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rich-nh 11-03-2012 10:04 AM

AC Return in exterior wall - mold
3 Attachment(s)
Hi - new to the forum. I found some mold in my attic, growing above an elbow of the AC return duct. I cleaned away the insulation and pulled away the duct to find the nothing but bare studs below on an exterior wall.

That makes sense to me now as each winter, I've got cold air blasting out of my return grate in my family room.

My first question is - is it OK to have no ductwork and use the uninsulated stud cavity on an external wall as part of my AC return path?

Secondly, thoughts on why the mold is growing right at the attic duct joint?

I've included 3 photos of the mold above the joint and the bare stud cavity.


joecaption 11-03-2012 11:07 PM

I see no one replyed back to you.
May want to go back and edit your profile with your location. I see NH in your name which may mean New Hampshire but who knows.
I've never once seen a return air vent run though an out side wall and if you are from up north you can see why it's a bad idea.

GBrackins 11-03-2012 11:41 PM

it is done, and I saw it done today for supply air from a furnance ..... :eek:

now I'm not that knowledgeable about HVAC so hopefully some of our more enlightened members will chime in.

I do not know what your code is, but this comes from the 2009 International Residential Code (your code my differ)

7. Stud wall cavities and the spaces between solid floor joists to be used as air plenums shall comply with the following conditions:
7.1. These cavities or spaces shall not be used as a plenum for supply air. 7.2. These cavities or spaces shall not be part of a required fire-resistance-rated assembly.
7.3. Stud wall cavities shall not convey air from more than one floor level.
7.4. Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fire blocking in accordance with Section R602.8.

now whether the attic is consider another floor level I cannot say as I'm not the building official .... but I would consider it as such

I would certainly say it's not the brightest ideal in the world to install it in an exterior wall that should have insulation in it.

Now that you've found it I'm sure you'll rectify the problem ....

as far as mold, you are returning warm moist air (humidity inside the home) through a space that is exposed to cold air. water vapor reaches its dewpoint and condenses from the air as a liquid. wood is a food source for mold (which is normal within a home, ever had molded bread?) so .....

Good luck! :thumbsup:

rich-nh 11-04-2012 07:09 AM

Yes, I am from New Hampshire.

And that 'ductwork' in my stud cavity is being used for a single floor - to get from the attic down to the first floor - bypassing the 2nd floor.

I'm curious about the M1601 building code as it's not specific about interior or exterior wall cavities. I know I wouldn't be having this issue if it was interior.


Gary in WA 11-04-2012 11:24 AM

Top four;

Photo 7 and 8;


rich-nh 11-04-2012 07:08 PM

Thank you GBR. Time to try to fix this now.

joecaption 11-04-2012 07:52 PM

Where you from in NH?
I'm from the Hillsboro-West Deering area.

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