mysterious mold spots on ceiling


We've lived in this house for almost 3 years now, and every now and then, a spot of mold (5-7" long, 2-5" wide) will appear on outside walls where the ceiling meets the wall. i've been all over the attic (not fun) looking for damp roofing and rafters, and I even opened up the soffits outside to look at the top of the drywall, and everything looks fine. I cannot figure out where the mold is coming from. any ideas?
my brother in law suggested my attic does not have enough ventilation. right now it has one fan in the middle of the house, and vents every 8' or so in the soffits in the overhangs.
thanks.
dave
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davethieben wrote:

Cold where you live? Insulation voids will create cold spots where moisture in the house can condense. If you use a humidifier in winter, that will increase the odds.
On a cold day, you may be able to feel the temp differential by moving the palm of your hand over the wall.
Jut one possibility to look at.
Jim
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yes, I live in Ohio. would that be insulation in the soffits outside, or insulation over that area in the attic?
thanks.
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davethieben wrote:

Could be attic or in the wall stud space. The latter because the existing insulation may have settled.
The suggestion to consider ice dams is worth looking at too.
Jim
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Since it is near an outside wall I suspect an ice dam is causing the problem. This is related to ventilation since ideal ventilation would minimize or eliminate the problem.
In the real world even well ventilated attics can have the ice dams if the conditions permit. Large snowfall followed by thawing and freezing creates an ice dam which holds snow melt back and prevent it from draining. A lot of water can collect behing the dam. At that point it can cause a leak which could explain the mold.
Is there snow and ice where you are? If so, perform an inspection of the roof and remove the snow if necessary. Then you can use an ice melting salt sprinkled on the ice at the edge of the roof.
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wrote:

It sounds like condensation, do you have humidity problems?
tom @ www.Consolidated-Loans.info
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wrote:

Hi Dave
I agree with your brother-in-law. At least a foot wide strip of your entire soffit width should be vented. Ensuring no insulation blocks the flow.
Near the ridge you should have 3 to 4 vents minimum. Or you could try the low profile ridge mesh product that runs the entire ridge.
Regards Dale
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yes, humidity is pretty high in our house. our weather station shows 65-75% most of the time, and I understand normal is around 40-50%.
I'm confused on one thing. it sounds like I need to insulate along the soffits to make sure there are no cold spots. but then Dale suggested venting the soffits ensuring no insulation blocks the flow. how am I to insulate the cold spots without blocking the flow?
what's the best way to reduce humidity in my house? would a single room dehumidifier work well enough? or will I need to have a dehumidifier in my furnace?
thanks.
dave
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davethieben wrote:

You don't already have a humidifier on the furnace, do you? Is humidity consistently that high? It may be far fetched, but perhaps a sealer on the outside of the brick would lessen humidity. Got a basement?
Are the mold spots on the ceiling or at the top of the wall?
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let's see:
house is in SW Ohio, so right now it is anywhere from the teens to 40-50s. we have quite a bit of condensation on the sliding glass door and some windows.
we do have exhaust fans in the bathrooms, but hardly ever use them.
I'm not aware of a humidifier. how can I tell? we don't have a standalone humidifier that we are using.
no basement, it's a ranch on a slab.
mold spots are on both the wall and ceiling, right at where they meet. i can take pictures if it would help.
i thought that humidity would be strange in the winter, since it's usually drier in the winter.
thanks guys.
dave
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davethieben wrote:

Any gas appliances? CO monitor? I couldn't follow the conversation about your attic and soffits, but that would be the starting point, assuming all appliances are operating properly. And, for sure, use the bathroom exhausts.
Should check the attic and vents, being sure the soffits aren't covered with insulation. Depending on the type of soffit vents, it might be easier to check if you just change out the covering on them. There is a formula for calculating attic ventillation which has been discussed a great deal over time on AHR; 1 sq. ft. per 300 sq. ft. of attic floor space, I believe. It doesn't calculate on ROOF area, and I think the logic is that the higher the roof (more roof area), the more volume of air in the attic so it would not become as hot.
Having a good HVAC service company check over the furnace would be a good idea if it hasn't been done in a while. We get a freebie yearly for our AC/heat pump because we have an appliance service contract. The co. isn't making any money on us. :o) Let us know how it goes.
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First thing I'd do is make sure the fans exhaust properly to the outside and then start using them after showering. You could also put them on a timer, so you set them on and they run for 15 mins or so.

I would only be on a forced air system. The humidifier, if there is one, would be mounted on a plenum on top of the furnace. Most have a round bypass hose going from it to the other plenum. And of course a small water line.

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