Mold on the roof

[david10708]

I noticed there is big patch of mold inside my roof. There is no mold anywhere else in the attic. The area is just right above the main bath room's bath tub. Since the bath tub has lower ceiling comparing to other part of the bath room, in the attic, I can see the thick red insulation under the white insulation in that area. So, I am sure it is the place.

I noticed the mold since year 2011. the mold is only between the two beams. I checked the roof and could not see obvious leak.

The pictures were taken yesterday with attic temperature around -9 celcius. It is very cold. There was no snow on the roof.

In one of the pictures, i can see a layer of frost on the wood. Again, that layer of frost only exists in that area. This area is facing north. you can see icicle on the tip of the nail, in other areas on the north side, almost all nails has such icicle. But on the south side of the house, no such icicle.

The house was built in 2001 and I bought it in 2009.

Just want to know what the problem could be. Is it roof problem?

Thanks,
David