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

Venting three bath fans at a porch roof

user-2310254 | Posted in Mechanicals on

I have three new Panasonic bath fans serving my top floor. The attic is now foamed and a semi-conditioned space.

The old fans were vented through the roof using three-inch flex duct. I was going to redo these lines with four-inch metal pipe. Now, I am considering a different approach that would be much easier to execute while allowing me to eliminate three problematic roof penetrations.

The top floor has a fairly sizable covered porch. There should be plenty of room to run the exhaust lines out over the porch and vent down to the outdoors. It will be much easier to run the lines horizontally across the attic instead of angling them up 12 to 15 feet to reach the roof line.

Thoughts? Acceptable plan or a headache in the making?

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. Yupster | | #1

    Just don't terminate the exhaust fans under the covered porch. The warm humid air that they exhaust gets "trapped" under the ceiling of the covered porch and will cause icicles to grow, not to mention sending some of the humid air right back into the attic if the covered porch ceiling is made of vented aluminum soffit material, as many here are. It needs to extend to where it can directly exhaust to the big wide world or use an clever termination like this at the edge of the covered porch

  2. user-2310254 | | #2

    Thanks, Yupster. Atlanta weather means it rarely gets cold enough for icicles to form, and my fairly minimal soffits were sealed as part of the foam insulation project. (The contractor also foam the top of the porch ceiling.) But I could put the vents terminations out near the edge of the porch where air pooling is less likely to be an issue.

    1. heidner | | #9

      Steve, I've done the venting out the side of the house toward a porch... I learned my lesson and had the venting changed so it goes up through the roof... Good roofers can ensure that the penetrations are not problems, and the will be no leaks.

      Venting to porches mean you also have the smells and as noted by others - moisture that gets trapped under the eaves and make it back up the underside of the roof.

  3. Expert Member
    BILL WICHERS | | #3

    It sounds like you’re going to vent through the wall ABOVE the porch, correct? If your soffit vents are sealed, then you’re probably ok. Just be sure to pitch the vent lines slightly so that any water condensing inside won’t pool in the lines.

    I think extending the lines out past the porch may be a problem. Even in Hotlanta you’ll probably have days where it’s cooler outside and then those long lines will condense more moisture out of the air the bathroom fans are exhausting. Lots of condensed water inside the vent piping can be a problem.


  4. user-2310254 | | #4

    Thanks, Bill. I am sure there are trade offs, but I do plan to pitch the vent lines so they could drain. (I am considering using thin-wall PVC instead of metal to simplify the installation process.)

    Whether going horizontally or vertically, it is a fair way to daylight since this is a three-story townhouse sandwiched between two other units.

  5. CMObuilds | | #5

    Why are the roof penetrations problematic?
    If it where my house I would rather a roof vent than a soffit/down facing horizontal penetration. I see a lot of roof sheathing damage from these.

  6. GBA Editor
    Martin Holladay | | #6

    I agree with those who vote against terminating these ducts in your porch ceiling. Either a gable wall termination or a roof termination is preferable.

  7. user-2310254 | | #7

    Thanks everyone for the input. The challenge here is working with three stories and coordinating trades with the HOA. From a practical standpoint, it would be much easier to run the vents to the porch. But it sounds like I may be trading one headache for another.

    1. Expert Member
      BILL WICHERS | | #8

      Just a thought, I have one first floor bathroom on my own house that I need to vent the fan through the wall which will be near the front door. I’ve been thinking about using one of the window planters on the second floor to hide the vent. Maybe some clever design details can help you make a stealthy vent to please your HOA?


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |