We live in a R2000 home, 5 years old with no central AC. We have an air exchanger but don't run it in summer because it doesn't cycle on and off. We keep the fan running on the furnace. We leave a couple of windows open a crack for air circulation. We run a dehumidifier in the basement during summer. Ceramics are installed on the basement floor, drywall is on the walls but not yet painted. There's mould spotted along most of the drywall along the bottom about 6" high.
How do I fix it and keep it from coming back?
Not sure if this has been fixed yet. But you do need to address not only the mould itself, but the source of the problem. Mould always requires high moisture content to grow. If you do not find the cause of the moisture in or on the drywall, it will very likely return.
Tight homes can in themselves be a source of humidity, and a tight home can retain that humidity. However, let's look at some potential areas.
The first might be condensation. As this is close to the floor, it will also be the coolest area and the most likely for any airborne moisture to condense. You do say you are running the furnace fan and dehumidifier though, have you measured humidity in basement area.
It is also possibly a capillary action or wicking of moisture from the backside of the plastic vapour barrier. If it is this, the drywall itself would almost certainly be moist throughout and may even be somewhat mushy. Remember water can move through concrete, check drywall to try and determine if any part of it is mushy or very moist. Is it coming from floor, wall or inside?
It is also possible, but less probable I think that it is coming from runoff or subsurface sources, with ceramic this would hide very well and not be necessarily noticeable unless it was sufficient to cause water on the floor surface. Again check for moist drywall, and if this is the source it will very likely only be present at times of the year when there is excessive moisture content in soil outside.
There are a few other possible reasons, but this covers the most probable ones. Good luck, but do find the source of moisture and fix that or you are wasting your money.
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