Why am I having an attic ventilation problem this year when I didn't last year?

Question

This winter in the attic I noticed ice on the roofing nails. Obviously warm moist air condensed and then froze on the cold nails. When the weather broke and warmed up a bit, I was back up in the attic and noticed black mold on certain areas of the lower attic ceiling going toward the eaves.

Some interesting facts:

• I have been in the house two years and did not notice this happening last year.
• This is only occurring on one side of the attic ceiling, the other side is completely fine (no rusted nails poking through, no mold)
• I recently put down some more plywood so I can have more area for storage, and before I did that I added some more insulation to the attic floor (rolls). I wonder if that has anything to do with it?
• It was also a colder drier winter this year so we had humidifiers in use more, so that may also have something to do with it.

I am just curious why it is happening this year and not last year, and why on only one side of attic ceiling (roof).

Important Information:

• For ventilation of the attic we have only two gable vents (one on each side of the house), about 1.5ft x 2ft rectangle each. These do not have any fans.
• There are no soffit vents or ridge or roof vents; just the 2 gable vents.
• Looking at the lower part of where the attic roof slopes down and meets the wall I notice there is a board that blocks the attic area from the soffit area/eave. So if I was to put in soffit vents from outside, I believe I would have to also drill holes from inside attic through this board so the outside air could actually pass into attic and not be trapped in this blocked off area.
• The attic space is about 600sq ft.
• The only insulation is in the attic floor and on there walls, there is no insulation on the attic ceiling.
• Bathroom fans are vented to outside the home (not into the attic or soffit)
• I live in Massachusetts where winters get cold and summers are hot and humid.

My questions:

1. What should I do?
2. Why was the gable venting system in place good enough last year but not this year?
3. Why only one side of attic affected?

I recently beefed up the insulation and seal on the pull-down attic entrance as this could be an area where warm moist air was entering into the attic (the entrance is on same side as the attic problem area). Will that fix problem, or do you recommend installing more ventillation anyways?

Will a fan on the gable vents do the trick, or do I need to install soffit vents anyway?

Will soffits with the gables work, or will ridge or roof venting also be required?

Answer 1

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.