Reusing Deteriorated Insulation
The home is in Livermore, California. Temps tend to range from 30-105 (110 - 120 on a bad year). Day/night temperature swings tend to stay around 30-40 degrees. We don't get much precipitation (15" a year), but morning fog/cloud cover from San Francisco bay isn't uncommon.
I don't think there is a pre-existing vapor barrier. If there was one, it has deteriorated along with the insulation. Also, given that the chunks of insulation are normally golf ball size or smaller, with the occasional fist sized piece. I was figuring that if small pieces of an old vapor barrier were mixed in, there would be enough openings in the reused pieces that they wouldn't trap moisture. But again, this is one of the reasons I wanted to ask about reusing the old insulation.
As for the need of a vapor barrier, I've noticed two interesting things. In some places, it appears that the insulation has bonded to the top paper of the ceiling sheet rock. Scraping with a plastic dust pan can sometimes break it free. If I'm not careful, bits of the paper come with it. I'm assuming that this sticking/bonding, is due to moisture getting into the insulation. Likewise, I consolidated insulation from two bays last weekend, but didn't have plastic for a vapor barrier on hand at the time. When I went back to add the barrier this weekend, the insulation felt like it had started to stick together. It was possible for me to pull up a bay wide section that was 2-3 inches long. However, the slightest jolt would cause it to break down again into small pieces. Again I'm assuming this is moisture getting up into the attic. I haven't seen signs of recent moisture, so I'm also assuming that any moisture is coming from the conditioned parts of the house. Unfortunately, the previous owners waited until the roof was leaking live a sieve before reroofing. So finding evidence of moisture build up is hampered by large areas where moisture got in but didn't damage the roof boards enough to warrant board replacement.
Ventilation wise, I have ~176 round soffit vents, which I believe give me ~132 square inches of venting. Additionally, I have 7x 80 sq. in vents near the top of the hip roof (2x2x2x1 configuration with 1 north facing). I know the soffit vents need to be enlarged to match the top vents. The whole house fan I have calls for 8 sq feet of total vent area if I remember correctly.