roof
I had a new roof put on last fall. I never noticed any hot spots on my roof until this winter. I had an inspection after the roof was put on and I was told my insulation looks okay. When they put on the new roof they closed two lower vents on my roof on the back side. There are canister vents on the back side near the top. My soffit covers are solid and I'm planning on adding soffit covers with holes. Thanks for any advice.
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.
Your inspector is correct. Air should enter at the soffit, and exit at either roof vents or a ridge vent. With the current setup air could be entering either the soffit vents or the roof vents, and exiting either the roof vents or the ridge vent. Since your soffits are covered with insulation (because there are no baffles), I'd guess that air is entering the roof vents and exiting the ridge vent. This means only the air near the top of the roof is cycling, which can lead to problems with the roof including a shortened roof life.
In my house there were originally roof vents in the roof above my garage. When they did a reroof they switched to a ridge vent, and as you can see they covered over the old roof vent openings.
On a side note: Inspectors are hired by you, and work for you. They have no reason to lie or mislead you, since they will not gain in any way from it. In fact, you can probably trust an inspector more than a random person on the internet. Seeing how you paid the inspector for advice, while I'm doing this for free.
Best Answer
Notice that deeper snow below and to the left of the bare area? It's deeper than it is anywhere else in the picture: why?
It's very unlikely that that area was much better insulated than the rest of the roof, and so has melted much less. Instead, it's almost certainly a wind-carved snow drift, especially when you notice that the deepest spot forms a ridge sloping away from the edge of the roof.
With that in mind, it's pretty clear that your bare spot is just an area where the wind during your latest snow storm either didn't deposit snow or actively removed it. Once the storm was done, the snow has been melting everywhere; this spot was the thinnest, and so has become bare before the rest of your roof.
Summary: nothing to worry about.