Without knowing more about the room(s) in question — particularly dimensions and
positions of door(s) and window(s), it's kinda difficult to advise on what treatment for
use with monitors is appropriate.
If it's for mixing/critical listening, rather
than recording space, I'd strongly recommend investing in a seriously good set of
headphones. That way, you'll take the various rooms out of the equation and have a
consistent mix reference without having to re-learn the room every time. You also get to
use smaller crappier rooms and therefore pay lower rent so they pay for themselves