Well, she either needs combustion air allowed into the crawlspace or a new high efficiency furnace with fresh air vent piping ran to the outside.

We’ve had houses that have existing 90+ percent furnaces in normal crawlspaces and they encapsulate after the fact. once the crawlspace is encapsulated we run an air intake to the outside. I see this as no different.

The fool tells me his reasons; the wise man persuades me with my own.