Hello again, as long as the furnace is vented to the outside.... and has its own air intake from the outside via plastic vent and intakes as with true sealed systems there will be no condensation in the attic.
a forced draft furnace still *must be vented to the outside, and still must have outside air intake vent from the outside
condensation is not an issue in any of these cases.
if the furnace exhaust
were vented illegally to the attic.. then you would have condensation and a very very dangerous installation for carbon monoxide poisoning of the occupants.
If the furnace has only an exhaust vent, as with most high efficiency furnaces
, its not a true sealed system and air intake from outside into the attic must be provided.
Leaving the top floor ceiling exposed to cooler air in the *winter, and only slightly warmer air in the summer due to the relatively small make up air opening required by law in that case will put a bit more load into the top floor rooms... in any event this would not be your problem.. its an engineering issue.
the engineer, not you, might owe the home owner 2 or 3 inches of insulation over the bottom of the attic space.
the Ruud unit specified by the way is not a sealed furnace by any stretch, so it must have outside air, and it *must be vented through the roof as you most likely have done.
refer to the furnace installation