I would apply straight bleach to the stains and if it goes away then you know you have mildew. If you do then wash the entire ceiling with a bleach and mild detergent mixture to remove the mildew and dirt. Then allow it to dry a day and prime with a pigmented shellac primer such as Zinsser BIN and then apply a latex flat paint.

You've had advice how to remove this, but not what causes it.

This usually happens because the steel is a thermal bridge, colder than the surronding materials so the SR right there is also cold. That makes moisture in the house condense on the colder suface. The presence of the moisture is condusive to mildew growth. it also provides a surface that smoke or soot is more likely to settle on. Any wood stove, cigarette smoke or candle smoke will add to the problem.

The problem should not happen tho in a new home. The insulation should be adequate to keep the steel from being copld in th efirst place. Even if there is enough thickness or depth, it may not be properly installed. Have an insulation contractor inspect it for you.

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