Why is there mold and moisture inside walls on the sheathing?

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Why is there Mould on particle board outside vapor barrier on new addition?

Christine ML Published: June 7, 2020, 1:35 p.m.Last updated: June 8, 2020, 12:43 p.m.

I build a second story addition on 100 yr old double brick bungalow 3 years ago. In the second summer the second floor bedrooms start to have a musty smell, particular smell coming through outlets. We cut a square into the dry wall, pushed back the insulation, cut through the vapor barrier and found mould on the exterior particple board. The are no leaks in the roof, or water running down the exterior walls. We wonder whether the mould is caused by condensation between the vapor barrier and the particle board. The house had air conditioning going all summer. The mould is on the particle board on the south side of the house. I have had the building inspector and the builder look at the issue. Both of them could not determine the source of the problem, nor suggest how to prevent the problem.  I am looking for a strategy to remediate the mould issue on the particle board. Thanks

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