I need to insulate the attic, and I have some questions/concerns I
hope someone can help me out.

I have 2x6 in the attic, and it is about 80-90% covered with really
old what appears to be unfaced R19 fiberglass batts. It looks very
dirty (I replaced the roof so lots of dust falled on it) and aged.
Some of it is only half the height of the 2x6.

I've gotten some bids, and it is about $1400 for blow-in fiberglass,
and $1100 for blow-in cellulose of R30. All of them said I can leave
what I have up there, and just blow on top of it. (I am in San
Francisco area and the recommended attic is R38)

1) Is it good to blow on top of the old/aged/molded fiberglass? Should
I remove it and start clean? Does it insulate better that way?

2) fiberglass guys claims that cellulose will rot the wiring and
plumbing if it get wet, and will degrade. It is also very dusty which
is a big concern for me. Cellulose guy says fiberglass has health
risks. I have heating duct in the attic, and I am concerned about the
fiberglass, or the cellulose dust being drawn into the duct and gets
everywhere. Fiberglass guy says because my house is relatively old
(1950ish), he wasn't sure if I have tube/wire electrical wire up there
and cellulose is not safe. The cellulose guy said I don't need to
worry about it.

I am thinking about replacing the fiberglass wrapping around my
heating duct with more modern ones to seal off any gaps before I blow
the insulation up there. Right now I am leaning toward fiberglass as
it won't age or settle, and if I get a roof leak it won't get
destroyed. But how do I know if the installer won't fluff the blow-in?

3) The cellulose guy quoted me 8.1" for R30. I just don't like that
8.1". I mean how the hell is he going to make sure that the attic is
8.1, not 8, or 7.5? by eye balling it? Seems like I am paying by the
inch yet I have no easy way of verifying it. (I can't really walk in
the attic after it's done..) One of the fiberglass guy said they've
done this for so long, that they just know if it's 12" or not. Is it
true? Or do they basically do "ah, it looks like 12", done, when in
fact, it may very well be just 10")

Which weights more? Cellulose or fiberglass? If I go with R38 (doesn't
add that much to the cost), Is the weight of all that insulation cause
any problem with the ceiling in the long run?

Thanks!

Raymond