Mold Between Drywall and Vapor Barrier

[Handyandy23]

My mother-in-law bought a townhouse in the summer with a partially finished basement, and I've been working on finishing the rest of it off. Today when I was removing the baseboards from the "finished" portion I found some black mold, and upon further removal found the mold was in the drywall and on the drywall side of the vapor barrier, but not behind it or in the insulation.

Anyone have any ideas what could have caused this? It's currently dry and I didn't find any current moisture or condensation. I've had the flooring ripped up for months now and haven't seen any hints of moisture anywhere.

Here are some observations I made, not sure if all of them are relevant but just listing off what I know:
- The mold was only at the base of the drywall, maybe a max of 12" up from the floor. Above that seems unaffected.
- As stated earlier, the mold was only on the inside / house side of the VB, nothing on the insulation.
- There is currently no moisture on either side of the VB, just the mold.
- This is on an exterior wall, but at the bottom of the poured concrete basement wall.
- The wall is framed and has batt insulation installed, but in some places it ends a few inches short of the base of the 2x4 stud.

My thoughts so far are that it's likely been caused either by water from inside the house at some point (maybe leaking plumbing or the hot water tank) that was repaired but previous owners didn't dry out properly, or it's from condensation forming due to the insulation not being sufficient and going fully to the bottom of the framing. I'm going to remove all of the moldy areas, beef up the insulation, and then patch with new drywall (likely the moisture resistant green stuff). I just want to make sure I'm not patching over a problem that is going to crop back up.

I'm assuming if this was an issue of water getting in from outside the house, that the mold wouldn't be on the inside of the VB and would instead be on the outside. And like I said before, no signs of moisture in the insulation. Also the fact that the mold is only near the floor where the insulation is lacking makes me think that is a likely cause. I'm just hoping for some validation of these assumptions, and any other ideas anyone might have!

Thanks in advance.

 

[RKA]

I think you're right on the money. Humidity from the house comes in contact with cold vapor barrier due to lack of insulation. It condenses and causes the mold. Fix the insulation and I think you'll be fine.

 

[b-boy]

I agree as well. Sounds like the insulation gaps are probably your culprit.

Do you run a dehumidifier?

 

[Handyandy23]

Thanks folks.

No dehumidifier, but the MIL has only been in the house since around August. So most of the time she's been in the house the air has been dry (Ontario, Canada). Probably a good idea to run one in the summer though.

 

[Jazz1]

I would guess water issue at some time. Seen it before on a friends house, he had to cut out the lower 2 feet of drywall around perimeter of basement after some minor flooding. When installing drywall in basement I would always leave a few inches gap to cement, hide with trim. I don't know why all drywall is not water resistant. I hauled flour into Quebec to a drywall factory,,that's the ingredient that causes the mold!

 

[Radix2]

Could be just the cool of the concrete in humid months. Esp in the spring when basements are still cool warm wet spring air coming into the house will condense on 50-60 degree concrete and easily. Could it wick up from the floor?

 

[Handyandy23]

That's a good point too, the drywall was installed pretty much tight to the concrete floor. When I replace I will leave it an inch or so up from the concrete floor to prevent the possibility of creating condensation off the cold floor.

I'm hoping there was just some previous water incident that caused this, but not knowing the full history I want to do my due diligence before patching it up and moving on.

 

[b-boy]

You have to leave at least 1/2 inch gap from the concrete to the drywall. Concrete is like a big sponge. It will wick water into your drywall for sure.

 

[Denwood]

There are two causes related to both insulation and humidity...and you mentioned Ontario.

1. The basement walls are vapour sealed really at the wrong place. The easiest way to fix this is apply closed cell spray foom to the bare concrete walls, and omit vapour barrier unde the drywall. This way the wall assembly can dry to the inside.

2. Because the basement insulation is likely not sufficient, and the slab may not be insulated, conductive ground temps in summer (cool) will lower the dew point of humid summer air and cause condenstation on the slab, behind trim, and between plastic/drywall. This will be worse at the base of the wall, and likely disappear as you move up.

My approach to the same problem was to sprayfoam the walls..but I also run a 70 quart dehumidifier set to 60% in summer as my slab is not insulated. The dehumidifier runs itelf and is plumbed into the sump...so we just plug it in and leave it.

If you pull the insulation from the concrete wall in winter, there is a good chance you will find the surface damp due to air leaks (moisture) from inside condensing on the cooler concrete surface. IMHO Fiberglass and plastic sheet have no place in a Canadian basement...it’s a proven recipe for mold.

 

[Handyandy23]

There are two causes related to both insulation and humidity...and you mentioned Ontario.

1. The basement walls are vapour sealed really at the wrong place. The easiest way to fix this is apply closed cell spray foom to the bare concrete walls, and omit vapour barrier unde the drywall. This way the wall assembly can dry to the inside.

2. Because the basement insulation is likely not sufficient, and the slab may not be insulated, conductive ground temps in summer (cool) will lower the dew point of humid summer air and cause condenstation on the slab, behind trim, and between plastic/drywall. This will be worse at the base of the wall, and likely disappear as you move up.

My approach to the same problem was to sprayfoam the walls..but I also run a 70 quart dehumidifier set to 60% in summer as my slab is not insulated. The dehumidifier runs itelf and is plumbed into the sump...so we just plug it in and leave it.

If you pull the insulation from the concrete wall in winter, there is a good chance you will find the surface damp due to air leaks (moisture) from inside condensing on the cooler concrete surface. IMHO Fiberglass and plastic sheet have no place in a Canadian basement...it’s a proven recipe for mold.

Thanks for the response! I'm far from an expert on the subject, but I'm a little confused at how moving the vapor barrier behind the insulation will help? Right now there is mold / aka moisture between the drywall and vapor barrier. If I moved that VB to the other side of the framing / batt insulation, wouldn't that just result in my insulation and studs getting moldy and wet?

I could see this as a solution more logically if I had the opposite problem. If I had water / moisture wicking through the concrete wall and saturating the studs and insulation, then by moving the VB out to the concrete wall I'd be blocking that moisture from getting in to my walls. But of that side is dry then moving the VB further out is just extending my condensation issue through more layers of the wall, isn't it?

But maybe I'm misunderstanding something here - if I am please help educate me on what I'm missing.