Recently we had the garage redone - into a private "apartment" (bedroom, bathroom/shower, oven/stove, sink) We've begun to smell a moldy smell, the kitchen hasn't been used yet, though the bathroom has. I have been sniffing, but unable to find a location where the smell is strongest I've also tried to correlate the intensity of the smell with rain, or with when the bathroom/shower is used - but no apparent correlation. I don't see any discoloration (which I guess makes sense as the drywall is new) I have crawled in the space above the "apartment" and don't see or smell anything. Actually don't smell the mold up there. How can I fix w/o ripping apart all the work that has been done.

  • Was the work permitted and inspected?
    – JACK
    Commented Dec 30, 2021 at 21:13
  • 1
    Were there any moisture prone spots before remodel, if so check those areas. Check around the foundation and see if something shows up. Is it possible some of the cold water side of the plumbing is dripping from condensate? You must have moisture to have mold.
    – Gil
    Commented Dec 30, 2021 at 22:02
  • No permit or inspection
    – Chaim
    Commented Dec 30, 2021 at 23:00
  • How can I tell if plumbing is dripping if sealed?
    – Chaim
    Commented Dec 30, 2021 at 23:00
  • 2
    Are you sure it smells like mold, not sewer gas?
    – r13
    Commented Dec 30, 2021 at 23:31

1 Answer 1


It appears to be sewer gas, the easiest way to test this is to dump a gallon or so of water in each drain every day for a week. By the time the week is over and the smell has disappeared and you have either solved your problem or you have determined it is mold.

The reason for this is the P-trap system used in drains.

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There is typically a "U" in the drain below each fixture that does not empty out when the fixture is emptied so it blocks the drain with water. Bacteria etc in the drains consume organic matter and emit gas some smelling very bad. This gas has to go somewhere or the pressure bill build up. This is a slow function and the pressure is very low and will eventually go out the vent pipe. However to do this it needs to build up a minimal amount of pressure to push the gas to the vent. This pressure is maybe 1/2" water or less. The trap "U" shaped thing has water blocking the pipe. Over time this water evaporates and the gas will then come out of the drain. It is a very small amount and spreads rapidly. You can visualize this with a piece of transparent tubing or a straw in a "U" shape with some water in it.

  • Thanks - I'll try this - Q - When you say to dump the water in each drain - do you mean the sink, shower and toilet of the garage?
    – Chaim
    Commented Jan 12, 2022 at 19:50
  • Yes all of them, especially ones that have not been used for a while. I will add more to the answer.
    – Gil
    Commented Jan 12, 2022 at 20:09
  • thx. I'll try it!
    – Chaim
    Commented Jan 13, 2022 at 2:09
  • 1
    OTOH, if the drains are being used on a regular basis (probably once/week would be sufficient unless you're in a very dry climate), the water in the traps shouldn't evaporate that quickly.
    – FreeMan
    Commented Jan 13, 2022 at 16:56

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