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attic mold

Matt_W | Posted in GBA Pro Help on

The north side of the attic roof has mold. I live in the northeast. Pictures are attached. After some research, I think that the problem is a combination of nails stick out of the plywood and air leaking to the attic. I plan to use a rotary tool to cut all the nails, then use vinegar to clean all the mold. Then I will air seal the attic. 

I have a few questions:
Does my assessment sound correct?
My basis is that the only possible source of moisture air other than air leaking is the one venting out of the 2nd floor bathroom. It is vented out to the roof side by a duct pipe. I didn’t see any obvious leaks from the pipe and the pipe is buried under a 6 inch thick insulation foam. The roof has a ridge vent and two windows on both sides. The entire attic is insulated by 6 inch foam. 

Should I cut the nails? Will it help? Do I need to add a layer of something to insulate the residual nail top?
My reasoning is that condensed water is visible on the nails. Cutting them will help. One strange thing is that no visible condensed water on nails on the south side of the roof, even at night. I guess that the south side is warmer than the north so less water the mold. Maybe I only need to cut the nail heads of the north side. 

Is it easy to air seal the attic? Should I find a professional? 
My state has a energy saving program and they provide free assessment of insulation and air leaking. I guess that I will wait for their assessment and then decide what to do. 

Thanks in advance for any suggestions.

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  1. Expert Member
    DCcontrarian | | #1

    Check out this article:

    The fact that it's the north side only makes me think the issue identified in the article may be your problem as well.

    1. Expert Member
      MALCOLM TAYLOR | | #2


      I'm not sure. RDH seems pretty clear it is a regionally specific problem to the PNW climate - and we see it regularly here. While night sky radiance may play a part, I think what the photos show is pretty typical of roofs everywhere with high air leakage and inadequate ventilation, and it shows up more on the north side mainly because the the sheathing doesn't get as hot, or the same chance to dry, on that face of the roof.

      Matt mentions ridge vents, but what is the eaves ventilation like? My suggestion would be to sort that out, and the air-sealing. The wet nails are a symptom, not a cause. There isn't any point in cutting them shorter.

  2. walta100 | | #3

    The first question is do you have any vents down low like in your soffits. Ideally you would have about 10% more ventilation in the soffits that the ridge has.

    Cutting off the nails is a bad idea. The moisture on the nails is rotting away the plywood holding the nail in. If you allow the situation to continue the wind is likely lift the shingles and pull a few out.

    Air sealing is not complex work but it is a thankless job that must be performed in the default conditions and the current insulation will need to be removed. If you higher the job done what you are looking for is called “Blower door directed air sealing” You want to know your ACH50 number before and after the work. If they can get your number under 3 That is about as good as can be expected in old work.

    I like the idea of getting an energy audit. Make sure your audit includes both a blower door test and inferred photography.


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