My nephew and his little daughter live in an old apartment that 'leaks like a sieve.' His heat bills were $220 a month last winter.

He doesn't have the ladder or the skills to get on the outside and caulk the windows. Would it be okay to advise him to buy some clear caulk and discreetly caulk the windows on the inside of the apartment? Or would that cause some kind of moisture problem in the walls of the place?