AC vent to nowhere, moldy drywall

[mound]

Ahh, the joys of peeling back the layers of time on a newly bought house..  

Just moved into this house, built in 1963

Noticed this patch of dark/discolored/moldy(?) looking drywall in the garage that was hidden by previous owners stuff before the sale/move. 

   

Notice how it's discolored running vertically, and then even worse toward the bottom bumping out to the right.. What the heck is behind here? I noticed that while the AC was running, this discolored area was cold to the touch, so I'm assuming its an HVAC run directly behind the wall, probably not insulated in any way and condensation is slowly seeping into the drywall. 

So I cut open the wall:

   

Right I was. Clearly the drywall is discolored inside the garage running up the wall directly in front of an HVAC line.  But what's up with the darker spot on the lower right? I peel back that bit of fiberglass insulation and see this:

   

An HVAC line to nowhere, blowing cold air out at me..   Switched on the heat instead of AC, and it's blowing hot air out on me. 

This line literally goes nowhere but into fiberglass insulation in the stud bay. 

The house was remodeled in the 90's and had an addition bumped out from this corner behind the garage, as well as another addition above the garage, and there is a complete HVAC line behind and to the right of this open ended line leading out to the addition. 

I can only imagine how much hot and cold air has been wasted, this must have been like this since the 90's. 

Anyway, what's the right fix for this?  That line to nowhere could simply be capped off I imagine, but what about the vertical line? A piece of foam insulation board of some kind between it and the drywall? Or perhaps capping off that line, and not pumping cold and hot air into that bay, would prevent the condensation problem from happening on the vertical line in the first place?


edit: OR, and this may be absurd, but I hope to use this space as a shop, maybe actually connect it to a register I can close, and use it to supply some cold and warm air into the garage as needed? That seems like a huge and possibly wasteful hack though

[barnowl]

....

edit: OR, and this may be absurd, but I hope to use this space as a shop, maybe actually connect it to a register I can close, and use it to supply some cold and warm air into the garage as needed? That seems like a huge and possibly wasteful hack though

This.

Buy a register boot, and install a register so that you can open and close it at will.

https://www.supplyhouse.com/Hart-Cooley-...AuEALw_wcB

[mound]

This.

Buy a register boot, and install a register so that you can open and close it at will.

https://www.supplyhouse.com/Hart-Cooley-...AuEALw_wcB

I may just do that! 
Do you think the mold is due only to that cold/warm air blowing into the stud bay and that the vertical run doesn't need any sort of insulation between it and the drywall?

[blackhat]

If the drywall side is unconditioned space, or outside the normal thermal envelope of the system, yes, it should be insulated.

[mound]

If the drywall side is unconditioned space, or outside the normal thermal envelope of the system, yes, it should be insulated.

One side of the wall is the garage (where the photos are) and the opposite side is conditioned living space.

[blackhat]

If the garage is not normally heated and or cooled by the HVAC system, it is outside the system envelope and duct should be insulated. The entire wall should be insulated as far as that goes. There are code restrictions on using the home’s HVAC  to serve a garage space. Entry of exhaust and or gasoline fumes into the home and depressurization of the home are both real possibilities.

[mound]

If the garage is not normally heated and or cooled by the HVAC system, it is outside the system envelope and duct should be insulated. The entire wall should be insulated as far as that goes. There are code restrictions on using the home’s HVAC  to serve a garage space. Entry of exhaust and or gasoline fumes into the home and depressurization of the home are both real possibilities.

Ah very good points.  Probably puts the kibosh on installing a register into the garage (my hack)

I am going to have the HVAC guys that service my units give me their expert opinion too on what the best course of action is.

[barnowl]

If the garage is not normally heated and or cooled by the HVAC system, it is outside the system envelope and duct should be insulated. The entire wall should be insulated as far as that goes. There are code restrictions on using the home’s HVAC  to serve a garage space. Entry of exhaust and or gasoline fumes into the home and depressurization of the home are both real possibilities.

Good points Ian.

[Snipe Hunter]

Like Blackhat said. You can't have an HVAC register in an unconditioned garage. A garage is basically a fire protected envelope and that would breach the envelope. The good news is you found your mold source. The wall is collecting condensation when it gets cold. Mold needs 2 things to grow, water and food. You can't get rid of the food but you can get rid of the water.