Common sense dictates that a 3 inch blanket of polystyrene around my home will make it much easier to heat, but can filling the cavities lead to moisture crossing to the inside wall?
A. With polystyrene beads this cannot happen because the material does not soak liquid. Drinking cups are made from it because of this quality. When you drink from such a cup you notice the heat from the drink cannot penetrate the thin wall of the cup. This will give you some idea of how little heat can pass through a 2½ – 4 inch thickness of the same material in a wall cavity.