Seems like a reasonable use of space and materials and saving in costs to use a stud cavity as a conditioned air return.  But is it a good idea?  The inside surface of naked, unpainted sheetrock is heaven for mold.  We know that these cavities are tough to get air tight, but if you can get the leakage down to meet codes and Standards, why is this a bad approach?

Has anybody used the Duct-EZ material?  And if you have, have you completely coated the inside of the cavity with the material or just the joints and seams?

Has anyone got pictures of a cavity that was used for ducting and been taken apart?

Use of the cavity as a supply duct is not permitted by code, but return flow use seems to be relatively common.  It would be great to have tangible evidence that it is a bad thing for IAQ.


Paul H. Raymer

Tags: Duct-EZ, IAQ, Stud, cavity, ducting, mold

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Has anyone documented mold inside a panned return?  We discourage the practice for different a reason.  We frequently see a 4 X 12 return grill = 48 in2 (less a bit for louvers), but the hole in the bottom plate of the wall is 2-1/2 X 10 = 25 in2  -  the return is the size of the smallest hole in the path back to the air handler.  And in many cases, such as where second floor wall stud spacing does not line up with the floor joist spacing below, much much smaller.  

In Canada, return air in stud cavities is the normal practice, as are "lined" joist bays which serve as return air routes. There are some practices which have to be respected. But if the supply & return air system is completely within the building envelope, this is a workable system. The inside of the cavities is not coated with any special material.

I have only seen mold in return cavities on homes that had excessive moisture levels, this is more moisture issue than it is a cavity return issue.

My biggest beef with cavity returns is the sizing of the hole at the bottom of the cavity, which i have found everything from a full 14.5"x 3.5" opening to no opening whatsoever. This is less of an issue if the return is in the floor of the first level, but when it has to go up the main floor walls, into the garage ceiling, and then up the bedroom wall into the attic, through 15+ ft of flex duct, my guess is IT WONT WORK! (As Joe Lstiburek says, "Don't do Stupid Stuff!")

Running a Manual D, fully ducting the returns accordingly, and sealing the ducts (Both Supply and Return) is optimal for performance.

The biggest problem is not actually the ductwork, it is builders that are not educated on the benefits of doing this properly, and are more interested in getting the keys turned over so they can get paid and start on the next project.

Paul, I have used stud cavity returns on interior walls only,but always metal lined to inhibit odor absorption by the materials. The standard size limitation of the cavity usually under sizes your return unless using multiple bays.

I don't think it a good idea. Every 5 years or so I have my ducts professionally cleaned. This involves putting spinning brushes down the ducts while a strong vacuum is pulling out what comes loose. If you use a stud cavity for the return, you can't clean your ducts.

Are you aware the EPA and Consumer's Union does not recommend duct cleaning on a routine basis?

Code has not allowed building cavities to be used as ducts  on either supply OR return since 2012 IECC.  See R403.3.5 in the 2015 IECC.  Some states have amended this language to allow.  In my opinion, it's setting the ducts up to fail.  It's possible you could still pass the duct tightness test, but this particular section of duct is almost guaranteed to leak.

It's also impossible to inspect from a code enforcement point of view.  Code requires ALL duct joints and seems to be sealed.  you would need to inspect duct sealing in a multi-stage process in order to be able to check cavity returns. 

Another problem you encounter is that cavity being used seems to sometimes get picked after it's already being used as a chase for something else (Electrical, plumbing, etc).  The cavity is closed up and no one notices there is a pipe running through part of the duct.  Can you imagine the air quality in a duct that has a pinhole leak in a copper pipe running through it?

I work with the State of DE on code language, and for 2012 there was was backlash from the HVAC guys about panned returns.  We managed to get language stating that they could be used only if the ducts are not required to be tested (i.e. all interior), and they were well sealed at all surfaces.  If a house is below 3 ACH50, you can be reasonably assured that there are no major leaks into the "stits" that might end up in the return system.  We prefer central returns, which makes them much easier to size and seal anyway.

And we should all re-read the EPA stance on duct cleaning

I discourage it because it's too easy to punch holes in drywall to hang pictures, or what have you. It only takes a few poorly sized holes to introduce enough leakage to foul the intended return path. Not to mention the other problems that have been mentioned. Also stud cavities usually communicate with adjacent cavities well enough to add to the problem.

CA code now requires supply ducts to be insulated even in conditioned space.

I live in a house that was built in 1963,  the builder used wall cavity between the first and second floor stair way as the return shaft for the furnace.  I'd could hear the furnance running louder because of the sound from the air movement was coupled to the sheet rock.  It always seemed like more "dirt" was being pulled in from somewhere -- and about 40 years after the house was built -- I found some pretty large leaks in that shaft that went all the way up into the attic and the blown in cellulose insulation.

When I replaced the furnace in 2015,  I insisted on bids that included changing that shaft to a fully lined sheet metal return duct, sealed with mastic,  using turning vanes along the length of the return duct all the way to the furnace.  I also raised the furnace TWO inches off the concrete floor, placing it onto of Dow pink foam board and 3/4" plywood platform.

I was surprised at the push back I got from the HVAC companies that I had bidding for the new furnace -- I had provided them with list of things to fix,  rough drawings to describe what I wanted - and included wording that they could use their best judgment so long as the shaft was lined.  I also had them bid on the extra work as separate line items -- and I told them I understood the cost could be significant.  The time to fix the problem was when the furnace was being replaced!

I also had requested bid on only the highest efficiency furnaces fully modulating.  Apparently most home owners don't care....about the ducting or the furnaces.

The sound transmission from the air in the shaft is gone,   the air filter stays cleaner longer,  I'm no longer sucking air in from the attic. 

As for the mold,  I don't think you'd see much mold in the return duct itself -- too much air movement in most climates -- unless one side of the shaft is on a cold wall.  Mold likes moisture and if you've got air movement and moisture controlled elsewhere .. it' not likely the source.  But pulling in dirty air from areas where the house isn't conditioned (the attic) through old insulation... where humidity levels may not have been controlled - that would be my concern.

That shaft would have also provided a real great chimney for a house fire to move between floors and the attic. It would have been difficult to fight without access to the attic via cutting holes in the roof.

When I looked at tightening up my house envelope - the connections between the floors for plumbing, HVAC  and electrical were something that can be a real eye opener.   I quickly made it a habit to seal around all openings - even if both sides were in conditioned air space.  I  used  Firestop foam to fill the holes... 

You really want the movement of the air to stay in the designed HVAC sheet metal and not use bypasses unknown.


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