[Greenbuilding] Exist insulation in ceiling

[conservationarchitect at rockbridge.net]

I have a client that has renovated a 1940's house by gutting the plaster and
installed close cell foam and fiberglass (flash and batt) against the
siding.  This is not what my choice would have been, but I was not involved
in that decision.  However, I expect that part of the thermal envelope to be
tight.  They are now living there.  They had the floor above the
basement-crawlspace and the ceiling above the second floor insulated with
fiberglass.  The ducts and air handling units are in the attic.  The roof is
metal with hips and roofing boards with gaps.  Since I expect the ceiling
with duct penetrations and fiberglass to be leaky, I am recommending foaming
the rafter spaces to bring the attic into the conditioned thermal envelope.
This will capture the likely heat losses (winter) from leaky ceiling and
ductwork.  However, I was told by an insulation contractor that leaving the
old fiberglass could create humidity problem.  He did not know why.
However, he had that experience.  I wonder if his experience was closed cell
foam that trapped the house humidity, therefore raising the dew point.
Without expressly heating the attic, it could drop below the higher dew
point from elevated humidity.  However, if we use a vapor permeable foam
such as open cell foam, the vapor would not be trapped.  My approach would
be to create ventilation above the insulation to allow vapor to continue to
vent to the highest point.   I expect that incidental leakage for the ducts
and the ceiling insulated with common practice fiberglass, would deliver
heat.  However, expressly providing conditioned air supply and return would
provide assurance of avoiding a problem.  They did a pretty good job
installing the insulation for that kind of system.  Since we do not need to
heat the attic to comfort, could that be an intermediate zone that helps
insulate the main living areas by leaving the ceiling insulation.  Or is
there a reason that we must remove the newly installed fiberglass batts in
the attic floor.

Hope you all are still out there.

Eli