extruded polystyrene (XPS), which is particularly moisture-resistant; and expanded polystyrene (EPS), which is often the cheapest. Cons: Foam board insulation can be harder to install snugly in ...

In his guide, he details the difference between XPS (expanded polystyrene) and EPS (extruded polystyrene) and how each product’s closed cell and open cell nature affects the final board.