On Mar 1, 4:03 pm, wrote:
dirty (I replaced the roof so lots of dust falled on it) and aged.

Don't worry about it, the attic is going to be a dirty place after all
these years. It should always be ventilated to the outside anyway.
Dirt won't affect the R-value.

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?

It won't insulate better than if you had all new fluffy insulation to
the final height, but so what, you can always blow extra. Are they
going to charge you for the final height, or just the addition? It
isn't practical to remove te old stuff, there's decades worth of dust
that'll ve kicked up. You hhave to wear a breathing filter even in a
settled attic, so this'll be a nightmare.

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

Is it supply or return? A supply is at positive pressure, so nothing
is getting drawn in unless there are holes (not likely).

everywhere. Fiberglass guy says because my house is relatively old
(1950ish), he wasn't sure if I have tube/wire electrical wire up there

Look into what you have. As another poster has mentioned, it is a good
time to upgrade wiring before you blow more insulation, if indeed it
needs upgrading. If it is conduit, you can upgrade wiring from the
inside of the house.

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.

Oh, you're willing to do some work yourself! In that case, take a can
of expanding foam such as Great Stuff! up there and lay a bead around
electrical box openings in the ceiling. Wear a nose mask.

You should also give consideration to blowing cellulose in yourself.
1400 sq ft is about 1.5 hours of actual blowing. You'll need a couple
of helpers, but you can blow as much as you want, and it is fun!