Your problem is very familiar to me. Let me explain:
Two and a half years ago, we had a new roof put on after a hail storm. The original sheet metal ridge vent was replaced with a plain Cobra vent, and we thought all was well.
In early December, I went into the attic for some Christmas stuff, and found the sheathing dripping wet (never happened before for 20+ years). Called the contractor, who maintained he'd used the same vent for years with no problems. By spring, we had a serious mold problem, and called him back. He took a look and said it was moisture from a separated bathroom duct he found, which he 'fixed' (for more $$). Had Servpro out to get rid of the mold (more $$$, insurance wouldn't cover). Thought all was well.
Next December, more wet sheathing. Called another contractor, who checked the bathroom ducts (2) and found them in horrible condition (crushed and full of holes). Had them replaced and hard-piped to soffits, and closed the soffit vents for 4-feet around to prevent back-drafts (more $$, insurance covered 'moisture damage', and mold was minimal because we got on it early). Put a temperature and humidity sensor in the attic (which later indicated summer attic temperatures as high as 150-160F), and thought all was well.
And this December? Yup. Wet again.
Did some digging online, and found some recent research that says:
o Ridge vents do not work because warm air rises, as is widely believed;
o Ridge vents work because of wind;
o Ridge vents without side baffles do not work, because the wind goes right through them;
o Ridge vents with side baffles work because the wind is forced over them, making a vacuum at the vent openings, and pulling the air out.
This is why our original baffled ridge vents worked, and the baffle-less Cobra vents do not.
We recently replaced the Cobra vent (yet more $$) with a baffled ridge vent, which is slowly drying the attic (and contents) from 30-40% moisture meter readings to 20-25% readings. Am hoping to get the moisture down to 16%, which is when mold doesn't grow.
My question is: what winter moisture and summer temperature levels should one expect for a well-ventilated attic.
I hope some of this is helpful to you all.