Joe, I don't want to argue with Peter, but if it is a plastic foam then it ought to be OK outside, UNLESS it is the one with a paper coating?
However if it doesn't have the paper coating, then it will still depend what you clad it with, of course, and if that is suitable. Mine are all finished with scribed surfaces to represent, brick, stone, timber etc and painted with ordinary emulsion paint from sample pots.
With one exception, all of my buildings are all made from foam board and I leave them outside permanently. In the pictures below, the station building is resin, but the shop and hotel at the rear are foamboard and have been out since late last spring and the bridge on the right is now on its third winter.
The thatched cottage has been outside since last June.
Windows, doors and frames are plasticard. I use one of the Evostick ultimate glues generally for this.
