Is it mold?

Below are 2 pictures of the paper side of some fiberglass insulation. Normally mold would never be a concern here, it's just not something you think about in the arid Southwest. However, I put a ventless propane fireplace in this building last winter, and they produce quite a bit of humidity. This is a nice bonus for the winter in New Mexico, where relative humidity is usually in the single digits. Buuuut now I am noticing this. Strangely, some of it has started recently, and humidity is around 20%. I fired up a dehumidifier and it hasn't generated a single drop of water. What do you think? Is this mold? Should I just replace those pieces of insulation and start running dehumidifiers for the winter?




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I am not a fan of ventless heaters--And even gas stoves without a power vent can make the kitchen walls a mess (natural gas leaves a sticky film on walls/vent hood that is not fun to clean).
Humidity wise... Mold will grow when it is >60% relative humidity for 3 days (the spores then "activate" and grow). So, if the humidity is over 60% for three days, you have an environment where mold will grow.
Note that cold objects (such as insulation and studs to exterior walls) can create local spots of higher than "room" humidity (as temperature drop, the RH goes up).
If this is mold (vs dust and static charge in dry air atrracting dust), you don't want to get that started. I have used spray bleach formulated for removing model on walls--Works very easily and quickly (must be much stronger than normal 5% laundry bleach).
https://www.homedepot.com/s/mold remover?NCNI-5
Used this on a room that was poorly ventilated and it worked great (and improved ventilation, removed potted plants, etc.). Mold did not return (was a person that like warm/humid room for sleeping--I guess).
Then there is the old dehumidifier:
https://www.homedepot.com/b/Heating-Venting-Cooling-Air-Quality-Dehumidifiers/Best-Rated/N-5yc1vZc4l8Zbwo5q
Can be a relatively big user of energy--But for some closets/storage areas/laundries/etc.--May be needed).
And for very air tight homes--An energy recovery ventilation system:
https://www.ecohome.net/guides/2362/ventilation-air-exchangers/
-Bill
https://www.certainteed.com/insulation/resources/do-i-need-vapor-barrier/
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I think your problem is you did not have drywall and you ran that heater, which is fine for construction but should be retired and replaced with a better choice, for obvious reasons. Good Luck !
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That had me scratching my head, so I had it tested. Not mold. It is actually the black asphalt glue that holds the paper to the fiberglass, and in very DRY environments the paper can wick the glue through, so it appears on the paper side. I'm pretty happy about that!
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My point is the code is there for a reason. Living with exposed fibreglass is not good for you either. Everytime a door is opened.....
I have seen alot in my years with all types of offgrid, not too many nightmares so that is good!
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