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
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.
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
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.
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.
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
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.
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?
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?
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.
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
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.
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
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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