Home Improvement Questions? Ask a Handyman for Answers
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You're seeing mold because you have condensation occurring. This is due to an inadequate vapor barrier between the living space and the new attic. Also where your insulation is installed is contributing to this problem by creating a warmer space in the attic. Insulation is never installed in the rafter space in an unheated attic.
Yes, a vapor barrier would help. And a poly barrier is best. But the rafter space insulation must be packed tight if that space is not vented. You don't want to trap moisture in the rafter space behind the vapor barrier
Yes, you've created a warmer space in the attic. The air there, since it's warmer, can hold more moisture which is condensing on the colder surface of the underside of the roof sheathing.
I wouldn't be too concerned about the mold at this point
If the moisture doesn't have a colder surface to condense on it just stays in the air. Sprayed foam insulation is best for un-vented rafter spaces because it is its own vapor barrier and the entire space is covered with insulation so no cold surfaces. So the same principles apply with what you're using.