"EXT" wrote in message
anews.com...
Because foam board is flammable, I understand that in all cases, it must
be covered with drywall to make it fire resistant. This certainly would
apply in a furnace room, probably more than any other room.


Not classified as flammable. It will only burn if there is another source of
ignition (such as a defective furnace) but will not support combustion on
its own. Take a small piece outside and set it on fire. Take away the
lighter and it will go out in a few seconds. .