Hi Willy - We are under contract on our house and this issue came up. The attic rafter which I thought came up with inspection and you didn't think it was mold. Do we need to have it looked into further?
Willy's Reply: The bad mold (if you had a problem) would be on the plywood roof sheathing and on the sides of the roof rafters. The picture you sent could be mold but it’s from the lumber when the house was built. In a lumber yard wood sits outside and gets rained on. The lumber is not kiln dried, it’s what they call surface green and mold as well as other fungi grow on and in this “NEW” lumber. The mold (if it is mold) is dead but the wood has been stained. If the stain concerns you, get some white primer paint like “Kilz Max or Zinnsers' mold killing primer” and paint over the black spots. When a serious mold problem does occur, professionals spray paint the entire attic with these type of products to kill the mold. It also helps to prevent it from growing back. But mold can grow on virtually any surface, so the best solution is to reduce the moisture and the mold, which is a plant, dies. The dead mold stain is what it looks like you have on your roof rafter from your photo. Hope that helped.
The vapor barrier on the insulation was installed backwards. The vapor barrier must face the warm side of the building to prevent moisture migration into the insulation and framing system.
Inadequate ventilation was also noted - you can see particles floating in view of the camera. Cellulose insulation visible in the attic area at the time of the inspection. It should be noted that this insulation material could be extremely flammable if it has not been treated with fire resistant chemicals