Wall cavity being used as return air plenum

1

I know its not allowed anymore, but I’ve never called this out in my reports on older homes… Until this one.
This one was in the wall between the garage and the home. So thermal imaging shows the cooler drywall surface on the garage side, as it pulls the cooler, interior air into the wall. So, first off, it obviously means there is no insulation inside this wall cavity.

But, that also got me thinking. What happens to the surface when cold air meets hot air?
Yep, condensation is possible.
So I checked with my moisture meter, and sure enough, there were elevated levels of moisture in the drywall.

Now it makes me wonder if I have ever missed a framing cavity being used as an air return that was inside an exterior wall. I think (hope) this is the first time I have ever seen it done like this.

20250802_124351
20250802_1243511920×2560 248 KB

20250802_124445
20250802_1244451920×2560 241 KB

20250802_124549
20250802_1245491920×2560 258 KB

download
download1500×2000 475 KB

6 Likes
2

Having the exterior wall cavity used for return air is pretty common on older homes.

2 Likes
3

Back in the 80’s we never put them in “exterior” walls, but garage walls were common.

2 Likes
4

I’m not sure I would trust the moisture meter 100%. In my opinion, the return air would have to be pretty cold in order to produce a significant amount of condensation in this instance. Were there any other indications of moisture? That would play into my decision if this was an actual issue.

My home was built in 1901 with no ductwork and all of the returns (added later of course) use the exterior wall cavities. I have never seen any indication of moisture from those returns.

1 Like
5

Thats a good question, because I didnt pick up high moisture everywhere. There was one area that readings got up to around 60%, but I would say maybe only 1 sf. All Other areas were about 16-18%, while the unaffected drywall was around 10-12.
But I did not see any visible signs of moisture. I made it more an issue of missing insulation, but listed the moisture as a possible concern

3 Likes
6

What was the temperature inside the home?

7

About 75. And as you can see, there is really only about 10 degree difference on the drywall

8

What was the relative humidity in the garage?

9

Seems like a good course of action. I have that same Extech moisture meter in my bag. Can’t say I trust it 100%. I think in this case I would have maybe pulled out the Flir with the long pins, since it’s just a garage wall.

10

Not sure, but weve been averaging about 80%

11

Same here. I should have checked with my Tramex, but the batteries died a couple days ago, and keep forgetting to replace

1 Like
12

It’s been about 20 years since I have had to figure this, so I hope my memory is right.

You have a garage temp of approximately 90 degrees with a 80% RH. The air going through the wall cavity should be around 75 degrees. The chart is showing you should not have a problem with condensation in the wall cavity.

Have no desire to try and relearn psychometric charts even though it is more accurate.

RH
RH1324×1165 254 KB

3 Likes
13

Is it possible that the return chase is made with duct board or even a metal duct? Both would fool the moisture meter. Also reflective tape such as aluminum duct tape can provide false positive readings. If you were finding higher readings in sporadic areas, I would say you are getting false positive readings.