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I have some patches of black mold (mould) on the finished surface of the drywall in my sunroom ceiling. It is a cathedral ceiling: behind the drywall is vapour barrier, about 6" of fibreglass insulation, then the roof -- no attic.

I cut away one of the moldy patches of drywall. There is no mold on the inside (non-exposed) surface of the drywall and no evidence of any moisture inside the insulation or vapour barrier. The mold seems to be exclusively on the exposed, finished surface of the ceiling. There is none on the walls.

The sunroom gets very hot in summer and very cold (just above freezing) in winter. I can't tell when the mold grew -- seems to have developed slowly over several months.

Any ideas? I'm guessing this has something to do with warm air from the house rising and condensing against the cold, snow-covered roof/ceiling in winter. If so, this sounds more like an annoyance/cleaning job rather than a structural problem.
 

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Not a biggie. Concrobium will take care of it. Spray it on, let it dry, and over the course of time it kills old and new spores. For short term, you could use some diluted bleach to kill it, but it will likely return.

Mold likes it damp so you may want to run a fan in there pointed up toward the ceiling to keep it dry during those transition times from warm to cold that produce moisture.
 

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MMM. clean, clean, clean again. all the cleaners and time. maybe fix it so it doesn't happen ?

I assume the room has some ventillation from the house right? You have to ventillate it "some".

get a thermometer and a humidity indicator. compare the readings with mold groth numbers. stay within the safe range.

it matters what temperatures are outside the drywall. If you have moist warm air in the sun-room contacting cooler drywall in the sun-room you may get water/condensation on the wall - which would be one factor of black mold growth.

black mold can't eat the drywall but it can eat the drywall paper and also the paint. a non-mold paint for indoor use might help if you can find any - but costs time and money also.

I would not "just clean it" with tide laundry detergent (trisodium phosphate). There IS NOT ANY SAFE PRODUCT that keeps mold away (for more than a day or two). Are you kidding? You wouldn't be able to google "mold" if a little laundry detergent solved the problem. It doesn't work. they are trying to sell the stuff for decks also: it doesn't work. What works: don't put linseed oil on your deck and the mold won't eat it :)

good news: it can be done, this is a work smarter not harder situation
 

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Often mold/mildew will grow on walls that have restricted airflow like when furniture is placed next to the wall. Anything that increases ventilation should help. Bath paints have extra mildewcide and should help some.
 

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If that room is not heated or minimally heated in the winter, and is not sealed to heated living space, that is your problem
In the winter, the air in the conditioned part of the house has significant amounts of moisture. If that air gets into a cold sunroom, water will condense on the cold drywall surface. Same way as water condenses on your windows. Water plus organic food source such as paper equals mold.
 
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