Q:

What is the purpose of a vapor barrier?

I live in southern Brazil and I don't really know if I need a vapor barrier, mainly because I'm still confused about its purpose.
The house in question have brick exterior walls and interior drywall. At first I thought that I might need the vapor barrier in the wall between batrooms-bedroom and bathroom-bathroom. But after some research, it seems to be that vapor barrier is meant to avoid condensation on colder climates in heated houses, is that so?
Here we don't have cold winters (around 10-15°C average) and or summers can get quite hot (35°C average).

A:

Yes vapor barriers are to prevent condensation inside of insulation, greatly reducing its efficiency. In your climate, if you use air conditioning and that space has insulated exterior walls, you will want a vapor barrier on the exterior side of the insulation to prevent outside humidity from condensing inside the cooler insulation. If your exterior walls are just solid masonry, no vapor barrier is required.

