'Tis the Season for Insulation: Vapor Barrier Needed for Attic?

We had a great question recently about insulation for the attic - should the attic have a vapor barrier and, if so, which direction to face it?

Insulation goes between you and the cold, right? It's the layer between you and the elements.

Well, vapor barrier is yet another layer. It's the layer between you and the insulation, and oddly enough, it's there to protect the insulation. Human bodies and just the general process of living, cooking, bathing all create moisture which can seep into the walls and affect insulation.

Image shows fiberglass insulation installed paper side down in an attic.

Image: Public Domain