Mildew on ceiling

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Have a spot of mildew on ceiling next to bathroom light/heater/vent fixtures. Is this a lack of insulation issue in attic or something else?

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Likely an air filtration issue around those fixtures more so than an insulation issue per se. Traditional fiberglass Insulation is a thermal barrier but not a air or vapor barrier.

Moisture is going condense and potentially form mildew and mold when the dew point temperature of the attic air entering the conditioned around those fixtures is higher than the air temperature in the conditioned area around the fixtures in the bathroom. And that would be exacerbated if you have an AC register blowing cold air in the vicinity of those ceiling fixtures where the attic air is leaking into the cooler conditioned space below.

Remember these thermodynamic principles, Heat moves to Cold, and Wet moves to Dry.

This is a long way of telling you to check and make the ceiling fixtures where mildew is forming are properly sealed from the attic side from the conditioned space below (usually can do that with cans of spray foam) and once properly sealed make sure those area in the attic are also insulated.

If you are handy this should be a DYI job you can handle yourself.

This is an good example where a thermal imaging camera in the post created by bapple below “ Ready to Burn $300? Handheld Klein Thermal Imaging Camera” LINK would help identify where the air leaks from the attic are occurring. Not implying you need one of those, it’s just a helpful tool for those types of issues.

Theres a spring valve inside the exhaust vent that goes up to the roof. If it wears out or gets stuck open, hot air from outside comes into the house through the vent. When the hot air hits your conditioned air, humidity is created and you get mildew around the vent.

In the attic, Look on the backside where the vent joins the exhaust tube and separate the two. You’ll see the valve. The valve will be on vent unit where it joins tubing to vent to roof.

Mine was stuck open. It wouldn’t close when the vent exhaust wasn’t on. I bought an inline one off of amazon and fixed the problem It was about $5.

Just be sure toinstall it so that it opens when wind pressure hits if from the exhaust fan side.

Same problem in my bathroom and around my atic stairs got
I am going to get up there this winter and do some weather striping and spray foaming