Basement – Wet smell, finished basement, aggravates when it rains -how to find the source

basement

I have a light wet smell in my basement and I am trying to find the source of it …not much luck so far.
The basement is finished (by the previous owner) and as far as I could see behind the finished walls where the water meter is or under the stairs, it was done using expanded polystyrene, some plastic sheeting which I believe it is the vapor barrier. I am not sure if batt insulation was used or not.

Anyway, in short, I don't want (if possible) to open all the walls to find out what is going on.
Is there any way to find out where the leak is without removing the drywall ?

thanks

Update: I have spoken with a civil engineer who explained me that the humidity circulates along gradient lines from wet do dry. The smell seems to originate from the furnace room. I had an old furnace that I just changed. The furnace room had a large opening outside, direct unobstructed opening for the old furnace to take in air. No pipe running from the old furnace to the opening. Now that opening is closed, they used that for the exhaust and air intake pipes for the new furnace. There were a couple of openings in the insulation so I guess the more humid air trapped between drywall/insulation/vapor barrier was getting out because the room was dryers hence the smell.
I am not sure why the smell stopped as soon as the weather got colder and it diminished to a hardly detectable smell after the furnace was replaced. I am now watching to see what is next 🙂 If you have questions post a comment here and I will get back with details. I am still trying to understand why a weather change and blocking that opening changed the situation. It is also possible that the high efficiency furnace is able to better dry the air …

Best Answer

Having vapor barriers in most basement environments is a really bad idea. Really these only are a good idea in extremely cold environments or when a basement is truly a "basement" - meaning it is completely below ground level.

It is highly likely that water is saturating your foundation and then not drying quickly because of the vapor barrier. But there are just a ton of reasons:

  1. Given you have carpet the moisture could be seeping in from the floor and staying in the carpet a bit. You may not even notice this but the carpet becoming constantly a little damp will make it smell.
  2. It could be just water trapped in the walls because of the vapor barrier or trapped in the insulation itself. This will not be totally fixed unless you tear down all the drywall - or enough to take vapor barrier and bottom insulation out. See people 10 years ago and even now thought that insulating your basement walls top to bottom with batts fiberglass was the best way to do it. Well it traps water and causes mold growth and provides no cost savings in most regions.
  3. You could have a leak. Basically you will probably need to untack carpet or flooring around the perimeter to see if water is leaking in. I doubt that you have a real leak and not see any moisture at all underneath your walls.

Dehumidifiers are for very small constant humidity problems. When you have a "smell" problem the dehumidifiers are hardly ever going to fix the issue. You either need to seal your basement better from the inside or outside, remove the vapor barrier (that is probably just amplifying a bigger issue), and the easiest and most helpful thing is to increase airflow. If you have windows trying to keep them open a lot. If you have stairs that are near an outside door trying to get the airflow circulating. I have even seen people run fan to their attic to recycle air in basements.

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