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 ?


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 ...

4 Answers 4


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.

  • I have to agree with everything here. I know we're not supposed to post 'I agree' comments. Just wanted to 'second' the advice. :)
    – DA01
    Oct 4, 2015 at 20:19

actually after of almost two years of observations and logical deductions, by eliminating all the possibilities by making different experiments I isolated the problem to a tube/drain that took the water from my furnace/AC condense water pump to the drain located on the other side of the building (still don't know where it joins the drain because the basement in finished. This tube had a leak (must have been a mouse or something) my guess is that it is plastic all the way to the drain and it was leaking somewhere in the ceiling but relatively close to the open ceiling area in the furnace room. The leak was not big enough to wet walls or something but big enough to generate the smell, I guess Since I disconnected that tube from the ceiling and since I started draining the water into a bucket (temporarily, till I build a proper drain) the humidity in the basement is controllable without much effort and the smell is gone


Basements are often moist. You may want to consider a dehumidifier, if you aren't running one already. Caveat: they burn a lot of electricity.

  • I am already running one but it does not help much.
    – MiniMe
    Jul 6, 2015 at 15:20

As DMoore points out, it appears there's some structural issues--namely the vapor barrier.

I'd bring in an inspector if you can after a rain period and see if they can narrow down where the leaks may be using their expertise and toolsets. Ultimately, it sounds like you're going to have to rip into the wall somewhere to find out the leak issue and being able to narrow it down a little bit would be a big help.

In addition to that, though, do a fully inspection around your house. Are all the gutters in working order? Is the ground surrounding your house all properly sloped AWAY from the foundation? Things like that.

  • I am going to post an update in the original post
    – MiniMe
    Oct 6, 2015 at 1:17

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