Expert: Dan Griffin - 11/13/2007

Question
QUESTION: I have seen this phenomenon a few times. However no one has been able to give me a reasonable answer.
What causes rafter shadows on ceiling in residential construction?

ANSWER: Thermal transfer through the framing.  The shadows are actually dirt - dust stuck to moisture line.

Think about this.  Make it bitter cold outside.  Provide excellent insulation between the joists.  The temperature of the finish face of the drywall is colder at the framing than at the insulation.  The effect is much more noticeable on cathedral ceilings and with metal framing.

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QUESTION: Will the installation of more insulation over the attic side of the rafters alleviate this condition?
Can the ceiling simply be painted in order to remove the stains?

Answer
YOu can certainly paint the ceiling.  If you don't change anything they will return.  Additional insulation would sure help. YOu never said if this were a cathedral ceiling or conventional attic.  Additional attic insulation is never a bad investment, no matter what your climate.