[edta1]

Mold doesn't just appear out of nowhere, it is the result of a water leak or other constant source of moisture. If the source of the moisture isn't repaired, the mold will return.

How do we find the source of the water leak/ moisture?

Our guess is we forgot to turn off the water valve (which is true and corrected this morning) to the humidifier from the winter time. So perhaps whenever the AC ran, cool air would circulate and start condensing on the wall? There was a plastic bag in front of the mold spot so perhaps that prevented air flow? This is just a guess. Plausible?

 

[edta1]

Edta1:

Before you start cutting holes in your drywall, check to see if moisture is coming THROUGH the drywall or is just on the surface of the drywall.

Simply remove that mold with some bleach. You can use it straight out of the jug without diluting it if you want; it won't harm the paint on your wall or baseboard. But, be careful not to drip any on your carpet because it could bleach the colour out of your carpet pile. Maybe cover the carpet with some cardboard or something. You can use bleach diluted with water. Four parts water to one part bleach will kill your mold and clean your wall, but it'll just take a bit more elbow grease.

IF (big if there) the moisture is coming through the drywall, then the drywall in that area will be softer than the surrounding drywall. If, after you clean the mold off and allow time for the surface of the paint to dry, you knock on the drywall with your forefinger knuckle, and the drywall where the mold was sounds the same as the drywall in the surrounding areas, then the drywall where the mold was WAS NOT softened by moisture.

In order for mold to grow, there has to be moisture present, but whether the moisture came THROUGH the drywall or was simply the result of condensation on the surface of the wall is of prime importance. In my humble opinion, you hit the nail on the head when you said: "There was a plastic bag in front of the mold spot so perhaps that prevented air flow?."

I have seen mildew grow on walls simply because of insufficient warm air flow. In my case there was a closet situated in the corner of a building and the baseboard radiators did not extend into the closet. So, with the closet sliding doors closed, the walls in the closet simply didn't recieve enough warm air circulation to keep them warm, and condensation formed on the walls and mildew growth followed. I solved that problem by painting the closet with Zinsser's Permawhite Bathroom Paint, which is a mildew resistant paint made specifically for Bathrooms. It has mildewcides in it which kill mildew spores before they can grow into mildew.

I fully expect the problem in your case was simply lack of sufficient air circulation to that area of the wall, and you simply need to remove the mold with some bleach and keep your stuff a few inches away from the walls from now on. If you have an unheated garage on the other side of that wall, that's the reason for the condensation, and hence mold growth, on the paint in that spot. Your walls might be perfectly well insulated, but if there's no air flow on the inside of the wall to keep the wall warm, but cold on the other side of the wall to cool it down, then your wall will cool down. Mildew and mold spores are obiquitous (sp?). They're everywhere and get carried by air currents throughout your house. All they need is moisture to come to life. But, if the source of moisture dries up, then the mold will go dormant after a while and die in a few years.

Thanks nestor, I'll give it a shot this weekend.