In case you haven't noticed, weather varies ;) This alone could account for the different effects each winter. But I think you're right, other factors are at play here.
Ventilation. Your gross vent area is 1/100 the attic area, accounting for louvers, let's say about 1/150th of attic area. If you do not consider the gable ends as cross ventilated, it is barely adequate. But as a cross ventilated arrangement, it is more than adequate. Will more help? Yes. Will it be effective? Harder to say. More on this later.
Moisture. Frost on nails alone is not necessarily cause for alarm. Black mold growing where the ventilation is less effective is definitely cause for concern. I'm no mold expert, but you should at least spray some moldicide on it while you devise a more permanent solution. Since the attic is always warmer than outside air, for there to be enough moisture to condense in the attic, the moisture had to come from either inside the house or from a roof or plumbing leak. Running a humidifier frequently would significantly contribute to the amount of moisture migrating into the attic. You don't mention anything about a vapor barrier, you quite likely do not know.The amount of moisture you observe tells me you don't have an effective moisture or vapor barrier. Not only can this cause mold, but it will also diminish the effectiveness of the insulation.
Why frost on only one side? Most likely solar access. The nails are colder on one side because the roof angles are different. This has significant effect on roof surface temperature, even under a good layer of snow.
Not only can vapor migrate through the ceiling, it can be carried in through air leaking around openings in the ceiling, in particular the pull down steps. If the perimeter is not weatherstripped, much moisture (and heat!) can escape this way.
What to do. Your primary focus should be to limit moisture migration into the attic. This will not only solve the condensation problem, but also limit heat loss. Seal all ceiling penetrations. Not only the steps, but plumbing and electrical penetrations as well. Apply an effective vapor barrier. This is not easy. You don't mention your ceiling finishes, this can make it easy or difficult or expensive. You could simply sandwich plastic between the current ceiling and a thin new ceiling finish. If you have an expensive existing finish, you may need to pull up the attic insulation and spray a barrier down on the back surface.
Note that if your household generates a lot of moisture, a good vapor barrier could cause you to have problems with condensation in your ceiling finish. The only solution to this is reduce moisture generation and increase whole house ventilation, ideally through a heat exchanger of some sort.
Once you've reduced the moisture migration, you may not have a mold problem in the attic any more. You may not need any more ventilation at all. The areas near the eaves obviously are not well ventilated, so the addition of soffit vents is the best approach if more is needed. You will need to drill many holes in the separation board to gain free communication with added vents.
If that works out to be too much effort, adding ventilation elsewhere can still benefit the lower areas, though not as well as soffit vents. Anything that increases ventilation will help. More static vents, power vents, ridge vents, turbine vents, they all have their own pros and cons, but they will all increase ventilation. You can do just one measure or a combination of measures. There is no right solution, but there is something that will make more sense than the other options.
Are you planning on replacing the roof? If so, are you redecking the roof or just replacing shingles? If you're only replacing shingles then there will be little or no additional opportunity to install soffit vents. If you are redecking, make sure to use radiant barrier decking. It adds about $2 a sheet to cost and has a large impact on attic heat. No roofer will tear off and reinstall old decking.
Soffit vents vs gable vents vs whirlybirds vs vent fans will make little difference in attic temperature in Texas. I have had houses with all combinations, and no vents at all, and the effectiveness is as follows:
- Soffit vents (almost no effect)
- Gable vents (little effect)
- Attic fan with gable vents (some effect, still easily 140+ in summer)
- Turbine vents with soffit and/or gable vents
I made the mistake of replacing turbines with attic fans on one house and the temp in the attic went up!
You can definitely hire someone to install soffit vent channels, and they will come rake all of the insulation out of the way and install the channels, then move the insulation back. The impact of soffit vents will be minor though.
Honestly, if you are are not having moisture issues in your attic (i.e. you don't live in Houston) and you have a house built before 1970 in Texas with a $150 summer electric bill then you probably want to stop worrying about the attic and start looking at other systems for improvement. If you had no attic insulation then you probably have no wall insulation. Having insulation blown into the walls will have a noticeable effect. Have you replaced the windows with low-e vinyls, esp. on the south and west walls? That will also have an impact on cooling costs. After all of those, start looking at A/C upgrades, pressure testing (most utilities will do this for free or low cost). Soffit vent channels and retrofit radiant barrier will have negligible effect if you already have several ft of insulation in the attic.
Best Answer
As mentioned by other comments and posts, the soffit needs to remain vented. For what it is worth, that one piece won't matter if it is perforated or not, (the whole soffit is open on the inside, that is a good thing) but use perforated soffit anyway so it matches. This material is available at any big box store.
The picture have captions in them that should explain what I seen needing to be done. What I have illustrated is the simplest way to proved a repair that will last without taking the fascia and soffit down. The only detail I left out was how to fasten the soffit panel in. Once the block is in that I drew in, add a piece of "F" track that will hold the soffit piece at the wall, or it can be screwed directly to the block. The outside egde can be screwed in too but the added fascia "extension lip" to replace the one that was torn off, is made to hold that end of the soffit in place.