You mentioned in one of your other posts about only having a very limited amount of soffit vents on your home.
Soffit fresh air intake is more vital to the attic air changes per hour than the exhaust venting and if you want to think of it in per centages, the most ideally recommended scenario is 60% Fresh Air Intake and 40% Hot/Humid Attic Air Exhaust. The perfect world results are not always obtainable though.
Since you are very concerned about the mold growth that has already occured, I will make sure you get the proper documentation in your hands to properly analyze the situation.
For instance, a 4" x 16" under soffit louvre vent only provides a grand total of 28 square inches of net free ventilating air flow intake for each vent. If you have 12 of these intake vents, you would currently have 336 square inches of intake venting. That would only be true though, if none of the vent louvres are clogged by any dust particles, cob webs. or attic insulation and also, if they have not been painted over, which would substantialy reduce the amount of intake NFVA flowage.
Typically, most soffit vents do not have the proper hole cut out in the first place and also have become restricted severely due to the other scenarios commonly discovered in the real world. They only perform to the rated NFVA if they are instaled correctly and Never get clogged. At best, they are usually only 50% efficient from the original rating.
Your upstairs section of your home is 30 foot by 40 foot. Lets do the actual math for the actual required “Minimum” amount of ventilation you will need.
30 x 40 = 1,200 square feet of attic floor space from corner to corner.
1,200 divided by 300 = 4 square Feet of attic ventilation required. This is the “Absolute Minimum” needed, and might need adjustments upwards, depending on the pitch of the roof or if their is no vapor barrier installed under the warm side of the insulation in the attic.
Translate 4 square feet into square inches, since that is how all of the ventilation products are rated. 1 square foot = 12" x 12" = 144 square inches.
4 square feet x 144 square inches = 576 square inches Minimum needed.
If you roof slope is above a 6/12 pitch through 9/12 pitch, then add 20%. If it from a 10/12 pitch to greater than a 12/12 pitch, then add 30%. This is to account for the additional cubic footage of air space in the attic.
If your attic is a steeper roof, like an 8/12, then add 20% for example. 576 x 1.20 = 692 square inches would then be required.
So, you at minimum realistically need 692 square inches.
Ridge line = 10 feet with a Shingle Vent II Ridge Vent with an internal filter and an external baffle providing 18 square inches per foot, so you can achieve 180 square inches of total exhaust venting.
You now need a minimum of 692 sq in - 180 sq in = 512 square inches more. This can be accompliched with continuous Fresh Air Intake Venting, such as the Smart Vent, by DCI Products, which provides 9 square inches per foot of intake. You would need just around 57 feet of intake venting to achieve this total NFVA.
This was with the asssumption that the old existing soffit vents were functioning at full efficiency though.
Another way to achieve the proper amount of air flowage instead of using the 10 feet of ridge vent, would be to use Only a Powered Attic Ventilator fan on the roof top, but definitely not using any additional roof top exhaust vents. They now have Solar Powered units, which provice a CFM rating of 800 cfm. Electric units actually cost more in the short and long term, due to increased energy usage and replacement costs for when they stop functioning.
If you have multiple types and placements of roof top exhaust vent products, they for sure would short circuit the entire flowage and you were NOT overthinking the scenario. I have case study stories analyzing this occurrence, which brings more problems than it solves.