I’ve seen this sort of thing before, and the cause is always the same. Without a doubt there’s a lack of attic insulation directly above the mold zone. This construction oversight allows condensation to form on the drywall, fostering mold growth. The problem is especially pronounced in corners because air circulation is often poor there.
Begin by going up into the attic to see what’s going on. Sometimes it’s easy to add more insulation to the troublesome spot and other times it’s not. There won’t be much room to work where the roof frame tapers down towards the edge of your house, so prepare for a struggle. But easy or not, more insulation is necessary to prevent condensation and mold from returning.
To repair the ceiling inside…
- Scrub the mold with a solution of 5 parts water to 1 part bleach and let it dry completely.
- Lightly sand the area.
- Coat it with a stain-blocking primer before applying several coats of paint on top.
If you’ve been successful in boosting attic insulation levels in the trouble zone, the mold won’t return. If you can direct the airflow from a heating register into the ceiling corner, it will also boost your chances of success.