pressure

My approach is to ignore #1 as I believe he is wrong as long as we meet our tightness target of 1.5ach. We can resolve #3 by showing him the installation manual. My concern is the code issue that he brings up. Is this true regarding the requirement of bathroom fans? Any suggestions as to where I can find this code information?

I realize the best answer is to find a different contractor but it has been terribly difficult to find any trades familiar with high-performance homes in our area (Detroit, MI).


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. David Meiland | | #1

    1. In a tight home, I think you are more likely to have problems with excess humidity, although I have lived in SE MI and it is of course very dry during the coldest months. Personally, I view a humidifier as a last resort and wouldn't install one, but if you're concerned about it, it's very easy to install one after the fact, as long as there is an appropriate section of ductwork to put it on, and water and power are available. Seems like you could easily make these provisions without actually installing the unit.

    2. Ask the inspector if s/he agrees. That would not be the case here, the HRV would suffice. Whether the HRV has enough capacity to remove all the humidity from a shower at a high enough rate is another question. Personally I like having an 80CFM exhaust fan in a bathroom, but I know plenty of people who don't agree (including, I think, Martin).

    What code is in force in your area?

    3. Don't agree. Personally, I think laundry rooms get enough exhaust airflow because there's a clothes dryer in there sucking out air when it runs, but maybe you line-dry laundry in yours (we do not, it's too small). I doubt that any of the many exhaust fans that I've installed in laundry rooms *ever* get used, and don't think that particular piece of code is too smart.

  2. GBA Editor
    Martin Holladay | | #2

    If your local jurisdiction follows the IRC, your bathroom is not required to have an exhaust fan if it has an operable window that measures at least 3 sq. ft. (Section R303.3 of the 2009 IRC).

    If your bathroom has no window or a window that is less than 3 sq. ft., it needs to have an exhaust fan. It is acceptable under the IRC to use an HRV to exhaust air from a bathroom.

    According to section M1507.3 of the 2009 IRC, "Ventilation systems shall be designed to have the capacity to exhaust the minimum air flow rate determined in accordance with Table M1507.3."

    That table requires that bathrooms and toilet rooms have "Mechanical exhaust capacity of 50 cfm intermittent or 20 cfm continuous." Most (if not all) HRVs can easily be commissioned to provide 20 cfm of continuous exhaust from a bathroom.

    The code requires that equipment be installed of a certain capacity. It does not dictate how that equipment is used, so if you choose to switch from continuous exhaust operation to intermittent exhaust operation, you are not violating the code.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |