I looked at a house in Sonoma County last week where there was no visible mold growth but the drywall was over 40% moisture content. I suspected that mold was growing in the wall cavity but the air sample results came back just fine. I recommended that the drywall be removed anyway because it was wet and mold just loves wet drywall. I went out today and saw the interior of the wall cavity and... what can I say... I was right. The question is, how can mold grow so heavy on the inside of the wall cavity when there is no sign of it on the painted surface? Is there a difference between the front side and back side of drywall? Yes, there is! The difference is the paint. In this case, the wall was painted with a semi-gloss enamel, which is not a great food source for mold growth. The back side of the drywall is basically paper, which is a great food source. The semi-gloss enamel paint acts as a vapor barrier trapping the moisture. Any time drywall is allowed to get wet, and stay wet, mold growth is a possibility... even if its hidden.