Help with understanding air leak in wall cavity
There is a closed cavity outlined in red that has plumbing for a tub. The cavity has six sides.
-The top is covered with wood and the edges where the wood meets the framing is foamed
-The bottom is the subfloor. The exterior facing edge was foamed
-One side is exterior wall facing and has thermaply that was foamed around the edges (in the picture the thermaply hasn’t been fastened or foamed yet)
-The other three sides have drywall. One of the sides has plumbing coming out of it over a bathtub.
When running a blower door, there is a strong draft coming out of the valve trim. I’m having trouble understanding why this would be the case since it seems like all the gaps in the cavity were foamed, including plumbing penetrations.
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Welcome to the world of complex three-dimensional air pathways. The space you have drawn in the photo is connected to either the exterior wall cavity or to the attic space (I'm assuming the attic is vented and unconditioned). There could also be a connection to whatever is under the space, an air pathway from a rim joist or crawlspace. Canned foams aren't the best product to seal small gaps such as the gap between wood framing and wall sheathing, or top plate and whatever product is used for the ceiling air barrier. A quality sealant or caulk works better at these locations. Two-part foams installed as a complete air barrier can work, but I have seen failures even with these products.
Diagnosing the leak can be done using either thermal imaging (if you have a good difference between indoor and outdoor temperatures) or by using smoke. I would start in the attic, remove the insulation in the area of the shower area and run the blower door. If the leak is in that area, you will probably be able to feel air movement, smoke can confirm.
Another option would be to use a blower door to pressurize the structure and inject smoke into the shower valve opening. It will exit someplace, either in the attic, somewhere along the exterior wall, or into the space below the shower area.
The tough air leaks to find are the ones where air moves from 10' away. I've found leaks that start in an exterior electrical box, found a pathway into a wall cavity, and comes out the ceiling light fixture in the middle of a room.
Randy
Thanks Randy. The top of the cavity interfaces with an unconditioned attic and one of the walls of the cavity interfaces with the outside.
This area has now been drywalled. With about a 40 degree temperature differential between the outdoors and inside, the cavity isn't reading any colder than the surrounding walls.
With that in mind, is it even worthwhile trying to fix this?
Tub drain area? I've seen some pretty huge holes cut there to accommodate dropping in the tub with the drain attached.
Yes, the tub overflow drain comes through that cavity. If you look closely in the photo, you can see red foam around the opening.
What I'm having trouble also understanding is would you expect the cavity to be cold if there was an air leak coming in? The temp isn't reading any different than the surrounding wall temps and there is a 40 degree temp differential between inside and outside
The blower door is moving much more air into or out of the home than the home normally sees, giving you the ability to detect air leaks. That being said, if a strong wind is blowing on that area of the home, you may be able to detect the temperature differential in the space. There is plumbing in that area, prolonged wind with cold temps can create issues you do not want.
If this is a customer's home, I'd fix it, if it is your personal home, I'd monitor the area
Randy.