I live on the second floor of a century-old house that doesn't have central air. We have a large window A/C unit in the main hall but our front door is at the bottom of the stairs rather than the top. So this means the base of the stairs gets chilly but our floor doesn't really get the full benefit of the A/C.

Are there any good ways to deal with this? We're thinking about putting curtains around the top of the stairs. We're tenants so more ambitious changes like installing a door at the top of the stairs aren't really possible.